More and more American school districts have dropped mask mandates in recent weeks as coronavirus cases plunged across the United States. But they remain a subject of debate among some students and their parents, and a study released on Tuesday by the Centers for Disease Control and Prevention suggested that those mandates had helped protect children and teachers from the coronavirus last fall.The study, examining public school districts in Arkansas from August to October as the Delta variant spread, found that districts with full mask requirements had 23 percent lower rates of the coronavirus among students and staff members than districts without the mandates.It was not clear whether the same would have been true once the Delta variant was overtaken by Omicron, which is more contagious and spread rapidly among children and adults alike.The C.D.C. has faced criticism from scientists in the past for overstating the benefits of school masking based on what some researchers have described as a flawed study out of Arizona. Some studies from abroad have also found that mask mandates were not associated with lower rates of the coronavirus in children.But some scientists said that the latest C.D.C. study had steered clear of the most serious methodological problems and had strengthened the evidence for masks protecting some children from the coronavirus.“It passes the smell test,” Louise-Anne McNutt, a former C.D.C. Epidemic Intelligence Service officer and an epidemiologist at the State University of New York at Albany, said of the study. “The estimates of the impact of masks are consistent with other studies that show masks have a modest, but important, reduction of SARS-CoV-2 transmission.”The study compared coronavirus case rates among 233 Arkansas districts. About a third of the districts had full mask mandates, a fifth required masks only in certain settings or situations, and half had no mask policies.It took into account staff and student vaccination rates and socioeconomic status. It also adjusted for coronavirus rates in the surrounding community — an attempt, the study’s authors said, to partly control for how much testing was happening in a given part of the state. Dr. McNutt, though, said that the study would have benefited from more details on statewide testing levels.Districts with full mask mandates had lower coronavirus rates relative to the case rates in the surrounding community than districts without the mandates, the study found. And among roughly two dozen districts that put in place mask mandates in the middle of the study period, case rates afterward dropped more than would have been expected from changes in community case rates at the same time, the study said.Partial masking policies did not show as strong an effect as full mask mandates.The study did not account for schools’ prevention efforts beyond masking, like ventilating classrooms. Jonathan Ketcham, an economist specializing in health care at Arizona State University, said that could be an “important flaw in the study itself.”Jason Abaluck, an economics professor at Yale University’s School of Management who helped lead a large trial on masking in Bangladesh, also cautioned that the schools with mask mandates could have differed from those without them in other ways, like adherence to distancing measures. He said that the study could have more closely matched nearby schools with different masking policies to study their effects.But Dr. Abaluck said the C.D.C. study was an improvement on previous research.“This study and the broader literature on masking suggests that in places where hospitalization and deaths are very high, the benefits of mask wearing in schools may be considerable,” he said.Still, he noted that masks can cause discomfort and make it harder for children to communicate. “Figuring out how severe an outbreak has to be to warrant mask mandates in schools,” he said, “requires making best guesses about the costs, which remain highly uncertain given existing evidence.”

A funding dispute with Republican senators may complicate the Biden administration’s efforts to guarantee supplies of free antiviral pills and monoclonal antibody treatments this year to Americans who test positive for the coronavirus.At issue is whether the White House has provided the level of detail desired by Republicans about the trillions of dollars in Covid relief spending that Congress previously authorized. A group of three dozen Republican senators, led by Mitt Romney of Utah, told the White House last week that they would not consider billions in new Covid relief spending without a more detailed accounting of how existing funds had been spent and whether any money was left over.“It is not yet clear why additional funding is needed,” they wrote in a letter to President Biden on Wednesday.Administration officials have requested $22.5 billion for additional vaccines, oral antivirals and monoclonal antibodies, testing and support for the global vaccination effort.Republican senators have suggested that the administration should divert money from the $1.9 trillion pandemic law to the areas of need, rather than have Congress spend billions of new funds.Jen Psaki, the White House press secretary, said on Friday that the current supply of monoclonal antibodies will run out by May and stocks of oral antivirals would be depleted by September without new purchases. “This is an urgent request,” she said.A breakdown of spending on Covid-related supplies like testing, vaccines and treatments provided to Congress by the Biden administration and obtained by The New York Times indicated that nearly all existing funds had been spent or committed in signed contracts, and that any remaining funds had been allocated. The administration offered some detail on its plans for new Covid spending in a letter to Congress on Wednesday.The funding dispute is playing out as the White House tries to put in place plans that it says are essential to the country’s efforts to return to a semblance of normalcy.Hundreds of pharmacies and other health facilities were prepared to launch a “test to treat” program this week allowing people to walk in for a coronavirus test and, if the results are positive, to leave with free antiviral pills, administration officials said.Last week, the Department of Defense told STAT, a medical news outlet, that it would exercise contract options to buy more of Pfizer’s antiviral treatment, known as Paxlovid, once funding became available.The White House also said that Americans who had already received a package of four free at-home coronavirus tests from the government would become eligible this week to place a second order on covidtests.gov.New virus cases in the United States have plunged in recent weeks, but more than 1,500 Americans are still dying from the virus each day on average.There are also signs that a particular version of the Omicron variant that is even more contagious than the one that tore through the United States this winter is becoming more prevalent, rising to roughly 8 percent of sequenced cases in the United States by late February. But overall cases remain on the decline and vaccines look to be equally effective against the Omicron subvariant, which does not appear to be any more severe.Biden administration officials hope to include the $22.5 billion in coronavirus aid, alongside humanitarian and military aid for Ukraine, in a sprawling catchall spending package that would fund the government for the remainder of the fiscal year. Congress has until March 11, when funding is set to lapse, to hammer out the details of a deal.

The Omicron variant of the coronavirus has been so contagious that it may have seemed a foregone conclusion that if one person in a household became ill, other people living there would catch the virus, too.But that turns out to be less certain: A small study of households by the Centers for Disease Control and Prevention released on Friday found that when the first person infected wore a mask and stayed in a separate room at least part of the time, the risk of other household members contracting the virus became markedly lower.Vaccinated people who became infected were also considerably less likely than unvaccinated people to spread the virus to other members of their households.Still, the study highlighted just how aggressively the Omicron variant had spread within a home, especially among people living with children under 5 who tested positive. Those children, who are not yet eligible for vaccines and often need to be in closer contact with their parents or relatives, spread the virus to 72 percent of household contacts identified in the study — the highest rate of any age group, the C.D.C. said.“These findings further highlight young children’s potential contribution to household transmission,” C.D.C. scientists wrote in the report.Federal regulators are waiting for data on how well three doses of the Pfizer-BioNTech coronavirus vaccine work in children under 5 before deciding whether to authorize the vaccine for that age group.The C.D.C. study was based on 183 households across four states where someone became infected with the Omicron variant from November to early February. After interviewing members of the households about their vaccination and infection histories, any precautionary measures in the home and whether they had tested positive or become ill, C.D.C. scientists determined that the variant had spread in roughly two-thirds of the households they identified.But when the first infected person was fully vaccinated, only about 44 percent of household members developed Covid, compared with 64 percent when the infected person was unvaccinated, the study said.And when the original infected household member stayed in a room alone at least some of the time, only 41 percent of other members of the household became infected, compared with 68 percent in situations without isolation. Masking by the infected person helped, too, reducing the likelihood of transmission to 40 percent from 69 percent.Determining the precise risk of the virus spreading in a home was difficult, the study’s authors noted. They excluded situations where it was unclear who had first developed Covid, and did not do the genetic sequencing necessary to know for certain that people had caught the virus from the infected person in their home rather than at other gatherings.

After the first person known to have become infected with the Omicron variant in the United States was revealed to have attended a 53,000-person anime convention in Manhattan, concerns quickly mounted that the event had sown the seeds of a major coronavirus outbreak.But a new study released on Thursday by the Centers for Disease Control and Prevention suggested that a combination of good air filtration, widespread vaccination and indoor masking had in fact helped prevent the anime convention in November from becoming a superspreader event.The share of attendee tests that came back positive was similar to the share of coronavirus tests that were positive across New York City around the same time, the C.D.C. said. What’s more, the few positive samples that were genetically sequenced were largely of the Delta variant, not Omicron.And conventiongoers who became infected were more likely than those who tested negative to have gone to bars, nightclubs or karaoke clubs.Still, the C.D.C. said that the virus’s spread at the convention could have been much worse had it been held after Omicron became dominant in the city, given that the variant is so contagious and capable of spreading among the vaccinated. At the anime convention, the C.D.C. said in a separate report on Thursday, the only documented Omicron infections were in a single cluster of at least 16 positive cases.The first study relied largely on people who came forward for testing after the event, which introduced potential biases: Those people could have been more cautious than the average convention attendee, or more inclined to report cautious behaviors. Health officials had urged attendees to get tested. The C.D.C. could only look for cases among people on a registration list of ticket buyers, which did not account for the full number of attendees.The agency’s findings matched those of New York City contact tracing officials, who said in early December that they had not found signs of widespread transmission at the anime convention.In the aftermath of the event, held at the sprawling Javits Center, the convention organizers came under scrutiny as people reported seeing attendees flouting masking rules and pushing past checkpoints.But among attendees who were tested, the C.D.C. said, “evidence of widespread transmission during the event was not identified.”The study drew on test results identified through health department surveillance systems for 4,560 attendees. Of those, 119 people — 2.6 percent — tested positive. Researchers also sent online surveys to attendees asking about their test results, symptoms and activities during the convention.The anime convention required attendees to have received at least one vaccine dose. The C.D.C. said that among attendees who could be matched with test and vaccination data in health department surveillance systems, 85 percent had completed their primary vaccination series, another 12 percent had received a booster dose and 3 percent were partly vaccinated.The C.D.C. study also credited the convention hall for being outfitted with HEPA filters, which have been shown to efficiently remove coronavirus particles from the air.

Such an extensive head injury would likely have left the actor confused, if not unconscious, experts said.Bob Saget, the comedian and actor, died after what appeared to be a significant blow to the head, one that fractured his skull in several places and caused bleeding across both sides of his brain, according to an autopsy report released on Friday.The findings complicated the picture of Mr. Saget’s death that has emerged in recent days: Far from a head bump that might have been shrugged off, the autopsy described an unmistakably serious set of injuries that would at the very least have probably left someone confused, brain experts said.The report, prepared by Dr. Joshua Stephany, the chief medical examiner of Orange and Osceola counties in Florida, ascribed Mr. Saget’s injuries to a fall.“It is most probable that the decedent suffered an unwitnessed fall backwards and struck the posterior aspect of his head,” Dr. Stephany wrote, referring to the back of the skull.Still, the autopsy left a number of unresolved questions about how exactly Mr. Saget, 65, was so badly hurt. He was found dead in a hotel room at the Ritz-Carlton Orlando, Grande Lake on Jan. 9 during a weekend of stand-up comedy acts. His family said this week that the authorities determined that he had hit his head, “thought nothing of it and went to sleep.”If the actor struck his head hard enough, and in just the wrong place, it is possible that fractures would have extended to other parts of his skull, brain injury experts said. Situations where someone cannot break their fall are even more dangerous.“It’s like an egg cracking,” said Dr. Jeffrey Bazarian, an emergency physician and concussion expert at the University of Rochester Medical Center. “You hit it in one spot, and it can crack from the back to the front.”But experts said that with such an extensive injury, it was unlikely that Mr. Saget would have intentionally ignored it. The injury would likely have left him confused, if not unconscious.“I doubt he was lucid,” Dr. Bazarian said, “and doubt he thought, ‘I’m just going to sleep this off.’”Some neurosurgeons said that it would be unusual for a typical fall to cause Mr. Saget’s set of fractures — to the back, the right side and the front of his skull. Those doctors said that the injuries appeared more reminiscent of ones suffered by people who fall from a considerable height or get thrown from their seat in a car crash.The autopsy, though, found no injuries to other parts of Mr. Saget’s body, as would be expected in a lengthier fall. The medical examiner ruled that the death was accidental. The local sheriff’s office had previously said there were no signs of foul play.“This is significant trauma,” said Dr. Gavin Britz, the chair in neurosurgery at Houston Methodist. “This is something I find with someone with a baseball bat to the head, or who has fallen from 20 or 30 feet.”Dr. Britz noted that the autopsy described fractures to particularly thick parts of the skull, as well as to bones in the roof of the eye socket. “If you fracture your orbit,” he said, referring to those eye bones, “you have significant pain.”The knock ruptured veins in the space between the membrane covering the brain and the brain itself, causing blood to pool, the autopsy indicated. The brain, secured in a hard skull, has nowhere to move, doctors said, and the result is a compression of brain centers critical for breathing and other vital functions.No alcohol or illegal drugs were detected in the actor’s system, according to the autopsy. But there were signs of Clonazepam, commonly known as Klonopin, a benzodiazepine that is used to prevent seizures and treat panic attacks. Tests also found Trazodone, an antidepressant, the report said.There was no indication in the autopsy findings that either of those drugs might have contributed to Mr. Saget’s injuries. But doctors said that they could make people sleepy and contribute to a fall.Benzodiazepines are widely prescribed for older people, despite warnings about the side effects. People who take them face increased risks of falls and fractures, of auto accidents and of reduced cognition.Use of multiple drugs “is a very dangerous cause of falls in the elderly,” said Dr. Neha Dangayach, the director of neuro-emergencies management and transfers for the Mount Sinai Health System. She said that some combinations could cause drops in blood pressure or confusion.The report noted that Mr. Saget had an enlarged heart, but did not suggest any link to his death. It also found signs of the coronavirus on a PCR test, but did not suggest that the virus contributed to Mr. Saget’s death. The actor said on a podcast in early January that he had contracted the virus, without specifying exactly when. PCR tests can show the presence of the virus days or even weeks after someone has recovered.Mr. Saget, best known for his role on the sitcom ‘Full House’ and for hosting ‘America’s Funniest Home Videos,’ thanked the “appreciative audience” of his stand-up comedy set in a Tweet early in the morning on Jan. 9, the day of his death.“I had no idea I did a 2 hr set tonight,” he said. “I’m happily addicted again to this.”

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.”