Boys between 12 and 15 years of age have a small but increased risk of heart problems after receiving the second dose of the Pfizer-BioNTech vaccine, Israeli researchers reported on Wednesday.Myocarditis, or inflammation of the heart muscle, occurred in 1 of 12,361 boys of this age group within a week of receiving the second dose, the study found.The cases were mild, and the side effect remains uncommon. Moreover, several studies have shown that the risk of myocarditis following Covid is much higher than that following vaccination.Still, the Israeli figure is higher than the Centers for Disease Control and Prevention estimate of 1 case per 16,129 vaccinated adolescents aged 12 to 17 years. Israeli researchers previously reported that nearly 11 of every 100,000 males between 16 and 29 years of age were at heightened risk of myocarditis after vaccination.Although the new results are based only on the Pfizer-BioNTech vaccine, other studies have found that the incidence of myocarditis may be higher with the other mRNA vaccine made by Moderna.Some European countries paused the use of the Moderna vaccine in male adolescents because of the risk. In the United States, only the Pfizer-BioNTech vaccine is authorized for use in adolescents and children 5 and older.Concern about heart inflammation may have prompted the Food and Drug Administration to ask Pfizer-BioNTech and Moderna to enroll more children in their vaccine trials. Even so, Pfizer’s trial of the vaccine included just 567 boys aged 12 to 15 years — too few to detect uncommon side effects.The new study tracked hospitalizations for myocarditis between June 2, 2021, and Oct. 20, 2021, through a surveillance system of the Israeli Ministry of health. During that time, 404,407 adolescents ages 12 to 15 years received the first dose of vaccine, and 326,463 adolescents received the second dose.The researchers found 18 reports of hospitalization for myocarditis. They excluded two of the cases because of “reasonable alternative diagnoses,” and two others because they occurred 46 and 70 days after the second dose — too long after the vaccination to be related, the researchers said.Of the remaining 14, one was in an unvaccinated adolescent, one in an adolescent who had received the first vaccine dose less than weeks earlier, and 12 cases within a week after the second dose. The results were published in The New England Journal of Medicine.All of the cases were clinically mild, and the adolescents were hospitalized for an average of three days. None was readmitted during 30 days of follow-up.The researchers estimated that myocarditis occurs in 0.56 male adolescents per 100,000 after the first dose, and in 8.09 per 100,000 after the second dose. The corresponding risk among girls of that age was negligible.The study may have underestimated the risk of myocarditis, because the researchers looked only at boys who were hospitalized for the condition. Myocarditis can manifest as chest pain, shortness of breath and the feeling of having a fast-beating, fluttering or pounding heart, according to the C.D.C.

Booster shots of the Pfizer-BioNTech and Moderna vaccines aren’t just preventing infections with the contagious Omicron variant — they’re also keeping infected Americans from ending up in the hospital, according to data published on Friday by the Centers for Disease Control and Prevention.The extra doses are 90 percent effective against hospitalization with the variant, the agency reported. Booster shots also reduced the likelihood of a visit to an emergency department or urgent care clinic. The extra doses were most effective against infection and death among Americans aged 50 and older, the data showed.Over all, the new data show that the vaccines were more protective against the Delta variant than against Omicron, which lab studies have found is partially able to sidestep the body’s immune response.It is generally accepted that booster shots keep people from becoming infected, at least for a while. Data from Israel and other countries have also suggested that boosters can help prevent severe illness and hospitalization, especially in older adults.“Data from other countries have also shown significant benefit of getting the booster, but this is really showing it in the U.S.,” Akiko Iwasaki, an immunologist at Yale University, said of the figures released on Friday. “These numbers should be very convincing.”On Thursday night, the C.D.C. published additional data showing that in December, unvaccinated Americans 50 years and older were about 45 times more likely to be hospitalized than those who were vaccinated and got a third shot. Yet less than 40 percent of fully vaccinated Americans who are eligible for a booster shot have received one.

The Omicron coronavirus variant is moving much faster than researchers can, worsening a longstanding problem: The agency must make tough decisions with scant data.The Centers for Disease Control and Prevention was long revered for its methodical and meticulous scientific approach. Agencies in other nations modeled themselves after the world’s most highly regarded public health authority, even adopting the name.At the outset of the pandemic, the C.D.C. moved at its accustomed pace. But this time, with a novel virus moving so quickly, the country paid a price: Testing and surveillance lagged as the agency tried to implement dated approaches with creaky infrastructure. Officials were late to recommend masking, in part because federal scientists took too long to recognize that the virus was airborne.Now the contagious Omicron variant is pushing the C.D.C. into uncharted territory. Because decisions must be made at a breakneck pace, the agency has issued recommendations based on what once would have been considered insufficient evidence, amid growing public concern about how these guidelines affect the economy and education.The agency’s director, Dr. Rochelle P. Walensky, has sometimes skipped much of the traditional scientific review process, most recently in shortening the isolation period for infected Americans.After the Trump administration’s pattern of interference, President Biden came to office promising to restore the C.D.C.’s reputation for independence and rigorous science. The challenge now for Dr. Walensky is figuring out how to convey this message to the public: The science is incomplete, and this is our best advice for now.For a bureaucracy staffed primarily by medical professionals, the change has not been easy.In recent interviews, some officials at the C.D.C. privately described the decisions as demoralizing, and worried about Dr. Walensky’s increasing reliance on a small group of advisers and what they saw as the White House’s heavy political influence on her actions.Yet others outside the agency commended Dr. Walensky for short-circuiting a laborious process and taking a pragmatic approach to managing a national emergency, saying she was right to move ahead even when the data was unclear and agency researchers remained unsure.There are policy considerations in a pandemic that are “not the sole purview of C.D.C.,” said Dr. Richard E. Besser, who served as interim chief of the agency during the H1N1 influenza virus outbreak of 2009. But, he added, “I think we need some more clarity” when policy and economics drive agency recommendations.As of Sunday, more than 800,000 Americans on average are infected daily, according to data gathered by The New York Times. Many schools and businesses are struggling to remain open; hospitals in nearly two dozen states are nearing capacity.At the end of December, Dr. Walensky announced that infected Americans would need to isolate for only five days, not 10, if they were no longer experiencing symptoms, and that a negative test result would not be required to end the isolation period.Critics complained that the virus might spread as contagious people were allowed to return to offices and schools. Many pointed out that the research supporting a shortened isolation period for Omicron infections was scant.But the recommendation had an important advantage: It could help keep hospitals, businesses and schools afloat through the worst of the Omicron surge.The recommendations for isolation are “basically correct,” said Dr. Thomas R. Frieden, who led the agency under President Barack Obama. “The problem is, they were not explained.”Dr. Walensky and the C.D.C. declined requests for comment on new tensions in the agency’s decision-making. But the director has frequently cited rapidly evolving science as justification for recommendations that proved to be confusing or unpopular.Testifying before the Senate on Tuesday, Dr. Walensky said that the agency’s new recommendations for shortened isolation periods represent “swift science-based action to address the very real possibility of staffing shortages.”It is has been something of a mantra for the director.Last March, the C.D.C. said that schoolchildren could safely sit three feet apart in classrooms, instead of six feet, although there was virtually no research to back up the recommendation. But the move did make it easier for administrators to consider opening schools.In May, Dr. Walensky cited scientific data when she told vaccinated people that they could take off their masks and mingle freely, much to the consternation of experts who said that the move ignored the possibility of breakthrough infections. (Those arrived with the Delta variant.)In August, Dr. Walensky joined President Biden in supporting booster shots for all Americans, well before scientists at the Food and Drug Administration or at her own agency had a chance to review the data on whether they were needed.The most recent example, the isolation advice, left turmoil within the agency over the way in which it was established and announced.On the Sunday night after Christmas, Dr. Walensky called an emergency meeting of the agency’s Covid response leaders. She told them the agency would shrink the recommended isolation period and would drop a negative test result as a requirement for leaving isolation, according to an official familiar with the video call who spoke on condition of anonymity because the individual was not authorized to speak on the matter.The new guidance would be made public the next day, Dr. Walensky said, and officials were not to discuss it until then.National Guard members helped staff a virus testing site in Columbus, Ohio, on Thursday.Maddie McGarvey for The New York TimesStunned, the scientists scrambled to gather the limited data to support the recommendations and to rewrite the hundreds of pages on the agency’s website that touch on quarantine and isolation.Before publishing a new recommendation, federal researchers normally pore over data, write a draft and fine-tune it based on comments from others. There was so little evidence for shortened isolation — and even that was based mostly on the Delta variant — that the “science brief” that typically accompanies guidance was downgraded to a “rationale” document.Some researchers bristled at being left out of the decision-making process and were enraged by the agency’s public statement the next day that the change was “motivated by science.”While some believed the new five-day cutoff was arbitrary, they also knew of data suggesting that rapid tests might miss some Omicron infections, and so mostly agreed with Dr. Walensky’s decision not to require a negative test result before ending isolation.But when Dr. Walensky informed staff of the new recommendations in the emergency meeting on Dec. 26, they were far from ready. Over the next week, C.D.C. scientists struggled to adjust hundreds of guidance documents on the agency’s website.About 2,000 health officials, public health lab directors and epidemiologists at the state and city levels join a weekly call with C.D.C. officials.On the call on Monday, Dec. 27, just hours before the C.D.C. released its statement, state and local officials peppered agency scientists with questions about the plans for isolation guidance for the general public.The Coronavirus Pandemic: Key Things to KnowCard 1 of 4The latest Covid data in the U.S.

The Centers for Disease Control and Prevention on Friday clarified its stance on various kinds of masks, acknowledging that the cloth masks frequently worn by Americans do not offer as much protection as surgical masks or respirators.While this disparity is widely known to the general public, the update marks the first time the C.D.C. has explicitly addressed the differences. The agency’s website also no longer refers to a shortage of respirators. The change comes as infections with the highly contagious Omicron variant continue to soar. Some experts have said that cloth masks are inadequate to protect from the variant, and have urged the C.D.C. to recommend respirators for ordinary citizens.The agency did not go that far. Its updated language now says that “a respirator may be considered in certain situations and by certain people when greater protection is needed or desired.” The previous version of the recommendations said individuals may choose to use a disposable N95 respirator instead of a mask “when supplies are available.” N95 respirators, so named because they can filter out 95 percent of all airborne particles when used correctly, were in short supply early in the pandemic. At the time, the C.D.C. and the World Health Organization both repeatedly said that ordinary citizens did not need to wear masks unless they were sick and coughing. The C.D.C. also said regular surgical masks were “an acceptable alternative” for doctors and nurses when interacting with a patient infected with the coronavirus — a move that angered medical personnel. Critics charged that the recommendations were based not on what would best protect Americans, and were instead prompted by a shortage of N95 respirators.When the C.D.C. finally recommended masks for ordinary Americans, it emphasized cloth face coverings. It took months more for the C.D.C. and the W.H.O. to concede that the coronavirus can be carried by tiny droplets called aerosols, which can linger indoors for hours. According to the C.D.C.’s new description of masks, loosely woven cloth products provide the least protection and layered finely woven products offer more. Well-fitting disposable surgical masks and KN95s — another type of respirator mask — are more protective than all cloth masks, and well-fitting respirators, including N95s, offer the highest level of protection.The agency urged Americans to “wear the most protective mask you can that fits well and that you will wear consistently.”

Some people with a weakened immune system can get a fourth dose of the coronavirus vaccine as early as this coming week, according to new recommendations from the Centers for Disease Control and Prevention.The C.D.C. endorsed a third dose of the Pfizer-BioNTech and Moderna vaccines for moderately or severely immunocompromised people on Aug. 13, but said this would be considered a part of the primary immunization, not a booster shot.In October, the agency said those immunocompromised people could receive a booster shot — a fourth dose of vaccine, six months after their third dose. These guidelines were consistent with its recommendation for other adults.Last week, hoping to stem the surge of infections with the highly contagious Omicron variant, the C.D.C. shortened that interval to five months for a booster shot for Pfizer-BioNTech or Moderna recipients.For immunocompromised people who received a single shot of the coronavirus vaccine made by Johnson & Johnson, the C.D.C. does not recommend additional primary doses, but advises that they get a booster shot of the Moderna or Pfizer-BioNTech vaccines two months after the first dose.Some people are born with absent or faulty immune systems, and in others, treatments for some diseases like cancer diminish the potency of immune defenses. The C.D.C. estimates there are about seven million immunocompromised individuals in the country.Many of them produce few to no antibodies in response to a vaccine or an infection, leaving them susceptible to the virus. When they do become infected, they may suffer prolonged illness, with death rates as high as 55 percent.It is unclear what proportion of those people are protected by additional doses. Still, with the Omicron variant surging in the country, some immunocompromised people sought out fourth or even fifth shots of the vaccines even before the C.D.C. changed its guidelines. While receiving multiple doses of vaccines in a short period is unlikely to be harmful, it may produce diminishing returns, according to some experts.The C.D.C. has said that any American 12 and older can receive a Pfizer-BioNTech booster — those 18 and older can alternatively receive a Moderna booster — five months after completing their initial shots with those vaccines. Israel has already begun offering fourth doses to high-risk groups including older adults. But the Biden administration has not yet said whether it plans to follow suit.When asked on Friday about the possibility of a fourth shot for the general population, Dr. Rochelle Walensky, director of the C.D.C., said that focus remained on Americans eligible for their third shots.She added that U.S. officials remained in close touch with Israel experts about their data. “We will be following our own data carefully as well, to see how these boosters are working in terms of waning effectiveness, not just for infection but, importantly, for severe disease,” she said.

Despite sharp criticism, the Centers for Disease Control and Prevention on Tuesday stood by its recommendation that Americans infected with the coronavirus end their isolation after five days without first obtaining a negative virus test.The agency guidelines, released last week, shortened the recommended isolation period from 10 days to five for infected people who do not have symptoms, or whose symptoms are resolving. But the agency did not recommend testing before leaving isolation, an omission that surprised and worried many outside experts.Dr. Rochelle Walensky, the C.D.C. director, told The New York Times last week that the recommendation was based on evidence showing that most people are no longer contagious five days after symptoms appear. But the agency did not share the data behind the decision.The guidelines suggested that following isolation, recovered people wear a mask around others for another five days. Dr. Walensky defended the decision to omit testing, saying that rapid tests are not reliable for determining when an individual is no longer contagious.The Food and Drug Administration also said last week that rapid tests might be less reliable at picking up Omicron, compared with infections caused by previous variants. But the agency declined to elaborate on the basis for that assertion.Many public health experts said ending isolation at five days was risky, and that the agency should at least urge people to test negative before mingling with others. They also derided the agency for what some saw as a capitulation to pressure from corporations buckling under staff shortages during the Omicron surge.Responding to some of the criticisms, Dr. Anthony S. Fauci, the nation’s top coronavirus adviser, said on Sunday that the C.D.C. was considering adding testing to its recommendations, and would clarify the guidance within days.The agency was widely expected to add testing to its guidance as early as Tuesday. But in a surprising move, officials updated the C.D.C. website without a testing requirement. The agency on Tuesday also provided scientific rationale for shortening the isolation period.The guidelines now say that an individual who “has access to a test and wants to test” at the end of the isolation period may do so, but stops short of a recommendation to test.The new guidelines “facilitate individual social and well-being needs, return to work, and maintenance of critical infrastructure,” according to an update posted on Tuesday to the agency’s website.For people exposed to the virus who are unvaccinated or have not yet received a booster dose of one of the vaccines, the agency now recommends a quarantine of five days, followed by strict mask use for another five.People who have received a booster shot do not need to quarantine following an exposure, but should wear a mask for 10 days afterward, according to the guidelines.

A booster shot of the Moderna coronavirus vaccine significantly raises the level of antibodies that can thwart the Omicron variant, the company announced on Monday.The news arrives as Omicron rapidly advances across the world, and most coronavirus vaccines seem unable to stave off infection from the highly contagious variant.Moderna’s results show that the currently authorized booster dose of 50 micrograms — half the dose given for primary immunization — increased the level of antibodies by roughly 37-fold, the company said. A full dose of 100 micrograms was even more powerful, raising antibody levels about 83-fold compared with pre-boost levels, Moderna said.Both doses produced side effects comparable to those seen after the two-dose primary series. But the dose of 100 micrograms showed slightly more frequent adverse reactions relative to the authorized 50-microgram dose.The results are based on laboratory tests that do not capture the full range of the body’s immune response against the virus. Although vaccines may not prevent infection from the variant, they are expected to prevent severe illness in the vast majority of people.The data have also not been published or reviewed by independent experts. Moderna said it was preparing a manuscript with the data that would be posted online.The pharmaceutical companies Pfizer and BioNTech announced earlier this month that a booster shot of their vaccine also increased the level of antibodies against Omicron.Moderna tested a third shot of several versions of its vaccine, each in 20 people. Before boosting, all the individuals had low levels of antibodies that can prevent Omicron infection. At Day 29, after receiving a third shot, the 50-microgram and 100-microgram doses of the current vaccine both sharply increased antibody levels.The company also tested “multivalent” booster shots that incorporate mutations seen in the Beta and Delta variants, many of which are also present in Omicron. Those continuing trials each have 300 to 600 people enrolled in them. The 50-microgram and 100-microgram doses of the multivalent boosters increased antibody levels to similarly high levels, Moderna said.Given how quickly Omicron is marching through the world, Moderna said, the company will focus its near-term efforts on extra shots of the original vaccine. It also plans to test a booster shot that is specific to the Omicron variant early next year and to include Omicron in a multivalent booster.“To respond to this highly transmissible variant, Moderna will continue to rapidly advance an Omicron-specific booster candidate into clinical testing in case it becomes necessary in the future,” said Stéphane Bancel, Moderna’s chief executive officer.

A new mathematical analysis strengthens concerns about the effects of the new variant on the pandemic’s course.Underscoring increasing concerns about Omicron, scientists in South Africa said on Friday that the newest coronavirus variant appeared to spread more than twice as quickly as Delta, which had been considered the most contagious version of the virus.Omicron’s rapid spread results from a combination of contagiousness and an ability to dodge the body’s immune defenses, the researchers said. But the contribution of each factor is not yet certain.“We’re not sure what that mixture is,” said Carl Pearson, a mathematical modeler at the London School of Hygiene and Tropical Medicine who led the analysis. “It’s possible that it might even be less transmissible than Delta.”Dr. Pearson posted the results on Twitter. The research has not yet been peer-reviewed nor published in a scientific journal.On Thursday, researchers reported that the new variant may partly dodge immunity gained from a previous infection. It’s still unclear whether, or to what degree, Omicron may evade protection conferred by the vaccines.But some experts said they would expect the outcome to be similar.“It’s scary that there are so many reinfections happening, which means that vaccine-induced immunity may also be impacted in similar way,” said Akiko Iwasaki, an immunologist at Yale.The Omicron variant has appeared in nearly two dozen countries. The United States has identified at least 10 cases in six states. President Biden reiterated on Friday morning that his administration’s newest pandemic measures, which were announced this week, should be sufficient to blunt the spread of Omicron.A Covid testing station at the O.R. Tambo International Airport in Johannesburg on Thursday.Joao Silva/The New York TimesThe variant was first identified in South Africa on Nov. 23 and has quickly come to account for about three-quarters of new cases in that country. South Africa reported 11,535 new coronavirus cases on Thursday, a 35 percent jump from the day before, and the proportion of positive test results increased to 22.4 percent from 16.5 percent.“It is actually really striking how quickly it seems to have taken over,” said Juliet Pulliam, the director of an epidemiological modeling center at Stellenbosch University in South Africa, who led the earlier research on immunity.Omicron cases are doubling roughly every three days in Gauteng province, which is home to South Africa’s densely populated economic hub, according to the new estimates by the researchers.In a mathematical analysis, they estimated the variant’s Rt — a measure of how quickly a virus spreads — and compared it with the metric for Delta. They found that Omicron’s Rt is nearly 2.5 times as high as that of Delta’s.That figure depends on not just how contagious the variant may be, but also its ability to sidestep the body’s immune defenses once it reaches a new host.Based on the mutations Omicron carries, some researchers had warned that the variant may turn out to be highly transmissible, and that the current vaccines may not be as effective against it as against previous variants.In the research published on Thursday, Dr. Pulliam and her colleagues estimated the new variant’s ability to evade immunity by looking at confirmed cases in the country through late November.They reported an uptick in reinfections among people who had tested positive for the virus at least 90 days earlier, suggesting that the immunity gained from a previous bout with the virus was no longer as protective as it had appeared. The increase in reinfections coincided with Omicron’s spread in the country.A quirk of Omicron’s genetic code made it easy to distinguish the variant from Delta in diagnostic tests, and that helped scientists quickly spot its steep rise, Dr. Pulliam said.“If we hadn’t had that, we probably would be several weeks behind where we are now in terms of recognizing that there was a new variant,” she added.The team did not confirm that the reinfections they observed were caused by the new variant, but said it was a reasonable assumption. A similar spike did not occur when the Beta and Delta variants were dominant, the scientists noted.The Coronavirus Pandemic: Key Things to KnowCard 1 of 5The Omicron variant.

The Omicron variant of the coronavirus has alarmed many scientists because of the sheer number of genetic mutations it carries — about 50 in all, including at least 26 that are unique to it. But more does not necessarily mean worse: Mutations sometimes work together to make a virus more fearsome, but they may also cancel one another out.“In principle, mutations can also work against each other,” said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle. “However, in this case evolutionary selection is more likely to lead to the spread of a new variant with favorable than unfavorable combinations of mutations.”Still, this phenomenon, called epistasis, is why scientists are reluctant to speculate on Omicron’s attributes, even though individual mutations in the variant are associated with greater transmissibility or with an ability to dodge the body’s immune defenses.“It is important to get a sense of the full virus,” said Penny Moore, a virologist at the National Institute for Communicable Diseases in South Africa.Dr. Moore’s team is among dozens worldwide trying to understand whether current vaccines will work against Omicron. The researchers are creating artificial versions of the virus that contain all of Omicron’s mutations, rather than making judgments based on a subset of mutations.It’s a lesson researchers learned last year, when the Beta variant emerged in South Africa. They estimated that variant’s ability to evade immunity based on one particular mutation, E484K. But Beta also had two other mutations that turned out to affect sensitivity to vaccines.“The combination of those three mutations was more resistance than a virus that contained only E484K,” Dr. Moore said. Studying the single mutation “turned out to be misleading.”Omicron carries a mutation called N501Y, which is thought to allow the virus to bind to human cells more tightly. This mutation was also present in the Alpha variant and was linked to its contagiousness.“Nonetheless, it ended up being Delta, which doesn’t have that particular mutation, that was more even more transmissible than Alpha,” Dr. Bloom said. “That’s because Delta had other mutations that enhance transmissibility.”A variant’s contagiousness depends on how well the virus binds to receptors on human cells, but also on the stability of the virus, where in the airways it replicates and how much of it is exhaled.Omicron has a cluster of mutations that are all linked to tighter binding to human cells. “But acting together, they might have a somewhat different effect,” Dr. Bloom said. For that reason, he added, he cannot predict how the variant will act in the body.That will require laboratory studies, which are underway across the globe.

A “Frankenstein mix” of mutations raises concerns, but the variant may remain vulnerable to current vaccines. If not, revisions will be necessary.As nations severed air links from southern Africa amid fears of another global surge of the coronavirus, scientists scrambled on Sunday to gather data on the new Omicron variant, its capabilities and — perhaps most important — how effectively the current vaccines will protect against it.The early findings are a mixed picture. The variant may be more transmissible and better able to evade the body’s immune responses, both to vaccination and to natural infection, than prior versions of the virus, experts said in interviews.The vaccines may well continue to ward off severe illness and death, although booster doses may be needed to protect most people. Still, the makers of the two most effective vaccines, Pfizer-BioNTech and Moderna, are preparing to reformulate their shots if necessary.“We really need to be vigilant about this new variant and preparing for it,” said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle.“Probably in a few weeks, we’ll have a better sense of how much this variant is spreading and how necessary it might be to push forward with a variant vaccine,” Dr. Bloom said.Even as scientists began vigorous scrutiny of the new variant, countries around the world curtailed travel to and from nations in southern Africa, where Omicron was first identified. Despite the restrictions, the virus has been found in a half-dozen European countries, including the United Kingdom, as well as Australia, Israel and Hong Kong.Already, Omicron accounts for most of the 2,300 new daily cases in the province of Gauteng, South Africa, President Cyril Ramaphosa announced on Sunday. Nationally, new infections have more than tripled in the past week, and test positivity has increased to 9 percent from 2 percent.Scientists have reacted more quickly to Omicron than to any other variant. In just 36 hours from the first signs of trouble in South Africa on Tuesday, researchers analyzed samples from 100 infected patients, collated the data and alerted the world, said Tulio de Oliveira, a geneticist at the Nelson R. Mandela School of Medicine in Durban.Within an hour of the first alarm, scientists in South Africa also rushed to test coronavirus vaccines against the new variant. Now, dozens of teams worldwide — including researchers at Pfizer-BioNTech and Moderna — have joined the chase.They won’t know the results for two weeks, at the earliest. But the mutations that Omicron carries suggest that the vaccines most likely will be less effective, to some unknown degree, than they were against any previous variant. Cape Town International Airport in South Africa on Sunday. Many countries restricted travel from the country over the weekend.Nic Bothma/EPA, via Shutterstock“Based on lots of work people have done on other variants and other mutations, we can be pretty confident these mutations are going to cause an appreciable drop in antibody neutralization,” Dr. Bloom said, referring to the body’s ability to attack an invading virus.South African doctors are seeing an increase in reinfections in people who already had a bout of Covid-19, suggesting that the variant can overcome natural immunity, said Dr. Richard Lessells, an infectious diseases physician at the University of KwaZulu-Natal.Omicron has about 50 mutations, including more than 30 in the spike, a viral protein on its surface that the vaccines train the body to recognize and attack.Some of these mutations have been seen before. Some were thought to have powered the Beta variant’s ability to sidestep vaccines, while others most likely turbocharged Delta’s extreme contagiousness.“My best guess is that this combines both of those elements,” Penny Moore, a virologist at the National Institute for Communicable Diseases in South Africa, said of the new variant.But Omicron also has 26 unique spike mutations, compared with 10 in Delta and six in Beta. Many of them seem likely to render the variant more difficult for the immune system to recognize and thwart.“There are many we’ve never studied before, but just looking at the location on the spike, they are in regions that we know are immuno-dominant,” Dr. Moore said, referring to parts of the spike protein that interact with the body’s immune defenses.Dr. Moore’s team is perhaps the furthest along in testing how well the vaccines hold up against Omicron. She and her colleagues are preparing to test blood from fully immunized people against a synthetic version of the Omicron variant.Creating such a “pseudovirus” — a viral stand-in that contains all of the mutations — takes time, but results may be available in about 10 days.To more closely mimic what people are likely to encounter, another team led by Alex Sigal, a virologist at the Africa Health Research Institute, is growing live Omicron, which will be tested against the blood of fully immunized people, as well as those who were previously infected.Those results may take longer but should provide a fuller picture of the vaccines’ performance, Dr. Sigal said.If the vaccines prove to be much less potent against Omicron, they may need to be tweaked to enhance their effectiveness. Preparing for the worst, Moderna, Pfizer-BioNTech and Johnson & Johnson are planning to test an artificial version of Omicron against their vaccines.The mRNA vaccines in particular — Moderna’s and Pfizer-BioNTech’s — were built with technology that should permit rapid modification. Pfizer’s scientists “can adapt the current vaccine within six weeks and ship initial batches within 100 days in the event of an escape variant” that eludes the immune system, said Jerica Pitts, a spokeswoman for Pfizer.Moderna’s work began on Tuesday, immediately after its scientists learned of Omicron — the fastest the company has ever responded to a variant, said Dr. Stephen Hoge, Moderna’s president.Even without data on Omicron’s spread, it was obvious the variant would be a formidable threat to vaccines, he said.“This thing is a Frankenstein mix of all of the greatest hits,” Dr. Hoge said, referring to the variant’s many concerning mutations. “It just triggered every one of our alarm bells.”Moderna could update its current vaccine in about two months and have clinical results in about three months if necessary, he said.Both companies also plan to test whether booster shots will bolster the immune system enough to fend off the new variant. Boosters of the Pfizer-BioNTech and Moderna vaccines have been shown to raise antibody levels significantly.But those antibodies may not be broadly effective against every iteration of the virus, and may not be enough to neutralize Omicron entirely, said Michel Nussenzweig, an immunologist at Rockefeller University in New York.People who recover from Covid and then receive even one dose of a vaccine tend to produce a broader range of antibodies, capable of recognizing more versions of the virus, than do people who are only vaccinated.A Pfizer research and development facility in Chesterfield, Mo. A revised version of its coronavirus vaccine could be ready in six weeks if needed, a spokeswoman said. Whitney Curtis for The New York Times“It’s clear that hybrid immunity, the kind that people get when they are both infected and vaccinated, is superior, and that is very, very likely to take care of this thing, too,” Dr. Nussenzweig said.“After two doses of vaccine, we did not see that. But we’re hoping that after three doses, maybe there’ll be some catching up,” he said..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-1kpebx{margin:0 auto;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-1kpebx{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-1kpebx{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-1kpebx{font-size:1.25rem;line-height:1.4375rem;}}.css-1gtxqqv{margin-bottom:0;}.css-1g3vlj0{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-1g3vlj0{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-1g3vlj0 strong{font-weight:600;}.css-1g3vlj0 em{font-style:italic;}.css-1g3vlj0{margin-bottom:0;margin-top:0.25rem;}.css-19zsuqr{display:block;margin-bottom:0.9375rem;}.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. Nussenzweig and his colleagues are preparing to test Omicron against the mRNA vaccines, as well as the vaccines made by Johnson & Johnson and AstraZeneca. They hope to have results within a month.Omicron-specific vaccines created in just weeks would be a miraculous feat. But the prospect of producing and distributing them raises daunting questions.If new versions are required to protect people everywhere, companies should make them available to the African countries that most need them and can least afford them, Dr. de Oliveira said.“South Africa at least has managed to procure their own vaccines,” he said. But poorer countries like Sudan, Mozambique, Eswatini and Lesotho will need low-cost options.Pfizer did not respond to a question about low-cost vaccines for African nations. Dr. Hoge, of Moderna, said the company already had an agreement with the African Union to deliver 110 million doses at $3.50 per half dose of vaccine.Dr. Hoge said he recognized that 110 million was less than 10 percent of Africa’s population. But, he noted, “we’re also the smallest of all manufacturers out there, and so 10 percent hopefully is useful.”Despite the frustration that South African scientists have expressed about vaccine inequity and punishing travel restrictions, they have been inundated with requests for genetic sequences of Omicron from Italy, Germany, Australia and New Zealand, as well as labs in North America.The more teams involved, the better, said Dr. Moore, who received about 50 requests just on Saturday. As the virus moves across the globe, it is likely to keep changing. “Getting the right combination of mutations in itself is a moving target,” she said.Researchers everywhere want to avoid drawing conclusions prematurely, a mistake they made when the Beta variant surfaced. Preliminary tests of that variant took only one known mutation into account and underestimated its ability to evade the immune system, Dr. Moore recalled. (Fortunately, the variant also turned out to be less contagious.)To get a full picture of the effectiveness of the vaccines against Omicron, scientists must look not just at antibody levels but also at immune cells that can recognize and destroy infected cells. Immune cells called T cells are crucial for preventing an infection from progressing to serious illness and death.Some of Omicron’s mutations occur in parts of the virus targeted by T cells, meaning the variant may be more difficult for T cells to recognize.Already, a computer simulation has predicted that those mutations may alter about six of the hundreds of regions that T cells can recognize, said Wendy Burgers, an immunologist at the University of Cape Town.That may not seem like much. But people make varying sets of T cells, so depending on which targets the mutations knock out, some people may barely be affected by Omicron — and others may be left vulnerable.Dr. Burgers is hoping to obtain blood from 50 people infected with the variant to gauge how the mutations will play out across a population. Once the samples are in hand, results will be available after “probably a week of very late nights and analysis,” she said.Even if the vaccines hold up against Omicron, new versions will probably be needed at some point, and perhaps soon. The virus is acquiring mutations much faster than expected, Dr. Bloom said.Seasonal influenza is the often cited example of a virus that mutates quickly, requiring regular updates to vaccines. But the coronavirus is “at least comparable and possibly even faster than that,” Dr. Bloom said. “There’s always going to be new variants arising.”Lynsey Chutel