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.
The 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.
They 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 Know
The Omicron variant. The highly transmissible Covid strain appears to be less severe than previous variants, according to new studies. Research also suggests many non-mRNA vaccines offer almost no defense against infection, though the Pfizer and Moderna boosters, which are mRNA-based, likely provide strong protection.
The team was able to tease apart the outcomes of Omicron infection on people who were vaccinated, previously infected or lacking any immunity at all. They confirmed the suggestion of the South African researchers that acquired immunity was helping to make Omicron infections milder on average.
The difference between the results of the English and Scottish studies also may be due in part to differences in what was measured. The Imperial College London team included people who just visited a hospital, in addition to those who had to be put in a bed for a more serious illness. The Scottish researchers looked only at hospital admissions.
Both teams of scientists cautioned that their results were still preliminary. For one thing, Omicron is still working its way through Britain and has yet to make much headway among older people who might be at greater risk of hospitalization.
And even if individual cases are mild, Omicron still poses a serious risk to hospitals because cases are exploding so quickly. “We’re not at a place to treat this as a cold,” Azra Ghani, a co-author of the Imperial College London study, said.
William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said that the new findings made it clear that people who were unvaccinated and had not yet had Covid were at especially high risk.
Because Omicron is spreading fast, the chances are high that they will be infected in the months to come, he said. If so, they will not have any immunity to blunt the severity of the infection.
“If you are unvaccinated and you have never been infected, it is a little less severe than Delta,” Dr. Hanage said. “But that’s a bit like saying you’re being hit over the head with one hammer instead of two hammers. And the hammers are more likely to hit you now.”
While the past few weeks have brought a flood of new data on Omicron, there are still many questions about how the coming weeks will unfold and how high hospitalization rates will climb.
Chris Robertson, an epidemiologist at the University of Strathclyde and a co-author of the Scottish study, warned that the holiday season would slow the gathering of data that he and others would need to update their findings.
“Normally, I would be doing this analysis every week, but now it’s going to be two to three weeks before there’re any other updates on it,” he said.
Nicholas Reich, a biostatistician and infectious disease modeler at the University of Massachusetts Amherst, said: “We need to be willing to wait for answers and be willing to swim in the uncertainty a little bit.”
But a team of modelers who run a project called the Covid-19 Scenario Modeling Hub issued a statement on Wednesday making it clear that even as they fine-tuned their projections, the writing was already on the wall.
“The best information we have at the moment indicates the threat posed by Omicron is substantial and imminent, and individuals and governments should be prepared to respond accordingly,” they warned.