The data offer a rare bit of good news. Still, hospitalizations are likely to increase simply because the variant is so contagious.Three separate teams of scientists on two continents have found that Omicron infections more often result in mild illness than earlier variants of the coronavirus, offering hope that the current surge may not be quite as catastrophic as feared despite skyrocketing caseloads.The researchers examined Omicron’s course through populations in South Africa, Scotland and England. The results in each setting, while still preliminary, all suggested that the variant was less likely to send people in hospitals.“Given that this is everywhere and given that it’s going to be so transmissible, anything that would lower severity is going to be better,” said Natalie Dean, a biostatistician at Emory University in Atlanta.Since the discovery of the Omicron variant in southern Africa in November, scientists have struggled to learn whether it causes more severe illness compared with other variants — and if so, in whom.The new research suggests that the variant may have biological features that make it somewhat less dangerous than Delta, the variant that dominated the world from summer till now.But Omicron’s lowered risk of hospitalization in all three countries also appears to be due in large part to immunity in those populations. Many of the infected already had protection against severe disease, either because of previous infections or vaccinations.While the new research is heartening, experts warn that the surge coming to many countries still may flood hospitals with Omicron cases, simply because the variant spreads so much more easily than previous versions of the coronavirus.“I don’t want to be alarmist, but I don’t think that you can let your guard down,” said Christina Ramirez, a biostatistician at the University of California, Los Angeles.Americans should take rapid tests before holiday events and should gather outside, open windows or find other ways to improve ventilation, Dr. Ramirez said.While Omicron may be causing milder disease on average, it’s also spreading faster than any variant yet. It is now the dominant variant in the United States, Europe and many other regions of the world.Yet encouraging news came from South Africa on Wednesday, suggesting that these surges may not last as long as previous ones. Omicron has fueled a record number of daily new cases, but officials reported on Wednesday that the wave of infections may have peaked.As infections in the United States rose to 154,000 daily cases on average over the past week, federal health officials added a defense against the rising threat to hospitals. The Food and Drug Administration authorized the first pill to treat Covid-19 on Wednesday, offering a highly effective treatment against severe illness.Boxes of Paxlovid, the new Covid pill treatment, were prepared for shipment at a facility in Memphis. Tens of millions of Americans will be eligible for the pill.Pfizer, via ReutersThe drug, developed by Pfizer and known as Paxlovid, is authorized for Covid patients 12 and older who are at risk of becoming severely ill. Pfizer has reported that its pills are likely to work against the Omicron variant.The three studies from the United Kingdom and South Africa confirm that Americans will gain some protection against severe disease from vaccinations and booster shots. More than 200 million Americans — over 60 percent of the population — are now fully vaccinated.But the United States lags behind other countries, some of which have inoculated over 80 percent of their populations. And only 63 million Americans have received boosters, which provide the strongest protection against both serious disease and infection from Omicron.The South African study focused on the surge of Omicron cases since November. The risk of hospitalization was about 70 percent lower among people infected with Omicron, compared with those infected with other variants of the coronavirus, the researchers found.The authors speculated that the milder cases might be due in part to the fact that Omicron was more successful at reinfecting people who had already had Covid-19. While the variant can evade the antibodies from previous infections and establish itself in the body, it may not be able to escape the powerful but slower immune responses that prevent serious disease.In South Africa, researchers estimate that about 70 percent of people had Covid infections before the Omicron wave. About 30 percent have been vaccinated. The authors were not able to separate the protection afforded by infections from that resulting from vaccinations.A researcher at the Africa Health Research Institute in Durban, South Africa, worked on research related to the Omicron variant this month.Jerome Delay/Associated PressThey also cautioned that their data were gathered during an early stage of the Omicron wave, when the overall infection rate was quite low. Infected people with relatively mild symptoms might have been more likely to be admitted to hospitals back then, before the wards filled up.In Scotland, researchers examined Delta and Omicron cases in November and December, looking at how many patients with each variant were admitted to a hospital. Omicron infections are associated with a two-thirds reduction in the risk of hospitalization compared with the Delta variant, the researchers found.Mark Woolhouse, a professor of infectious diseases at the University of Edinburgh and a co-author of the new study, said that the dramatic surge of Omicron meant that even if these preliminary results held up, the variant would still put a lot of people into hospitals very quickly.Also on Wednesday, a team of researchers at Imperial College London compared Omicron and Delta cases in the first two weeks of December and reported a reduction in hospital visits, albeit a smaller reduction than that found by their Scottish colleagues.Initial estimates suggest that compared with Delta variant cases, individuals infected with Omicron are 15 to 20 percent on average less likely to turn up in hospitals overall and 40 to 45 percent less likely to be hospitalized for a night or more.The Coronavirus Pandemic: Key Things to KnowCard 1 of 4The Omicron variant.

Health authorities in Europe are warning of a sharp increase in Omicron cases, adding to an existing surge from the Delta variant.Public health authorities in Denmark and Norway on Monday released grim projections for the coming wave of the Omicron coronavirus variant, predicting that it will dominate both countries in a matter of days. Although scientists don’t yet know how often the variant causes severe disease, they say its rapid rate of spread will lead to an explosion of cases and could potentially increase pressure on hospitals, even if it proves to be mild.The reports follow similarly worrisome findings from England released over the weekend, although researchers caution that the trend could change as the variant comes into clearer view. It’s not yet certain how often Omicron infections will send people to the hospital, or how many hospitalized patients are likely to die. And while Omicron can partly evade immune defenses, researchers have yet to determine how well vaccinations and previous infections will protect people against severe disease.The authors of both new reports also observed that swift actions now, such as booster campaigns and reducing opportunities for Omicron to spread, could lessen the variant’s impact.American researchers have yet to release models of Omicron’s rise in the United States. But experts point out that the country is similar to Norway and Denmark in terms of vaccination levels and certain Covid risk factors, like the average age of the population.“It would be naïve to think the United States would be any different than Denmark,” Mads Albertsen, a microbiologist at Aalborg University, said. “Denmark is likely a best-case scenario.”In recent weeks, many epidemiologists have been paying close attention to Denmark, a country of 5.8 million residents, about the population of Wisconsin.Early in the pandemic, the country set up a sophisticated surveillance system combining large-scale coronavirus testing with genetic sequencing of many samples. That strategy has allowed Denmark to spot newly emerging variants, even when they’re at low levels, and adjust public health policies to prepare for new surges.The first Omicron sample from Denmark was sequenced on Dec. 3. The specimen was collected on Nov. 23, around the same time researchers in South Africa first told the world about a rise in cases there.The Statens Serum Institute in Copenhagen, where researchers on Monday released an estimate that Omicron cases in Denmark were doubling every two days.Mads Claus Rasmussen/Ritzau Scanpix, via Agence France-Presse — Getty ImagesBecause sequencing genetic material from coronavirus samples can take days, Danish researchers developed a quick genetic test that picks up a few key mutations found only in Omicron. Every positive test result in Denmark is now screened for the new variant, resulting in an exceptionally comprehensive picture of Omicron’s spread.In the report released on Monday by the Statens Serum Institute in Copenhagen, researchers estimated that Omicron cases in Denmark were doubling every two days. Omicron is spreading much faster than Delta, which means that the new variant will become dominant by midweek, the report found.Three-quarters of the Omicron cases are in people who have received two vaccine doses, which is about the same fraction of the entire country that’s fully vaccinated. That high percentage indicates that vaccines are providing little protection from infection, though most scientists believe that the shots will still fend off severe disease and death.The Danish data are consistent with a smaller report of Omicron infections in the United States. Out of 43 documented cases, 34 — or about 79 percent — were people who were fully vaccinated.“This thing can spread, and it can spread whether or not you were vaccinated,” Christina Ramirez, a biostatistician at the University of California, Los Angeles, said.In England, researchers also found that full vaccination provided low protection against a breakthrough infection. But they found that booster shots restored defenses to much higher levels.In these European countries, Omicron will not simply replace Delta: It will drive up cases. Currently, Denmark is seeing around 6,000 cases a day — already a record for the country and driven almost entirely by Delta. The Danish researchers project that Omicron will drive the daily cases to 10,000 by the end of the week, and the numbers will continue to climb from there.The model predicts that the explosive growth could send large numbers of people to the hospital, even if the Omicron variant turns out to be milder than earlier variants. The Danish researchers also warned that Omicron outbreaks at hospitals, even if mild, could lead to dangerous drops in staffing as doctors and nurses are sent home to quarantine.Standing in line for Covid booster shots in London on Monday.Andy Rain/EPA, via ShutterstockThe authors of the new report cautioned that their model was preliminary. It did not take into account the potent protection that boosters can afford, for example. Right now, 21 percent of people in Denmark have gotten a booster shot, and the country is pursuing an aggressive booster campaign.Even so, Troels Lillebaek, the director of the Statens Serum Institute, said that the next few weeks would be a major challenge for the country’s hospitals.“Regardless of the uncertainty about the precise severity and contagiousness of Omicron, there is a very high risk of an increasing number of admissions,” he said.In Norway, researchers have also observed a rapid rise of Omicron in recent days. “The Omicron variant is becoming established in Norway and will soon dominate,” the Norwegian Institute of Public Health said in a statement on Monday.The Coronavirus Pandemic: Key Things to KnowCard 1 of 5U.S. nears 800,000 Covid deaths.

A panel of advisers to the Centers for Disease Control and Prevention on Friday voted to recommend lifting the pause on the Johnson & Johnson Covid-19 vaccine for all adults and adding a label about an exceedingly uncommon, but potentially dangerous, blood clotting disorder.The vote came after a C.D.C. scientist presented nine new confirmed cases of the disorder, bringing the total to 15. All the cases have been in women, and 13 have been in women between 18 and 49 years old.Federal health officials are expected to respond quickly and formally recommend that states lift the pause.Administration of the vaccine ground to a halt last week after reports emerged of a rare blood clotting disorder in six women who had received the vaccine. The disorder combines blood clots, often in the brain, and a low level of platelets, blood cells that typically promote clotting.“This pause was essential to our ability to inform the public,” Dr. José R. Romero, the chair of the advisory panel, said at the meeting.The overall risk of developing the clotting disorder is extremely low. Women between 30 and 39 appear to be at greatest risk, with 11.8 cases per million doses given. There have been 7 cases per million doses among women between 18 and 49.Three women have died from the rare clots and seven remain hospitalized, four of whom are in the intensive care unit, a C.D.C. scientist said at Friday’s meeting.“These cases are not just numbers to any of us, and we take them very seriously,” Dr. Joanne Waldstreicher, the chief medical officer of Johnson & Johnson, said at the meeting. “These are people.”The company supported adding a warning label to the vaccine, and has agreed upon language for a potential label with the F.D.A., she said. The label she presented notes that “most cases” of the clotting disorder have occurred in women between 18 and 49 years old.Nearly 8 million doses of the Johnson & Johnson vaccine have now been administered. Among men and women who are 50 or over, there has been less than one case per million doses.The clotting disorder is “rare but clinically serious,” Dr. Tom Shimabukuro, the deputy director of the C.D.C.’s immunization safety office, said at the meeting.Roughly 10 million doses or more of the Johnson & Johnson vaccine, produced at the company’s factory in the Netherlands, are sitting on shelves across the United States and could be deployed immediately.The vaccine has immense potential benefits for the population. If vaccinations resume for all adults, 26 to 45 cases of the clotting disorder would be expected over the next six months, according to a model presented by Dr. Sara Oliver, a C.D.C. scientist. However, 600 to 1,400 fewer Covid-19 related deaths would be expected over the same time period.The vaccine, which is easy to store and requires just one shot, is also especially well-suited for use in hard-to-reach populations, including people who are homebound, homeless, or incarcerated.Additional potential cases of the clotting disorder, including some in men, are currently being reviewed. The C.D.C. scientist also mentioned one case that developed in a 25-year-old man who participated in a clinical trial of the vaccine.Twelve of the 15 women in the confirmed cases developed blood clots in the brain. Many also had clots elsewhere. Initial symptoms, which include headaches, typically begin six or more days after vaccination, Dr. Shimabukuro said. As the disorder develops, it can cause more severe headaches, nausea and vomiting, abdominal pain, weakness on one side of the body, speech difficulties, loss of consciousness and seizures.Dr. Shimabukuro noted that seven of the women were obese, two had hypothyroidism, two had high blood pressure, and two were using oral contraceptives. It is not yet clear whether any of these factors might increase the risk of developing the clotting disorder after vaccination.The patients’ symptoms closely resemble a rare syndrome that can be caused by heparin, a widely used blood thinner, Dr. Michael Streiff, a hematologist at Johns Hopkins University, told the panel. Heparin, which might typically be used to treat blood clots, should not be used to treat these patients, he said.Doctors should consider the rare clotting disorder in patients who present with blood clots and low levels of platelets within three weeks of receiving the Johnson & Johnson vaccine, Dr. Streiff said.“Recognition that this syndrome exists is helping to improve outcomes,” he said.The meeting comes as the federal government is also investigating problems at a Baltimore factory that was slated to help satisfy the country’s vaccine demand. Emergent BioSolutions, the plant’s operator, has produced tens of millions of doses of Johnson & Johnson’s vaccine, but they cannot be distributed until regulators certify the plant.After Emergent had to discard up to 15 million possibly contaminated doses of the vaccine last month, federal regulators conducted an inspection that found a series of problems, including the risk that other batches could have been contaminated.