Since Americans first began rolling up their sleeves for coronavirus vaccines, health officials have said that those who are immunized are very unlikely to become infected, or to suffer serious illness or death. But preliminary data from seven states hint that the arrival of the Delta variant in July may have altered the calculus.Breakthrough infections in vaccinated people accounted for at least one in five newly diagnosed cases in six of those states and higher percentages of total hospitalizations and deaths than had been previously observed in all of them, according to figures gathered by The New York Times.The absolute numbers remain very low, however, and there is little doubt that the vaccines remain powerfully protective. This continues to be “a pandemic of the unvaccinated,” as federal health officials have often said.Still, the trend marks a change in how vaccinated Americans might regard their risks.“Remember when the early vaccine studies came out, it was like nobody gets hospitalized, nobody dies,” said Dr. Robert Wachter, chairman of the department of medicine at the University of California, San Francisco. “That clearly is not true.”The figures lend support to the view, widely held by officials in the Biden administration, that some Americans may benefit from booster shots in the coming months. Federal officials plan to authorize additional shots as early as mid-September, although it is not clear who will receive them.“If the chances of a breakthrough infection have gone up considerably, and I think the evidence is clear that they have, and the level of protection against severe illness is no longer as robust as it was, I think the case for boosters goes up pretty quickly,” Dr. Wachter said.The seven states — California, Colorado, Massachusetts, Oregon, Utah, Vermont and Virginia — were examined because they are keeping the most detailed data. It is not certain that the trends in those states hold throughout the United States.In any event, scientists have always expected that as the population of vaccinated people grows, they will be represented more frequently in tallies of the severely ill and dead.“We don’t want to dilute the message that the vaccine is tremendously successful and protective, more so than we ever hoped initially,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston.“The fact that we’re seeing breakthrough cases and breakthrough hospitalizations and deaths doesn’t diminish that it still saves many people’s lives.”The C.D.C. declined to comment on the states’ numbers. The agency is expected to discuss breakthrough infections, hospitalizations and vaccine efficacy at a news briefing on Wednesday.Most analyses of breakthrough infections have included figures collected through the end of June. Based on the cumulative figures, the C.D.C. and public health experts had concluded that breakthrough infections were extremely rare, and that vaccinated people were highly unlikely to become severely ill.The states’ data do affirm that vaccinated people are far less likely to become severely ill or to die from Covid-19.

Federal health officials said the new data justified a campaign of booster shots. But some scientists disagreed, saying not every American needs another dose.The Centers for Disease Control and Prevention released three studies on Wednesday that federal officials said provided evidence that booster shots of the Pfizer-BioNTech and Moderna coronavirus vaccines would be needed in the coming months.But some experts said the new research did not back up the decision to recommend booster shots for all Americans.Taken together, the studies show that although the vaccines remain highly effective against hospitalizations and deaths, the bulwark they provide against infection with the virus has weakened in the past few months.The finding accords with early data from seven states, gathered this week by The New York Times, suggesting a rise in breakthrough infections and a smaller increase in hospitalizations among the vaccinated as the Delta variant spread in July.The decline in effectiveness against infection may result from waning vaccine immunity, a lapse in precautions like wearing masks or the rise of the highly contagious Delta variant, experts said — or a combination of all three.“We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease, hospitalization and death,” Dr. Vivek Murthy, the surgeon general, said at a White House news briefing on Wednesday.Citing the data, federal health officials outlined a plan for Americans who received the two vaccines to get booster shots eight months after receiving their second doses, starting Sept. 20.People who received the Johnson & Johnson vaccine may also require additional doses. But that vaccine was not rolled out until March 2021, and a plan to provide boosters will be made after reviewing new data expected over the next few weeks, officials said.Some scientists were skeptical of the administration’s new initiative.“These data support giving additional doses of vaccine to highly immunocompromised persons and nursing home residents, not to the general public,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center and a former adviser on the pandemic to the administration.Boosters would only be warranted if the vaccines were failing to prevent hospitalizations with Covid-19, she said.“Feeling sick like a dog and laid up in bed, but not in the hospital with severe Covid, is not a good enough reason” for a campaign of booster shots, Dr. Gounder said. “We’ll be better protected by vaccinating the unvaccinated here and around the world.”Pharmacists prepared to give vaccinations at a nursing home in Brooklyn in January. Dr. Gounder said the data supported boosters for nursing home residents and immunocompromised people, not the general public.Yuki Iwamura/ReutersIt’s unclear whether a third dose would help people who did not produce a robust immune response to the first two doses, said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.And the recommendation for boosters may also end up undermining confidence in the vaccines, he warned: “A third shot will add to skepticism among people yet to receive one dose that the vaccines help them.”Together, the new studies indicate overall that vaccines have an effectiveness of roughly 55 percent against all infections, 80 percent against symptomatic infection, and 90 percent or higher against hospitalization, noted Ellie Murray, an epidemiologist at Boston University.“Those numbers are actually very good,” Dr. Murray said. “The only group that these data would suggest boosters for, to me, is the immunocompromised.”The apparent reduction in vaccine effectiveness against infection could instead have been caused by increased exposure to the highly contagious Delta variant during a period of unfettered social interactions, she added: “This seems to me like a real possibility, since many early vaccinated were motivated by a desire to see friends and family and get back to normal.”Dr. Murray said a booster shot would undoubtedly boost immunity in an individual, but the added benefit may be minimal — and obtained just as easily by wearing a mask, or avoiding indoor dining and crowded bars.The administration’s emphasis on vaccines has undermined the importance of building other precautions into people’s lives in ways that are comfortable and sustainable, and bolstering capacity for testing, Dr. Murray and other experts said.“This is part of why I think the administration’s focus on vaccines is so damaging to morale,” she added. “We probably won’t be going back to normal anytime soon.”Dr. Vivek Murthy, the surgeon general, last month. “We are concerned that this pattern of decline we are seeing will continue in the months ahead,” he said Wednesday.Doug Mills/The New York TimesBefore people can begin to receive boosters, the Food and Drug Administration must first authorize a third dose of the vaccines made by Pfizer-BioNTech and Moderna, and an advisory committee of the C.D.C. must review the evidence and make recommendations.One of the new C.D.C. studies analyzed the effectiveness of vaccines among residents of nearly 4,000 nursing homes from March 1 to May 9, before the Delta variant’s emergence, and nearly 15,000 nursing homes from June 21 to Aug. 1, when the variant dominated new infections in the country.The vaccines’ effectiveness at preventing infections dropped from about 75 percent to 53 percent between those dates, the study found. It did not evaluate the vaccines’ protection against severe illness.Nursing homes were required to report the number of immunized residents only after June 6, which “makes comparisons over time very challenging,” Dr. Murray said. “It’s fully possible that the vaccine effectiveness reported here hasn’t actually declined over time.”.css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}The decline in effectiveness also could have resulted from the spread of the Delta variant, Dr. Gounder said.“It makes sense to give an extra dose of vaccine to vaccinated nursing home residents, but what will have an even bigger impact on protecting those nursing home residents is to vaccinate their caregivers,” she said. Many heath aides in long-term care facilities remain unvaccinated.A second study evaluated data from New York State from May 3 to July 25, when the Delta variant grew to represent more than 80 percent of new cases. The effectiveness of vaccines in preventing cases in adults declined from 91.7 percent to 79.8 percent during that time, the study found. But the vaccines remained just as effective at preventing hospitalizations.During those weeks, New York recorded 9,675 breakthrough infections — roughly 20 percent of total cases in the state — and 1,271 hospitalizations in vaccinated people, which accounted for 15 percent of all Covid-19 hospitalizations.Although fully immunized people of all ages got infected with the virus, vaccine effectiveness showed the sharpest drop, from 90.6 percent to 74.6 percent, in people aged 18 through 49 — who are often the least likely to take precautions and the most likely to socialize.A pop-up Pfizer-BioNTech vaccination site in Corona, Queens, this month. Byron Smith for The New York TimesData from Israel has suggested that immunity against infection has waned in vaccinated adults who are 65 or older. But in the New York data, the effectiveness of the vaccines in that group barely budged.Adults ages 65 or older were more likely to be hospitalized than other age groups, regardless of vaccination status. But the vaccines did not show a decline in effectiveness against hospitalizations in any of the age groups.The third study from the C.D.C. found that the vaccines showed 90 percent effectiveness against hospitalizations in the country, “which is excellent,” Dr. Gounder noted.The vaccines were less protective against hospitalization in immunocompromised people. “But not all immunocompromised persons will respond to an additional dose of vaccine,” Dr. Gounder noted.To protect these vulnerable individuals, everyone around them should be vaccinated and should continue to wear masks, she added.The vaccines may appear to be less effective than they did in the trials that led to their authorization because those studies were conducted before the emergence of the Delta variant.Statistically, the vaccines can appear to lose relative effectiveness as more unvaccinated people become infected, recover and gain natural immunity. And scientists always expected that as more people became vaccinated, the proportions of vaccinated people among the infected would rise.If preventing infection is the goal, it would be wiser to develop a booster of a nasal spray vaccine, which is better at inducing immunity in the nose and throat, where the virus enters the body, Dr. Gounder said.

The Centers for Disease Control and Prevention released three studies on Wednesday that federal officials said provided evidence that booster shots of the Pfizer-BioNTech and Moderna coronavirus vaccines would be needed by all Americans in the coming months.But some experts said the new research did not support the decision to recommend booster shots for all Americans.Taken together, the studies show that although the vaccines remain highly effective against hospitalizations, the bulwark they provide against infection with the virus has weakened in the past few months.It’s unclear whether the decline in protection against infection is the result of waning immunity, a drop in precautions like wearing masks, or the rise of the highly contagious Delta variant — or a combination of all three.“We are concerned that this pattern of decline we are seeing will continue in the months ahead, which could lead to reduced protection against severe disease hospitalization and death,” Dr. Vivek Murthy, the surgeon general, said at a White House news briefing on Wednesday.Citing the data, federal health officials outlined a plan for Americans who received the two vaccines to get booster shots eight months after receiving their second doses, starting Sept. 20.Some scientists were deeply skeptical of the new plan.“These data support giving additional doses of vaccine to highly immunocompromised persons and nursing home residents, not to the general public,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center and a former adviser on the pandemic to the administration.Boosters would only be warranted if the vaccines were failing to prevent people from ending up hospitalized with Covid-19, she said.“Feeling sick like a dog and laid up in bed, but not in the hospital with severe Covid, is not a good enough reason,” Dr. Gounder said. “We’ll be better protected by vaccinating the unvaccinated here and around the world.”It’s unclear whether a third dose would help people who did not produce a robust response to the first two doses, said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.The recommendation for boosters may also end up undermining confidence in vaccines, he warned: “A third shot will add to skepticism among people yet to receive one dose that the vaccines help them.”Together, the new studies indicate overall that vaccines have an effectiveness of roughly 55 percent against infections, 80 percent against symptomatic infection, and 90 percent or higher against hospitalization, noted Ellie Murray, an epidemiologist at Boston University.“Those numbers are actually very good,” Dr. Murray said. “The only group that these data would suggest boosters for, to me, is the immunocompromised.”The drop in protection against infection could be the result of increased exposure to a highly contagious variant during a period of unfettered social interactions, she added: “This seems to me like a real possibility, since many early vaccinated were motivated by a desire to see friends and family and get back to normal.”Dr. Murray said boosters would undoubtedly boost immunity in an individual, but the benefit may be minimal — and obtained just as easily by wearing a mask, or avoiding indoor dining and crowded bars.The administration’s emphasis on vaccines has undermined the importance of building other precautions into people’s lives in ways that are comfortable and sustainable, and on building capacity for testing, she and other experts said.“This is part of why I think the administration’s focus on vaccines is so damaging to morale,” she added. “We probably won’t be going back to normal anytime soon.”Before people can begin to receive boosters, the Food and Drug Administration must first authorize a third dose of the mRNA vaccines made by Pfizer-BioNTech and Moderna, and an advisory committee of the C.D.C. must review the evidence and make recommendations.One of the new C.D.C. studies analyzed the effectiveness of vaccines among residents of nearly 4,000 nursing homes from March 1 to May 9, before the Delta variant’s emergence, and nearly 15,000 nursing homes from June 21 to Aug. 1, when the variant dominated new infections in the country.The vaccines’ effectiveness at preventing infections dropped from about 75 percent to 53 percent between those dates, the study found. It did not evaluate the vaccines’ protection against severe illness.Nursing homes were required to report the number of immunized residents only after June 6, which “makes comparisons over time very challenging,” Dr. Murray said. “It’s fully possible that the vaccine effectiveness reported here hasn’t actually declined over time.”.css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}The decline in effectiveness also could have resulted from the spread of the Delta variant, Dr. Gounder said.“It makes sense to give an extra dose of vaccine to vaccinated nursing home residents, but what will have an even bigger impact on protecting those nursing home residents is to vaccinate their caregivers,” she said. Many heath aides in long-term care facilities remain unvaccinated.A second study evaluated data from New York State from May 3 to July 25, when the Delta variant grew to represent more than 80 percent of new cases. The effectiveness of vaccines in preventing cases in adults declined from 91.7 percent to 79.8 percent during that time, the study found. But the vaccines remained just as effective at preventing hospitalizations.During those weeks, New York recorded 9,675 breakthrough infections — roughly 20 percent of total cases in the state — and 1,271 hospitalizations in vaccinated people, which accounted for 15 percent of all Covid-19 hospitalizations.Although fully immunized people of all ages got infected with the virus, vaccine effectiveness showed the sharpest drop, from 90.6 percent to 74.6 percent, in people aged 18 through 49 years — who are often the least likely to take precautions and the most likely to socialize.Data from Israel has suggested that immunity against infection has waned among in vaccinated adults who are 65 or older. But in the New York data, the effectiveness of the vaccines in that group barely budged.Adults ages 65 or older were more likely to be hospitalized than other age groups, regardless of vaccination status. But the vaccines did not show a decline in effectiveness against hospitalizations in any of the age groups.The third study from the C.D.C. found that the vaccines showed 90 percent effectiveness against hospitalizations in the country, “which is excellent,” Dr. Gounder noted.The vaccines were less protective against hospitalization in immunocompromised people. “But not all immunocompromised persons will respond to an additional dose of vaccine,” Dr. Gounder noted.To protect these vulnerable individuals, everyone around them should be vaccinated and should continue to wear masks, she added.The vaccines may appear to be less effective than they did in the clinical trials because the trials were conducted before the emergence of the highly contagious Delta variant. The vaccines can also seem to lose effectiveness as more unvaccinated people become infected with the virus and gain natural immunity.If preventing infection is the goal, it would be wiser to offer a booster of a nasal spray vaccine, which is better at inducing immunity in the nose and throat, where the virus enters, Dr. Gounder said.

Americans who got the Pfizer-BioNTech and Moderna coronavirus vaccines will be able to obtain booster shots eight months after receiving their second doses, the Biden administration announced on Wednesday.Health care workers, nursing home residents and other older adults who were vaccinated early will be first in line, starting Sept. 20. “We are starting to see evidence of reduced protection against mild and moderate disease,” officials of several federal agencies said in a prepared statement.Protection conferred by the vaccines against severe disease, hospitalization and death could diminish in the months ahead, especially among high-risk groups who were vaccinated early, the officials said. “For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability.”People who received the Johnson & Johnson vaccine may also require additional doses. But that vaccine was not rolled out until March 2021, and a plan to provide boosters for those individuals will be made after reviewing new data expected over next few weeks, officials said.Some experts immediately pushed back against the decision, saying only some older adults and people with weakened immune systems needed extra protection. The World Health Organization has asked that wealthy countries defer distributing booster shots until the end of September.“We will also continue to expand our efforts to increase the supply of vaccines for other countries, building further on the more than 600 million doses we have already committed to donate globally,” federal officials said.Before Americans can begin to receive boosters, the Food and Drug Administration must first authorize a third dose of the mRNA vaccines made by Pfizer-BioNTech and Moderna, and an advisory committee of the Centers for Disease Control and Prevention must review the evidence and make recommendations.Federal officials plan to begin by offering booster shots directly to residents of long-term care facilities, since the vaccines were distributed to this population early in the rollout and the virus poses a particular threat to the elderly.“We will continue to follow the science on a daily basis, and we are prepared to modify this plan should new data emerge that requires it,” federal officials said.

With the arrival of the contagious Delta variant, Covid hospitalizations and deaths among vaccinated Americans also may have increased, according to preliminary figures.Since Americans first began rolling up their sleeves for coronavirus vaccines, health officials have said that those who are immunized are very unlikely to become infected, or to suffer serious illness or death. But preliminary data from seven states hint that the arrival of the Delta variant in July may have altered the calculus.Breakthrough infections in vaccinated people accounted for at least one in five newly diagnosed cases in six of these states and higher percentages of total hospitalizations and deaths than had been previously observed in all of them, according to figures gathered by The New York Times.The absolute numbers remain very low, however, and there is little doubt that the vaccines remain powerfully protective. This continues to be “a pandemic of the unvaccinated,” as federal health officials have often said.Still, the rise indicates a change in how vaccinated Americans might regard their risks.“Remember when the early vaccine studies came out, it was like nobody gets hospitalized, nobody dies,” said Dr. Robert Wachter, chairman of the department of medicine at the University of California, San Francisco. “That clearly is not true.”The figures lend support to the view, widely held by officials in the Biden administration, that some Americans may benefit from booster shots in the coming months. Federal officials plan to authorize additional shots as early as mid-September, although it is not clear who will receive them.“If the chances of a breakthrough infection have gone up considerably, and I think the evidence is clear that they have, and the level of protection against severe illness is no longer as robust as it was, I think the case for boosters goes up pretty quickly,” Dr. Wachter said.The seven states — California, Colorado, Massachusetts, Oregon, Utah, Vermont and Virginia — were examined because they are keeping the most detailed data. It is not certain that the trends in these states hold throughout the country.In any event, scientists have always expected that as the population of vaccinated people grows, they will be represented more frequently in tallies of the severely ill and dead.“We don’t want to dilute the message that the vaccine is tremendously successful and protective, more so than we ever hoped initially,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston.“The fact that we’re seeing breakthrough cases and breakthrough hospitalizations and deaths doesn’t diminish that it still saves many people’s lives,” he added.The C.D.C. declined to comment on the states’ numbers. The agency is expected to discuss breakthrough infections, hospitalizations and vaccine efficacy at a news briefing on Wednesday.Most analyses of breakthrough infections have included figures collected through the end of June. Based on the cumulative figures, the C.D.C. and public health experts had concluded that breakthrough infections were extremely rare, and that vaccinated people were highly unlikely to become severely ill.The states’ data do affirm that vaccinated people are far less likely to become severely ill or to die from Covid-19. In California, for example, the 1,615 hospitalizations of people with breakthrough infections as of Aug. 8 represents just 0.007 percent of nearly 22 million fully immunized residents, and breakthrough deaths an even smaller percentage.But in six of the states, breakthrough infections accounted for 18 percent to 28 percent of recorded cases in recent weeks. (In Virginia, the outlier, 6.4 percent of the cases were in vaccinated people.) These numbers are likely to be underestimates, because most fully immunized people who become infected may not be taking careful precautions, or may not feel ill enough to seek a test.“There’s just a lot more virus circulating, and there’s something uniquely infectious about the variant,” said Natalie Dean, a biostatistician at Emory University in Atlanta.Breakthrough infections accounted for 12 percent to 24 percent of Covid-related hospitalizations in the states, The Times found. The number of deaths was small, so the proportion among vaccinated people is too variable to be useful, although it does appear to be higher than the C.D.C. estimate of 0.5 percent.If breakthrough infections are becoming common, “it’s also going to demonstrate how well these vaccines are working, and that they’re preventing hospitalization and death, which is really what we asked our vaccines to do,” said Anne Rimoin, an epidemiologist at the University of California, Los Angeles.A vast majority of vaccinated people who are hospitalized for Covid-19 are likely to be older adults or those who have weakened immune systems for other reasons. C.D.C. data show that 74 percent of breakthrough cases are among adults 65 or older.Most states do not compile the numbers by age, sex or the presence of other conditions. But in Oregon, which does, the median age for a breakthrough-associated death is 83 years.Workers set up overflow tents outside a hospital in Houston. Godofredo A. Vásquez/Houston Chronicle, via Associated PressThe numbers suggest that people who are at higher risk for complications from Covid-19, and anyone who lives with someone in that group, “really needs to seriously consider the risks that they’re taking now,” said Dr. Dean Sidelinger, a state epidemiologist and state health officer for Oregon.Especially for high-risk groups, “the most important message is that if you do get Covid, then take it seriously,” Dr. Dryden-Petersen said. “Don’t assume that it’s going to be mild. And seek out therapies like monoclonal antibodies if you’re high-risk, to try to prevent the need for hospitalization.”The figures also underscore the urgency of vaccinating all nursing home residents and staff members.The states’ numbers come with many caveats. Immunized adults greatly outnumber unvaccinated adults in most states, and their ranks are growing by the day. So the proportional representation of the vaccinated among cases, hospitalizations and deaths would also be expected to rise.Breakthrough infections are also likely to be most severe among older adults or those who have conditions like obesity or diabetes. These individuals have the highest rates of vaccination, and yet the highest risk of weak or waning immunity.Their representation among the hospitalized may skew the percentages, making it seem that vaccinated Americans overall are hospitalized more often than is really the case..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}“People who are older are both more likely to be vaccinated and more likely to be hospitalized given a breakthrough,” Dr. Dean noted.Registering for a vaccination at a church in the Bronx. C.D.C. data show that 74 percent of breakthrough cases are among adults 65 or older.James Estrin/The New York TimesTo draw more direct conclusions about breakthrough infections, she and other experts noted, states would need to collect and report timely and consistent data to the C.D.C.Instead, each state slices its data set differently, in different time frames, and many still don’t record mild breakthrough cases because of a directive from the C.D.C. in May. “This is a microcosm of the larger challenges that we’ve had getting data together,” Dr. Dean said.Studies are also needed on how often people with breakthrough infections spread the virus to others, including to unvaccinated children, and how many of them have persistent symptoms for months after the active infection has resolved, Dr. Rimoin said.Some scientists noted that while the vaccines are highly effective, people ought to be more cautious, including wearing masks in public indoor spaces, than they were earlier this summer. As more vaccinated people comply, the incidence of cases and hospitalizations may decrease.In the meantime, the trend in breakthrough infections, if it holds up nationwide, is likely to intensify the debate around boosters.Most experts still say that boosters are unlikely to be needed in the near future for the general population. But a rise in hospitalizations among the vaccinated may indicate that the boosters are required for some high-risk groups.Data from Israel and from a handful of studies have suggested that immunity to the virus may wane after the first few months in some groups and may need to be supplemented with booster shots.Among vaccinated Americans, 72 percent of those who are 65 or older already say they want a booster shot, according to one recent survey.“When boosters become available, barring arguments about ethics about global supply of vaccines, you should go and get a vaccine,” said Dr. Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health.Ideally, Dr. Mina said, doctors would track their patients’ antibody levels over time to assess who needed a booster shot, much as they do for measles and rubella vaccines in health care workers. But the C.D.C. and the Food and Drug Administration have said that available antibody tests are not accurate enough for that purpose.Dr. Dryden-Peterson said it was hard for him to reconcile the idea of boosters for Americans with his work in Botswana, where vaccines are mostly unavailable.“Even just one dose helps a lot in terms of preventing death,” he said. “We have done an incomplete job of vaccinating the United States, and that should probably be our focus rather than moving on to boosters.”

For every one million Americans immunized with a coronavirus vaccine, about 60 develop temporary heart problems, according to a study published on Wednesday in the journal JAMA Network.The complications were all short-lived, the researchers found. And these heart problems are far more common among patients who develop Covid-19, outside experts noted.Analyzing the medical records of just over 2 million people who had received at least one dose of a coronavirus vaccine through May 2021, the new study found 20 cases of myocarditis, or inflammation of the heart muscle, and 37 cases of pericarditis, inflammation of the membrane surrounding the heart.Patients who were hospitalized were discharged after only a few days, and none died.The incidence of myocarditis in the study, at 10 cases per million vaccinated, is higher than the Centers for Disease Control and Prevention’s estimate of 4.8 cases per million, suggesting that there may be more cases than are reported to the federal database for tracking so-called adverse events following vaccinations.“We see that these adverse events are leading to very short and unremarkable hospital stays,” said Dr. Jeremy Faust, an emergency medicine physician at Brigham and Women’s Hospital in Boston who was not involved in the study. “The same can’t be said of hospitalizations for Covid-19 in this or any age group so far.”“When people are hospitalized for Covid, the consequences are far more severe,” added Dr. Faust, who has compared rates of myocarditis following vaccination to those among Covid-19 patients.The researchers, with the Providence health care system, evaluated medical records from 40 hospitals in Washington, Oregon, Montana and Los Angeles County, Calif.They found that myocarditis developed a median of 3.5 days after vaccination, mostly after the second dose, and in people with a median age of 36 years. Three-quarters of the 20 cases were in men.The 19 patients who were admitted to the hospital were discharged after a median of two days. About three weeks after vaccination, 13 patients had recovered from their symptoms and the remaining seven were improving.Pericarditis affected older patients, at a median age of 59 years, and later, about 20 days after vaccination, the researchers found. Pericarditis, too, was more common in men. Of the 37 cases identified, 13 were admitted to the hospital; the median stay was one day.A separate study, posted online last week, suggested that the incidence of myocarditis in boys ages 12 to 17 who had Covid-19 was 876 per million; in girls of the same age group with Covid-19, the incidence was 213 cases per million.The study has not yet been peer reviewed or published in a scientific journal.

The C.D.C.’s new masking advice was based in part on data showing that the virus can thrive in the airways of vaccinated people. The findings are expected on Friday. The recommendation that vaccinated people in some parts of the country dust off their masks was based largely on one troublesome finding, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.New research showed that vaccinated people infected with the Delta variant carry tremendous amounts of the virus in the nose and throat, she said in an email responding to questions from The New York Times. The finding contradicts what scientists had observed in vaccinated people infected with previous versions of the virus, who mostly seemed incapable of infecting others.That conclusion dealt Americans a heavy blow: People with so-called breakthrough infections — cases that occur despite full vaccination — of the Delta variant may be just as contagious as unvaccinated people, even if they have no symptoms.That means fully immunized people with young children, aging parents, or friends and family with weak immune systems will need to renew vigilance, particularly in high-transmission communities. Vaccinated Americans may need to wear masks not just to protect themselves, but everyone in their orbit.There are 67,000 new cases per day on average in the United States, as of Thursday. If vaccinated people are transmitting the Delta variant, they may be contributing to the increases — although probably to a far lesser degree than the unvaccinated.The C.D.C. has not yet published its data, frustrating experts who want to understand the basis for the change of heart on masks. Four scientists familiar with the research said it was compelling and justified the C.D.C.’s advice that the vaccinated wear masks again in public indoor spaces.The research was conducted by people outside the C.D.C., the scientists said, and the agency is working quickly to analyze and publish the results. The agency expects to publish the research on Friday, one official said. Some of the research may be related in part to an outbreak in Provincetown, Mass., where Fourth of July festivities have led to 882 cases as of Thursday. Nearly three-quarters of those people were fully vaccinated. The agency also has tracked data from the Covid-19 Sports and Society Workgroup, a coalition of professional sports leagues that is testing more than 10,000 people at least daily and sequencing all infections.It’s still unclear how common breakthrough infections are and how long the virus persists in the body in those cases. Breakthroughs are rare, and unvaccinated people account for the bulk of virus transmission, Dr. Walensky said.Dr. Rochelle Walensky, the C.D.C. director, appearing before a Senate committee last week.Stefani Reynolds for The New York TimesRegardless, the data that the C.D.C. is reviewing suggest that even fully immunized people can be unwilling vectors for the virus. “We believe at individual level they might, which is why we updated our recommendation,” Dr. Walensky said in her email to The Times.The conclusion also suggests that vaccinated people who are exposed to the virus should get tested, even if they feel fine. (In Britain, vaccinated people who are contacts of a known case are required to isolate for 10 days.)The new data do not mean that the vaccines are ineffective. The vaccines still powerfully prevent severe illness and death, as they were meant to, and people with breakthrough infections very rarely end up in a hospital.About 97 percent of people hospitalized with Covid-19 are unvaccinated, according to data from the C.D.C. But scientists warned even last year that the vaccines might not completely prevent infection or transmission. (Immunity from natural infection may offer even less protection.)Previous versions of the virus rarely broke through the immunization barrier, which prompted the C.D.C. to advise in May that vaccinated people could go mask-free indoors. But the usual rules don’t seem to apply to the Delta variant.The variant is twice as contagious as the original virus, and one study suggested that the amount of virus in unvaccinated people infected with Delta might be a thousand times higher than seen in people infected with the original version of the virus. The C.D.C. data support that finding, said one expert familiar with the results.Anecdotes of clusters of breakthrough infections have become increasingly frequent, with groups of vaccinated people reporting sniffles, headache, sore throat, or a loss of taste or smell — symptoms of an infection in the upper respiratory tract..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}But the overwhelming majority do not end up needing intensive medical care, because the immune defenses produced by the vaccine destroy the virus before it can get to the lungs.“We’re still going to see a huge, huge, huge impact on severity of disease and hospitalization,” said Michal Tal, an immunologist at Stanford University. “That’s really what the vaccine was made to do.”The Delta variant seems to flourish in the nose, and its abundance may explain why more people with breakthrough infections are experiencing cold-like symptoms.Jim Wilson/The New York TimesThe coronavirus vaccines are injected into muscle, and the antibodies produced in response mostly remain in the blood. Some antibodies may make their way to the nose, the main port of entry for the virus, but not enough to block it.“The vaccines — they’re beautiful, they work, they’re amazing,” said Frances Lund, a viral immunologist at the University of Alabama at Birmingham. “But they’re not going to give you that local immunity.”When people are exposed to any respiratory pathogen, it may find a foothold in the mucosal lining of the nose — without causing any harm beyond that. “If you walked down the street and swabbed people, you would find people that had viruses in their mucosa who were asymptomatic,” said Dr. Michael Marks, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Our immune system is mostly fighting these things off most of the time.”But the Delta variant seems to flourish in the nose, and its abundance may explain why more people than scientists expected are experiencing breakthrough infections and cold-like symptoms.Still, when the virus tries to snake down into the lungs, immune cells in vaccinated people ramp up and rapidly clear the infection before it wreaks much havoc. That means vaccinated people should be infected and contagious for a much shorter period of time than unvaccinated people, Dr. Lund said.“But that doesn’t mean that in those first couple of days, when they’re infected, they can’t transmit it to somebody else,” she added.To stop the virus right where it enters, some experts have advocated nasal spray vaccines that would prevent the invader from gaining purchase in the upper airway. “Vaccine 1.0 should prevent death and hospitalization. Vaccine 2.0 should prevent transmission,” Dr. Tal said. “We just need another iteration.”

Infections in vaccinated Americans also may be as transmissible as those in unvaccinated people, the document said, and lead more often to severe illness.The Delta variant is much more contagious, more likely to break through protections afforded by the vaccines and may cause more severe disease than all other known versions of the virus, according to an internal presentation circulated within the Centers for Disease Control and Prevention.Dr. Rochelle P. Walensky, the director of the agency, acknowledged on Tuesday that vaccinated people with so-called breakthrough infections of the Delta variant carry just as much virus in the nose and throat as unvaccinated people, and may spread it just as readily, if less often.But the internal document lays out a broader and even grimmer view of the variant.The Delta variant is more transmissible than the viruses that cause MERS, SARS, Ebola, the common cold, the seasonal flu and smallpox, and it is as contagious as chickenpox, according to the document, a copy of which was obtained by The New York Times.The immediate next step for the agency is to “acknowledge the war has changed,” the document said. Its contents were first reported by The Washington Post on Thursday evening.The document’s tone reflects alarm among C.D.C. scientists about Delta’s spread across the country, said a federal official who has seen the research described in the document. The agency is expected to publish additional data on the variant on Friday.“The C.D.C. is very concerned with the data coming in that Delta is a very serious threat that requires action now,” the official said.There were 71,000 new cases per day on average in the United States, as of Thursday. The new data suggest that vaccinated people are spreading the virus and contributing to those numbers — although probably to a far lesser degree than the unvaccinated.Dr. Walensky has called transmission by vaccinated people a rare event, but other scientists have suggested it may be more common than once thought.The agency’s new masking guidelines for vaccinated people, introduced on Tuesday, were based on the information presented in the document. The C.D.C. recommended that vaccinated people wear masks indoors in public settings in communities with high transmission of the virus.But the internal document hints that even that recommendation may not go far enough. “Given higher transmissibility and current vaccine coverage, universal masking is essential,” the document said.The agency’s data suggest that people with weak immune systems should wear masks even in places that do not have high transmission of the virus. So should vaccinated Americans who are in contact with young children, older adults, or otherwise vulnerable people.There are roughly 35,000 symptomatic infections per week among 162 million vaccinated Americans, according to data collected by the C.D.C. as of July 24 that was cited in the internal presentation. But the agency does not track all mild or asymptomatic infections, so the actual incidence may be higher.Infection with the Delta variant produces virus amounts in the airways that are tenfold higher than what is seen in people infected with the Alpha variant, which is also highly contagious, the document noted..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}The amount of virus in a person infected with Delta is a thousandfold more than what is seen in people infected with the original version of the virus, according to one recent study.The C.D.C. document relies on data from multiple studies, including an analysis of a recent outbreak in Provincetown, Mass., which began after the town’s Fourth of July festivities. By Thursday, that cluster had grown to 882 cases. About 74 percent were vaccinated, local health officials have said.Detailed analysis of the spread of cases showed that people infected with Delta carry enormous amounts of virus in their nose and throat, regardless of vaccination status, according to the C.D.C. document.“This is one of the most impressive examples of citizen science I have seen,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “The people involved in the Provincetown outbreak were meticulous in making lists of their contacts and exposures.”Infection with the Delta variant may be more likely to lead to severe illness, the document noted. Studies from Canada and Scotland found that people infected with the variant are more likely to be hospitalized, while research in Singapore indicated that they are more likely to require oxygen.Still, the C.D.C.’s figures show that the vaccines are highly effective in preventing serious illness, hospitalization and death in vaccinated people, experts said.“Overall, Delta is the troubling variant we already knew it was,” said John Moore, a virologist at Weill Cornell Medicine in New York. “But the sky isn’t falling and vaccination still protects strongly against the worse outcomes.”

In communities with growing caseloads, vaccinated and unvaccinated people should return to masking in indoor public areas, health officials said.Revising a decision made just two months ago, the Centers for Disease Control and Prevention said on Tuesday that people vaccinated against the coronavirus should resume wearing masks in public indoor spaces in parts of the country where the virus is surging.C.D.C. officials also recommended universal masking for teachers, staff, students and visitors in schools, regardless of vaccination status and community transmission of the virus. With additional precautions, schools nonetheless should return to in-person learning in the fall, according to agency officials.The recommendations are another baleful twist in the course of America’s pandemic, a war-weary concession that the virus is outstripping vaccination efforts. The agency’s move follows rising case counts in states like Florida and Missouri, as well as growing reports of breakthrough infections of the more contagious Delta variant among people who are fully immunized.“The Delta variant is showing every day its willingness to outsmart us,” Dr. Rochelle Walensky, director of the C.D.C., said at a news briefing on Tuesday. Data from several states and other countries show that the variant behaves differently from previous versions of the coronavirus, she added: “This new science is worrisome and unfortunately warrants an update to our recommendation.”“This is not a decision we at CDC have made lightly,” Dr. Walensky added. “This weighs heavily on me.” Jen Psaki, the White House press secretary, said at a news conference on Tuesday that changing the guidance was crucial to “battling an ever-evolving virus” and that the Biden administration supported the effort.“Their job is to look at evolving information, evolving data, an evolving historic pandemic, and provide guidance to the American public,” Ms. Psaki said.“The virus is changing, we are dealing with a dynamic situation,” said Dr. Anthony S. Fauci, the Biden administration’s top pandemic adviser. The C.D.C. is correct to revisit its recommendations as the virus evolves, he said.“I don’t think you can say that this is just flip-flopping back and forth,” he added. “They’re dealing with new information that the science is providing.”The vaccines remain remarkably effective against the worst outcomes of infection with any form of the coronavirus, including hospitalization and death. But the new guidelines explicitly apply to both the unvaccinated and vaccinated, a sharp departure from the agency’s position since May that vaccinated people do not need to wear masks in most indoor spaces.Those recommendations, which seemed to signal a winding down of the pandemic, were based on earlier data suggesting that vaccinated people rarely become infected and almost never transmit the virus, making masking unnecessary.But that was before the arrival of the Delta variant, which now accounts for the bulk of infections in the United States. C.D.C. officials were persuaded by new scientific evidence showing that even vaccinated people may become infected and may carry the virus in great amounts, Dr. Walensky acknowledged at the news briefing.“The big concern is that the next variant that might emerge — just potentially a few mutations away — could evade our vaccine,” she said.Some public health experts welcomed the agency’s decision to revise its guidelines. Based on what scientists are learning about the Delta variant’s ability to cause breakthrough infections, “this is a move in the right direction,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York.The American Federation of Teachers and the National Education Association, the two leading teachers’ unions, strongly endorsed the C.D.C.’s move to universal masking in schools.“Masking inside schools, regardless of vaccine status, is required as an important way to deal with the changing realities of virus transmission,” said Randi Weingarten, president of the A.F.T. “It is a necessary precaution until children under 12 can receive a Covid vaccine and more Americans over 12 get vaccinated.”Dr. Rochelle Walensky, the C.D.C. director, during a Senate hearing earlier this month.Stefani Reynolds for The New York TimesWhether state and local health officials are willing to follow the agency’s guidance is far from certain. And there is sure to be resistance from pandemic-fatigued Americans, particularly in regions of the country where vaccination rates are low and concerns about the virus are muted.Some jurisdictions, like Los Angeles County and St. Louis County, Mo., have already reinstated mask mandates in response to rising cases. But Arkansas, one of the states with the highest numbers, has retained a ban on mask mandates even as vaccination rates lag.As recently as last week, a C.D.C. spokesman said that the agency had no plans to change its guidance, unless there were a significant change in the science. Researchers have begun to turn up disturbing new data.The Delta variant is thought to be about twice as contagious as the original version of the virus. Some research now suggests that people infected with the variant carry about a thousandfold more virus than those infected with other variants, and may stay infected for longer.C.D.C. officials were swayed by new research showing that even vaccinated people may carry great amounts of the variant virus in the nose and throat, hinting that they also may spread it to others.Large so-called viral loads may help explain reports of breakthrough infections in groups of vaccinated people. For example, an outbreak that began in Provincetown, Mass., after Fourth of July festivities there has grown to include at least 765 cases, according to Steve Katsurinis, chair of the Provincetown Board of Health.Of the 469 cases reported among Massachusetts residents alone, 74 percent were in people who were fully immunized, Mr. Katsurinis said.Smaller clusters of breakthrough infections have been reported after weddings, family reunions and dinner parties. Some of the infected had symptoms, but the vast majority were not seriously ill, suggesting that immunity produced by the vaccines quickly curbs the virus.Vaccines “are not a force field,” said Jennifer Nuzzo, an epidemiologist at Johns Hopkins University’s Bloomberg School of Public Health. Instead, vaccination trains the immune system to recognize cells that become infected with the virus..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}“The term breakthrough infection is probably a bit misleading,” she said. “It’s probably more realistic that we talk about breakthrough disease and how much of that is occurring.”Dr. Walensky on Tuesday again urged people to get vaccinated, noting that the rise of cases and hospitalizations is greatest in places with low vaccination rates and among unvaccinated people. She acknowledged that some vaccinated people can become infected with the Delta variant and may be contagious, but maintained that it was a rare event. But the C.D.C. is tracking only breakthrough infections that lead to hospitalization or death among vaccinated Americans. Officials have previously said that vaccinated people account for just 3 percent of hospitalizations.Dr. Gounder and other experts said that it is unclear how often vaccinated people transmit the virus to others, but it may be more common than scientists had predicted as the original virus spread.“We’ve seen increasing numbers of breakthrough infections, and it seems like most of those may be happening in places where people are exposed to a lot of Covid,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston, who has been reviewing breakthrough infections in Massachusetts.Masks can augment the effectiveness of the vaccine, said Dr. Scott Dryden-Peterson, an infectious disease physician. “Almost everywhere in the U.S. it’s a good idea,” he said.Brittainy Newman for The New York TimesVaccinated people — particularly people with weak immune systems or otherwise at high risk — should consider wearing masks even in areas of low transmission, he said: “Masks can effectively reduce the amount of virus that we breathe in and prevent us from getting sick, and so they augment the impact of our vaccine. Almost everywhere in the U.S., it’s a good idea.”Infections have been rising swiftly in the United States, to more than 56,000 daily cases on average, as of Tuesday, more than four times the number a month ago. Hospitalizations have also been ticking up in nearly all states, and deaths have risen to an average of 275 per day.“Given what we’re seeing, that’s absolutely needed right now to slow and curb transmission,” Dr. Robby Sikka, a physician who worked with the N.B.A.’s Minnesota Timberwolves, said of the new masking guidance.“Not everyone who has a breakthrough infection will be at risk for transmission, but it’s imperative to note that there is a risk of transmission,” he said.But Dr. Sikka noted that relying on states or localities to set masking rules will require more testing than is being done now to identify people with mild or asymptomatic infections. “That’s something that we’re probably not totally prepared to do,” he said.Given that the virus seems likely to become endemic, permanently embedded in American life, federal officials need to articulate an even clearer plan for long-term masking, Dr. Nuzzo said.“The question is, what are the off ramps for masking? It’s really important for us to define that,” she said. “If we want to continue to ask people to step up, we need to give them a vision of what we’re working toward.”The C.D.C. should have simply made a universal recommendation and told all Americans to wear masks indoors, said Ali Mokdad, an epidemiologist at University of Washington and former C.D.C. scientist.“The director said the guidance is for people in areas of high transmission, but if you look at the country, every state is seeing a rise in transmission,” Dr. Mokdad said. “So why not say, ‘Everybody in the U.S. should be wearing a mask indoors?’ The whole country is on fire.”

In communities with growing caseloads, vaccinated and unvaccinated people should return to masking in indoor public areas, health officials said.Revising a decision made just two months ago, the Centers for Disease Control and Prevention said on Tuesday that people vaccinated against the coronavirus should resume wearing masks in public indoor spaces in parts of the country where the virus is surging.C.D.C. officials also recommended universal masking for teachers, staff, students and visitors in schools, regardless of vaccination status and community transmission of the virus. With additional precautions, schools nonetheless should return to in-person learning in the fall, Dr. Rochelle Walensky, the agency’s director, said at a news briefing on Tuesday.The recommendations are another baleful twist in the course of America’s pandemic, a war-weary concession that the virus is outstripping vaccination efforts. The agency’s move follows rising case counts in states like Florida and Missouri, as well as growing reports of breakthrough infections of the more contagious Delta variant among people who are fully immunized.Jen Psaki, the White House press secretary, said at a news conference on Tuesday that changing the guidance was crucial to “battling an ever-evolving virus” and that the Biden administration supported the effort.“Their job is to look at evolving information, evolving data, an evolving historic pandemic, and provide guidance to the American public,” Ms. Psaki said.The vaccines remain remarkably effective against the worst outcomes of infection with any form of the coronavirus, including hospitalization and death. But the new guidelines explicitly apply to both the unvaccinated and vaccinated, a sharp departure from the agency’s position since May that vaccinated people do not need to wear masks in most indoor spaces.Those recommendations, which seemed to signal a winding down of the pandemic, were based on earlier data suggesting that vaccinated people rarely become infected and almost never transmit the virus, making masking unnecessary.But that was before the arrival of the Delta variant, which now accounts for the bulk of infections in the United States. C.D.C. officials were persuaded by new scientific evidence showing that even vaccinated people may become infected and may carry the virus in great amounts, according to three federal officials with knowledge of the discussions.Some public health experts welcomed the agency’s decision to revise its guidelines. Based on what scientists are learning about the Delta variant’s ability to cause breakthrough infections, “this is a move in the right direction,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York.The American Federation of Teachers and the National Education Association, the two leading teachers’ unions, strongly endorsed the C.D.C.’s move to universal masking in schools.“Masking inside schools, regardless of vaccine status, is required as an important way to deal with the changing realities of virus transmission,” said Randi Weingarten, president of the A.F.T. “It is a necessary precaution until children under 12 can receive a Covid vaccine and more Americans over 12 get vaccinated.”Dr. Rochelle Walensky, the C.D.C. director, during a Senate hearing earlier this month.Stefani Reynolds for The New York TimesWhether state and local health officials are willing to follow the agency’s guidance is far from certain. And there is sure to be resistance from pandemic-fatigued Americans, particularly in regions of the country where vaccination rates are low and concerns about the virus are muted.Some jurisdictions, like Los Angeles County and St. Louis County, Mo., have already reinstated mask mandates in response to rising cases. But Arkansas, one of the states with the highest numbers, has retained a ban on mask mandates even as vaccination rates lag.As recently as last week, a C.D.C. spokesman said that the agency had no plans to change its guidance, unless there were a significant change in the science. Researchers have begun to turn up disturbing new data.The Delta variant is thought to be about twice as contagious as the original version of the virus. Some research now suggests that people infected with the variant carry about a thousandfold more virus than those infected with other variants, and may stay infected for longer.C.D.C. officials were swayed by new research showing that even vaccinated people may carry great amounts of the variant virus in the nose and throat, hinting that they also may spread it to others, according to three federal officials familiar with the matter.Large so-called viral loads, particularly in the nose and throat, may help explain reports of breakthrough infections in groups of vaccinated people. For example, an outbreak that began in Provincetown, Mass., after Fourth of July festivities there has grown to include at least 765 cases, according to Steve Katsurinis, chair of the Provincetown Board of Health.Of the 469 cases reported among Massachusetts residents, 74 percent were in people who were fully immunized, Mr. Katsurinis said.Smaller clusters of breakthrough infections have been reported after weddings, family reunions and dinner parties. Some of the infected had symptoms, but the vast majority were not seriously ill, suggesting that immunity produced by the vaccines quickly curbs the virus.Vaccines “are not a force field,” said Jennifer Nuzzo, an epidemiologist at Johns Hopkins University’s Bloomberg School of Public Health. Instead, vaccination trains the immune system to recognize cells that become infected with the virus.“The term breakthrough infection is probably a bit misleading,” she said. “It’s probably more realistic that we talk about breakthrough disease and how much of that is occurring.”.css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}Dr. Walensky on Tuesday again urged people to get vaccinated, noting that the rise of cases and hospitalizations is greatest in places with low vaccination rates and among unvaccinated people. She acknowledged that some vaccinated people can become infected with the Delta variant and may be contagious, but maintained that it was a rare event. But the C.D.C. is tracking only breakthrough infections that lead to hospitalization or death among vaccinated Americans. Officials have previously said that vaccinated people account for just 3 percent of hospitalizations.Dr. Gounder and other experts said that it is unclear how often vaccinated people transmit the virus to others, but it may be more common than scientists had predicted as the original virus spread.“We’ve seen increasing numbers of breakthrough infections, and it seems like most of those may be happening in places where people are exposed to a lot of Covid,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston, who has been reviewing breakthrough infections in Massachusetts.Masks can augment the effectiveness of the vaccine, said Dr. Scott Dryden-Peterson, an infectious disease physician. “Almost everywhere in the U.S. it’s a good idea,” he said.Brittainy Newman for The New York TimesVaccinated people — particularly people with weak immune systems or otherwise at high risk — should consider wearing masks even in areas of low transmission, he said: “Masks can effectively reduce the amount of virus that we breathe in and prevent us from getting sick, and so they augment the impact of our vaccine. Almost everywhere in the U.S., it’s a good idea.”Infections have been rising swiftly in the United States, to more than 56,000 daily cases, on average, as of Tuesday, more than four times the number four weeks ago. Hospitalizations have also been ticking up in nearly all states, and deaths have risen to an average of 275 per day.“Given what we’re seeing, that’s absolutely needed right now to slow and curb transmission,” Dr. Robby Sikka, a physician who worked with the N.B.A.’s Minnesota Timberwolves, said of the new masking guidance.“Not everyone who has a breakthrough infection will be at risk for transmission, but it’s imperative to note that there is a risk of transmission,” he said.But Dr. Sikka noted that relying on states or localities to set masking rules will require more testing than is being done now to identify people with mild or asymptomatic infections. “That’s something that we’re probably not totally prepared to do,” he said.Given that the virus seems likely to become endemic, permanently embedded in American life, federal officials need to articulate an even clearer plan for long-term masking, Dr. Nuzzo said.“The question is, what are the off ramps for masking? It’s really important for us to define that,” she said. “If we want to continue to ask people to step up, we need to give them a vision of what we’re working toward.”