People who find themselves alone after a significant knock to the head are at higher risk of harm.It appeared to be an ordinary fall: Bob Saget, the actor and comedian, knocked his head on something and, perhaps thinking nothing of it, went to sleep, his family said on Wednesday.But the chilling consequences — Mr. Saget, 65, died some hours later on Jan. 9 from blunt head trauma, a medical examiner ruled — have underscored the dangers of traumatic brain injuries, even those that do not initially seem to be causes for alarm.Some 61,000 deaths in 2019 were related to traumatic brain injuries, according to the Centers for Disease Control and Prevention, and nearly half of head trauma-related hospitalizations result from falls.Brain injury experts said on Thursday that Mr. Saget’s case was relatively uncommon: People with serious head trauma would be expected to have noticeable symptoms, like a headache, nausea or confusion. And they can generally be saved by surgeons opening up their skull and relieving pressure on the brain from bleeding.But certain situations put people at higher risk for the sort of deterioration that Mr. Saget experienced, doctors said.As serious a risk factor as any, doctors said, is simply being alone. Someone with a head injury can lose touch with their usual decision-making capacities and become confused, agitated or unusually sleepy. Those symptoms, in turn, can stand in the way of getting help.And while there was no indication that Mr. Saget was taking blood thinners, experts said the medications can greatly accelerate the type of bleeding after a head injury that forces the brain downward and compresses the centers that regulate breathing and other vital functions. More Americans are being prescribed these drugs as the population ages.Mr. Saget had been in an Orlando hotel room during a weekend of stand-up comedy acts when he was found unresponsive. The local medical examiner’s office announced on Wednesday that his death resulted from “blunt head trauma,” and said that “his injuries were most likely incurred from an unwitnessed fall.”There was no evidence of illegal drugs in his system, the medical examiner said.“If you have a head injury, you never — and I mean never — be by yourself for the first 24 hours,” said Dr. Gavin Britz, the chair in neurosurgery at Houston Methodist.Dr. Britz said that he would counsel people who get a significant knock to the head to see a doctor or, short of that, to ask someone to track their symptoms and even wake them up occasionally at night for monitoring.Brain injury experts also emphasized that the presence of symptoms usually indicated whether medical help was needed.“There’s no need to call the doctor after a little bump,” said Dr. Jeffrey Bazarian, an emergency physician and concussion expert at the University of Rochester Medical Center. On the other hand, he said, “If you hit your head and have lingering symptoms, like a headache or confusion, that requires medical attention — especially if you’re on a blood thinner.”The C.D.C. warns that traumatic brain injuries can be overlooked in older people when the symptoms overlap with those seen in other common ailments, like dementia.People 75 and older account for roughly one-third of head trauma-related hospitalizations, the agency said, though experts said they tend to apply extra caution to any patients who are at least 60 years old. C.D.C. data indicate that men are at higher risk than women.Neither Mr. Saget’s family nor the medical examiner offered details on Wednesday about precisely how the head injury had killed him.The family specified that Mr. Saget had accidentally hit the back of his head. Injuries to the sides and back of the skull may be associated with significant brain bleeding, said Dr. Angela Lumba-Brown, an associate professor of emergency medicine at Stanford University School of Medicine.Still, she said, significant blows to any part of the head can cause problems.Doctors said that there were a number of possible scenarios for how Mr. Saget died. They also cautioned that crucial details of his case were missing, like whether or not he had underlying conditions and the precise nature of his injuries.In one scenario, they said, more common among younger patients, someone receives a blow to the head serious enough that their skull fractures, rupturing a blood vessel or artery between the skull and the thick lining covering the brain. The result is an epidural hematoma, and the bleeding can be deadly. In some such cases, there is an interval when patients feel fine.That was the injury that killed the actress Natasha Richardson in 2009 after what appeared to be a minor fall on a beginner’s ski slope.“Maybe there’s a bit of a headache, and you go to bed, and the blood clot expands,” Dr. Britz said. “Over time, it gets so big that you get brain stem compression.”Another scenario is a fall that ruptures small veins between the membrane covering the brain and the brain itself, doctors said. That kind of injury is more common in older patients — a result, in part, of brains shrinking as people age, doctors said, and the prevalence of blood thinners.In those cases, known as subdural hematomas, symptoms can develop quickly or over the course of weeks.Brain experts said that as Americans age and, in many cases, go on blood thinners after a heart attack or stroke, the risks of head injuries become more pronounced.“As our population is aging, we have to be aware of the risks that come with being on blood thinners,” said Dr. Neha Dangayach, the director of neuroemergencies management for Mount Sinai Health System. “If you fall or hit your head, don’t write it off.”Still, doctors said, those head injuries were often easily treatable and deaths usually completely preventable.Dr. Jamshid Ghajar, a neurosurgeon at Stanford University School of Medicine who has worked on guidelines for treating brain injuries, said that he had operated on a 100-year-old with a serious head injury.“I took the blood out,” Dr. Ghajar said, “and he was awake right away after.”

Third shots of coronavirus vaccines significantly reduced the risk that people with weakened immune systems would be hospitalized with Covid, the Centers for Disease Control and Prevention reported on Thursday, reinforcing the case for additional doses in that group.The Pfizer-BioNTech and Moderna vaccines were roughly 88 percent effective against hospitalizations in immunocompromised people who had received a third dose, compared with 69 percent effective in immunocompromised people with only two doses, the agency said.The data came from a study of people treated at 21 American hospitals from August to mid-December, a period when the Delta variant was dominant. But third doses have also been shown to bolster people’s defenses against severe outcomes from Omicron, even if the overall protection against that variant is weaker.The latest C.D.C. study also added to the considerable evidence that third doses bolster the defenses of people with stronger immune systems. Three doses of the Pfizer-BioNTech or Moderna vaccines were roughly 97 percent effective in preventing hospitalizations in that group, compared with 82 percent effective among two-dose recipients.But there have been fewer studies on the real-world impact of additional doses in people with weakened immune systems. Those people — the C.D.C. estimates there are about seven million in the country — can be born with faulty immune systems or, in other cases, experience a weakening of their immune defenses because of treatments for diseases like cancer.Many of them produce fewer antibodies in response to a vaccination or an infection, leaving them susceptible to the virus and to higher risks of serious illness.In August, federal regulators authorized a third Pfizer-BioNTech or Moderna dose at least four weeks after the second dose for some immunocompromised people. Health authorities considered the additional dose for this group an integral part of their primary immunization series, rather than a booster shot.In October, the C.D.C. said that immunocompromised people could also get a booster shot as soon as early 2022 — a fourth dose of vaccine — as a way of enhancing or restoring protection. But some of those people have said that pharmacies or hospitals have refused them the additional doses.For immunocompromised people with a single shot of the Johnson & Johnson vaccine, the C.D.C. advises a booster dose of the Moderna or Pfizer-BioNTech shots.The new study evaluated roughly 1,100 adults with weakened immune systems and 1,900 other adults. The C.D.C. said that future studies should look at the protection afforded by additional doses against the Omicron variant in immunocompromised people, as well as the durability of that protection.

Some laboratory studies suggest that molnupiravir can insert errors in DNA, which could in theory harm a developing fetus, sperm cells or children.A new Covid-19 pill from Merck has raised hopes that it could transform the landscape of treatment options for Americans at high risk of severe disease at a time when the Omicron variant of the coronavirus is driving a surge of cases in highly vaccinated European countries.But two weeks after a Food and Drug Administration expert committee narrowly voted to recommend authorizing the drug, known as molnupiravir, the F.D.A. is still weighing Merck’s application. Among the biggest questions facing regulators is whether the drug, in the course of wreaking havoc on the virus’s genes, also has the potential to cause mutations in human DNA.Scientists are especially worried about pregnant women, they said, because the drug could affect a fetus’s dividing cells, theoretically causing birth defects. Members of the F.D.A. expert committee expressed those same concerns during a public meeting on Nov. 30.“Do we want to reduce the risk for the mother by 30 percent while exposing the embryo and the fetus to a much higher risk of harm by this drug?” Dr. James Hildreth, the president of Meharry Medical College in Tennessee, said at the meeting. “My answer is no, and there is no circumstance in which I would advise a pregnant woman to take this drug.”The F.D.A. advisers also noted that the risks could extend to other patients, including men wanting to become fathers, though those risks remain poorly understood and Merck said its own studies had turned up no evidence that the drug causes DNA mutations.Crucially, molnupiravir is expected to work against Omicron. But it has drawn concern from some scientists and regulators in Europe for being less effective than certain other treatments: It has been shown to reduce the risk of hospitalization and death by 30 percent if given within five days of symptoms emerging.Here’s what scientists know about how the drug works and its potential risks.How does molnupiravir work?When the drug is processed in the body, it creates compounds that closely resemble one of the building blocks of RNA, the genetic material inside the coronavirus.That causes problems for the coronavirus as it makes copies of itself: Once the virus enters a cell and starts replicating, the drug compound can slip into the virus’s RNA and insert enough errors that the virus cannot survive.“What molnupiravir does is it disguises itself,” Elizabeth Campbell, an expert in structural biology at the Rockefeller University who studies coronavirus antivirals, said in an interview. “It can propagate errors that are going to be sprinkled all over the genome.”Making more and more mistakes, the virus eventually grinds to a stop, Dr. Campbell said. That helps the body fight off the infection and potentially saves the patient’s life.The problem is that the same compound that interferes in the replication of the virus’s genetic material can also be transformed into one that resembles a building block of DNA. Some scientists are concerned that could cause errors in a patient’s own DNA, or in that of a developing fetus.“If cells are replicating, it means they’re uptaking a version of one of the DNA building blocks derived from molnupiravir and incorporating it,” Dr. Campbell said.How serious a problem is that?A team of researchers at the University of North Carolina studied the use of molnupiravir in isolated hamster cells over 32 days and found that the drug did induce mutations in DNA.Those mutations could “contribute to the development of cancer, or cause birth defects either in a developing fetus or through incorporation into sperm precursor cells,” the authors of that study wrote.The drug targets only dividing cells, which are relatively sparse in an adult. That poses a narrower risk than other so-called mutagens, like radiation, which can damage DNA in all types of cells.Still, Ronald Swanstrom, an H.I.V. researcher at the University of North Carolina at Chapel Hill who helped lead the hamster cell study, said that adults had enough dividing cells — in bones, for example, and in the lining of the gut — to cause concern. He also noted that men were constantly making dividing sperm cells that could carry potential mutations.“I don’t think anybody knows what this dose means in terms of human outcomes,” Dr. Swanstrom said. “I hope it’s trivial, but I don’t think anybody knows.”In a letter objecting to Dr. Swanstrom’s conclusions, Merck scientists said that hamster cells were exposed to the drug for considerably longer than Covid patients would be. The company said that it tested the drug in rodents and found no signs of DNA mutations (also known as mutagenicity).“We see this molecule as having a very low risk for mutagenicity,” Dr. Roy Baynes, Merck’s chief medical officer, said in an interview. “This drug is used for five days, and the goal is to eradicate the virus quickly, and this is not a long-term treatment.”Brianne Barker, a biologist at Drew University, said that Merck should publish its rodent data, but that the short course of treatment lowered the risks. She said that the isolated hamster cells were also “a bit different from cells you’d actually find in an organism,” making it difficult to know how serious the dangers would be in people.What are the risks during pregnancy?Cells in a fetus are dividing all the time, heightening the risk of mutations. As a result, Merck excluded pregnant and breastfeeding women, as well as women likely to become pregnant, from its clinical trial.“Human development in utero is an absolutely astounding sequence of events,” said Dr. John Mellors, an infectious disease specialist at the University of Pittsburgh Medical Center. “You start tinkering with that in any way, you can end up with a disaster.”Dr. Mellors noted that Merck had reported that high doses of the drug in pregnant rats could cause developmental abnormalities or the death of a fetus.In Britain, health authorities have said that Merck’s pill should not be given to pregnant or breastfeeding women. Women who could become pregnant, they advised, should use contraception while taking the drug and for four days after.“If I was pregnant, I wouldn’t take this,” Dr. Campbell said. “I probably would go so far as to say I wouldn’t give it to a child, a teenager, anyone whose cells are still dividing and differentiating at higher rates.”Scientists said that older antiviral pills offered lessons for safely prescribing molnupiravir. Before powerful hepatitis C drugs arrived in recent years, doctors frequently used a pill known as ribavirin as part of combination therapy to treat hepatitis C patients.The F.D.A. warns that ribavirin should not be given to pregnant women or to their male partners, and that pregnancy should be avoided during treatment and for months afterward.“There’s no doubt that if you read the label, you pause,” said Dr. Ashwin Balagopal, a researcher at Johns Hopkins University who has treated hepatitis C patients with ribavirin and is now leading a study of molnupiravir. “We were careful about how we did it, but we didn’t avoid using it because we thought, let’s wait for something better.”Coronavirus Drug and Treatment TrackerAn updated list of potential treatments for Covid-19.How are scientists weighing the drug’s benefits and risks?Within weeks, Pfizer is expected to receive the government’s green light for its own Covid pill, which appears to be more effective than Merck’s — and doesn’t carry the risk of mutations in human DNA.But molnupiravir is expected to be more readily available than Pfizer’s drug in the coming months, when the country may be facing a new surge in cases caused by the Omicron variant.The Coronavirus Pandemic: Key Things to KnowCard 1 of 4U.S. surpasses 800,000 deaths.

The first real-world study of how vaccines hold up against the Omicron variant showed a significant drop in protection against symptomatic cases caused by the new and fast-spreading form of the coronavirus.But the study, published by British government scientists on Friday, also indicated that third vaccine doses provided considerable defense against Omicron.Government scientists on Friday also offered the most complete look yet at how quickly Omicron was spreading in England’s highly vaccinated population, warning that the variant could overtake Delta by mid-December and, without any precautionary measures, cause Covid-19 cases to soar.Those warnings were reinforced by a computer modeling study of England released on Saturday suggesting that even in populations with high levels of immunity, Omicron could significantly disrupt life and overwhelm hospitals. Scientists cautioned that those projections could change as they learned more about the severity of Omicron infections.The vaccine study published Friday indicated reduced levels of protection. Four months after people received a second dose of the Pfizer-BioNTech vaccine, the shots were roughly 35 percent effective in preventing symptomatic infections caused by Omicron, a significant drop-off from their performance against the Delta variant, the scientists found.A third dose of the Pfizer-BioNTech vaccine, though, lifted the figure to roughly 75 percent.Two doses of the AstraZeneca vaccine appeared to offer virtually no protection against symptomatic infection caused by Omicron several months after vaccination. But for those recipients, an additional Pfizer-BioNTech dose paid big dividends, boosting effectiveness against the variant to 71 percent.Still, the study’s authors said they expected that the vaccines would remain a bulwark against hospitalizations and deaths, if not infections, caused by Omicron. And the researchers cautioned that even in a country tracking the variant as closely as Britain is, it was too early to know precisely how well the vaccines would perform.That study was released alongside new findings about how easily Omicron is managing to spread. Someone infected with the Omicron variant, for example, is roughly three times as likely as a person infected by the Delta variant to pass the virus to other members of his or her household, Britain’s Health Security Agency reported.And a close contact of an Omicron case is roughly twice as likely as a close contact of someone infected with Delta to catch the virus.Neil Ferguson, an epidemiologist at Imperial College London, said that Omicron’s ability to evade the body’s immune defenses accounted for most of its advantage over previous variants. But modeling work by his research team also suggested that Omicron was simply more contagious than Delta, by roughly 25 to 50 percent.“I think that there’s a significant amount of immune escape,” Dr. Ferguson said, referring to the virus’s ability to dodge the body’s defenses. “But it’s also more intrinsically transmissible than Delta.”He and other scientists have cautioned that evidence was still coming in, and that better surveillance in places where the Omicron wave is most advanced could affect their findings.The World Health Organization said this week that some evidence had emerged that Omicron was causing milder illness than Delta, but that it was too early to be certain. Still, scientists have warned that if the variant keeps spreading as quickly as it is in England, where cases are doubling every 2.5 days, health systems around the world may be deluged with patients.Even if Omicron causes severe illness at only half the rate of the Delta variant, Dr. Ferguson said, his computer modeling suggested that 5,000 people could be admitted to hospitals daily in Britain at the peak of its Omicron wave — a figure higher than any seen at any other point in the pandemic.Scientists said that widespread vaccination in countries like Britain and the United States would keep as many people from dying as have in earlier waves. But the experts also warned that patients with Covid and with other illnesses would suffer if hospitals became too full.“It only requires a small drop in protection against severe disease for those very large numbers of infections to translate into levels of hospitalization we can’t cope with,” Dr. Ferguson said.The Coronavirus Pandemic: Key Things to KnowCard 1 of 5U.S. nears 800,000 Covid deaths.

The first real-world study of how vaccines hold up against the Omicron variant showed a significant drop in protection against symptomatic cases caused by the new and fast-spreading form of the coronavirus.But the study, published by British government scientists on Friday, also indicated that third vaccine doses provided considerable defense against Omicron.Government scientists on Friday also offered the most complete look yet at how quickly Omicron was spreading in England’s highly vaccinated population, warning that the variant could overtake Delta by mid-December and, without any precautionary measures, cause Covid-19 cases to soar.Four months after people received a second dose of the Pfizer-BioNTech vaccine, the shots were roughly 35 percent effective in preventing symptomatic infections caused by Omicron, a significant drop-off from their performance against the Delta variant, the scientists found.A third dose of the Pfizer-BioNTech vaccine, though, lifted the figure to roughly 75 percent.Two doses of the AstraZeneca vaccine appeared to offer virtually no protection against symptomatic infection caused by Omicron several months after vaccination. But for those recipients, an additional Pfizer-BioNTech dose paid big dividends, boosting effectiveness against the variant to 71 percent.Still, the study’s authors said they expected that the vaccines would remain a bulwark against hospitalizations and deaths, if not infections, caused by Omicron. And the researchers cautioned that even in a country tracking the variant as closely as Britain is, it was too early to know precisely how well the vaccines would perform.That study was released alongside new findings about how easily Omicron is managing to spread. Someone infected with the Omicron variant, for example, is roughly three times as likely as a person infected by the Delta variant to pass the virus to other members of his or her household, Britain’s Health Security Agency reported.And a close contact of an Omicron case is roughly twice as likely as a close contact of someone infected with Delta to catch the virus.Neil Ferguson, an epidemiologist at Imperial College London, said that Omicron’s ability to evade the body’s immune defenses accounted for most of its advantage over previous variants. But modeling work by his research team and other groups in Britain also suggested that Omicron was simply more contagious than Delta, by roughly 25 to 50 percent.“I think that there’s a significant amount of immune escape,” Dr. Ferguson said, referring to the virus’s ability to dodge the body’s defenses. “But it’s also more intrinsically transmissible than Delta.”He and other scientists have cautioned that evidence was still coming in, and that better surveillance in places where the Omicron wave is most advanced could affect their findings.The World Health Organization said this week that some evidence had emerged that Omicron was causing milder illness than Delta, but that it was too early to be certain. Still, scientists have warned that if the variant keeps spreading as quickly as it is in England, where cases are doubling every 2.5 days, health systems around the world may be deluged with patients.Even if Omicron causes severe illness at only half the rate of the Delta variant, Dr. Ferguson said, computer modeling suggested that 5,000 people could be admitted to hospitals daily in Britain at the peak of its Omicron wave — a figure higher than any seen at any other point in the pandemic.Scientists said that widespread vaccination in countries like Britain and the United States would keep as many people from dying as have in earlier waves. But the experts also warned that patients with Covid and with other illnesses would suffer if hospitals became too full.“It only requires a small drop in protection against severe disease for those very large numbers of infections to translate into levels of hospitalization we can’t cope with,” Dr. Ferguson said.It will take several weeks to understand how the current surge in Omicron infections may translate into people needing hospital care. “I’m concerned that by the time we know about severity,” Dr. Ferguson said, “it may be too late to act.”

Regeneron said on Tuesday that its Covid-19 antibody treatment might be less effective against the Omicron variant of the coronavirus, an indication that the popular and widely beneficial monoclonal antibody drugs may need to be updated in case the new variant spreads aggressively.The company said that previous laboratory analyses and computer modeling of certain mutations in the Omicron variant suggest that they may weaken the effect of the treatment. But studies using the variant’s full sequences have not been completed, it said.The company said it had already been testing future antibody drug candidates, and that preliminary analyses indicated that some of those “may have the potential to retain activity against the Omicron variant.” More data is expected in the coming month, it said.“What we have to admit is, in the course of the past six days, our urgency has increased,” Dr. George Yancopoulos, Regeneron’s president and chief scientific officer, told The Wall Street Journal in an interview. “What started out as a backup plan has now been made a lot more urgent.”The Omicron variant has caused alarm among scientists because it contains mutations in the spike protein, the target of the government-supplied monoclonal antibody treatments made by Regeneron and Eli Lilly.Scientists have also been scrambling to gather data on how effective the current vaccines will be against Omicron. Antiviral pills, including drugs from Merck and Pfizer that federal regulators are considering authorizing soon, are expected to hold up well against the variant because they target a different site of the virus from where Omicron’s mutations are clustered.Monoclonal antibody treatments, given in a single infusion, use lab-made copies of the antibodies that people generate naturally when fighting an infection. They have been shown to significantly shorten patients’ symptoms. Regeneron’s cocktail reduces the risk of hospitalization by 70 percent.The company said the treatment was effective against the Delta variant, which remains the dominant form of the virus in the United States.

Coronavirus vaccines were significantly less effective in protecting people with weakened immune systems than they were for other people, the Centers for Disease Control and Prevention reported on Tuesday, buttressing the agency’s call for immunocompromised adults to receive third or fourth doses of vaccines.Two doses of either the Pfizer-BioNTech or Moderna vaccines were 77 percent effective against Covid-related hospitalization for immunocompromised people. That was a significant degree of protection, the agency said, but far lower than the shots’ benefit to people without immune deficiencies: In those people, the agency said, the vaccines were 90 percent effective against Covid hospitalizations.The Moderna vaccine also offered more protection to people with weakened immune systems than did the Pfizer shot, mirroring results seen across American adults. And certain people with immune deficiencies — especially organ or stem cell transplant recipients, who often take drugs to suppress their immune systems and prevent rejection of the transplant — showed weaker responses to Covid vaccines than other categories of immunocompromised people did.The study did not examine recipients of the Johnson & Johnson vaccine.To help immunocompromised people mount a more aggressive immune response, the C.D.C. suggests that they be given three doses of Pfizer or Moderna vaccines, plus an additional booster shot six months after the third dose. In addition, the agency’s scientists wrote, they should take precautions like wearing masks, and be considered for treatments like monoclonal antibody therapy as early as possible after a Covid diagnosis.The study released on Tuesday used a circuitous experimental design. The researchers examined roughly 20,000 immunocompromised adults and 70,000 people without immune deficiencies hospitalized this year with Covid-like illness. Of the immunocompromised patients in the study, 43 percent were fully vaccinated. Of the other participants, 53 percent were vaccinated.The researchers then determined how many of those hospitalized patients were indeed infected with the coronavirus, and compared the odds of a positive test results between fully vaccinated and unvaccinated patients.The immunocompromised patients included those with cancer, inflammatory disorders, organ or stem cell transplants and other immune deficiencies.The study’s authors cautioned that there could have been cases in which patients were misclassified as immunocompromised, and that there could have been biases in which patients sought out coronavirus tests.

A new study by the Centers for Disease Control and Prevention suggests that vaccination provides stronger and more reliable protection against the coronavirus than a past infection does, the agency said on Friday.Unvaccinated people who had previously recovered from a coronavirus infection were five times as likely to get Covid as people who had received both shots of the Pfizer-BioNTech or Moderna vaccines, the C.D.C. said.The study’s authors cautioned, however, that certain gaps in patient data and biases in their study participants could have influenced the results.“We now have additional evidence that reaffirms the importance of Covid-19 vaccines, even if you have had prior infection,” said Dr. Rochelle P. Walensky, the C.D.C. director, in a statement accompanying the release of the report.The question of whether people who have had Covid really need a shot has gained traction among some Americans as vaccine mandates take hold across the country. Scientists have urged Covid survivors not to skip the vaccine, noting that the strength and durability of so-called natural immunity depends heavily on people’s age and health, and the severity of an initial infection.The C.D.C. study used a roundabout experimental design. The researchers examined roughly 7,000 people hospitalized this year with Covid-like illness across nine states. They then looked at how many of those hospitalized patients were indeed infected with the coronavirus. The odds of testing positive for the virus were considerably higher among unvaccinated, previously infected patients than they were among vaccinated people.The study comes with several caveats, however. The researchers cautioned that the findings may not translate to non-hospitalized people with different levels of access to medical care, and that some patients in the vaccinated group may unknowingly have also had previous infections.The researchers also noted that separate research in Israel had failed to show that vaccinated people were better protected than those who had only been infected. In general, scientists said, studies on the topic had drawn contradictory conclusions.Still, some patterns have emerged. Two doses of an mRNA vaccine produce more antibodies, and more reliably so, than a coronavirus infection does. But the antibodies from prior infection are more diverse, potentially helping people fend off variants.Whatever the effect, doctors have warned that acquiring natural immunity is perilous and uncertain. Not everyone survives Covid in the first place, and those that do may not be able to count on a vigorous immune response.

A new study by the Centers for Disease Control and Prevention suggests that vaccination provides stronger and more reliable protection against the coronavirus than a past infection does, the agency said on Friday.Unvaccinated people who had previously recovered from a coronavirus infection were five times as likely to get Covid as people who had received both shots of the Pfizer-BioNTech or Moderna vaccines, the C.D.C. said.The study’s authors cautioned, however, that certain gaps in patient data and biases in their study participants could have influenced the results.“We now have additional evidence that reaffirms the importance of Covid-19 vaccines, even if you have had prior infection,” said Dr. Rochelle P. Walensky, the C.D.C. director, in a statement accompanying the release of the report.The question of whether people who have had Covid really need a shot has gained traction among some Americans as vaccine mandates take hold across the country. Scientists have urged Covid survivors not to skip the vaccine, noting that the strength and durability of so-called natural immunity depends heavily on people’s age and health, and the severity of an initial infection.The C.D.C. study used a roundabout experimental design. The researchers examined roughly 7,000 people hospitalized this year with Covid-like illness across nine states. They then looked at how many of those hospitalized patients were indeed infected with the coronavirus. The odds of testing positive for the virus were considerably higher among unvaccinated, previously infected patients than they were among vaccinated people.The study comes with several caveats, however. The researchers cautioned that the findings may not translate to non-hospitalized people with different levels of access to medical care, and that some patients in the vaccinated group may unknowingly have also had previous infections.The researchers also noted that separate research in Israel had failed to show that vaccinated people were better protected than those who had only been infected. In general, scientists said, studies on the topic had drawn contradictory conclusions.Still, some patterns have emerged. Two doses of an mRNA vaccine produce more antibodies, and more reliably so, than a coronavirus infection does. But the antibodies from prior infection are more diverse, potentially helping people fend off variants.Whatever the effect, doctors have warned that acquiring natural immunity is perilous and uncertain. Not everyone survives Covid in the first place, and those that do may not be able to count on a vigorous immune response.

The pandemic dealt a serious setback to global efforts to immunize children against diseases like measles and polio, the Centers for Disease Control and Prevention reported on Thursday, reducing worldwide coverage for some vaccines to levels not seen since more than a decade ago.The proportion of eligible children who received a polio vaccine fell to 83 percent in 2020 from 86 percent the year before, as did coverage with the third dose of the diphtheria-tetanus-pertussis vaccine, known as DTP3. Coverage with the measles vaccine also dipped slightly, to 84 percent last year from 86 percent in 2019.Those setbacks, while seemingly small, meant that millions more children missed out on routine immunizations during the pandemic, putting them and their communities at risk.Globally, nearly 23 million children targeted for the DTP3 shot were not vaccinated in 2020, compared with 19 million in 2019, the C.D.C. said. The vast majority of those had not received a single dose of diphtheria-tetanus-pertussis vaccine. Not since 2009 had coverage with that vaccine been so low.The C.D.C. scientists involved in the report called for action to be taken to address the immunity gaps of preventable diseases in countries already saddled with Covid outbreaks. Scientists from the World Health Organization and UNICEF also were co-authors on the study.The decline in vaccinations follows a decade of stagnant immunization levels. In 2019, measles deaths swelled to their highest levels in 23 years, a consequence of what public health experts described as insufficient vaccination coverage. Scientists said that the pandemic had hampered the tracking of measles outbreaks.The pandemic also disrupted immunization programs, the C.D.C. report said, interrupting the supply of basic vaccines and making it more difficult to administer them.Immunization levels for diphtheria-tetanus-pertussis and measles were lowest in much of Africa, the report said. The W.H.O. also said on Thursday that only five of Africa’s 54 nations were expected to reach a year-end goal of vaccinating 40 percent of their people against Covid. UNICEF, a United Nations agency working to distribute coronavirus vaccines, warned of a shortfall next year in syringes for both Covid and routine vaccinations.