A new study of nearly 70,000 Covid patients in California demonstrates that Omicron causes less severe disease than other coronavirus variants, results that align with similar findings from South Africa, Britain and Denmark, as well as a host of experiments on animals.
Compared with Delta, Omicron infections were half as likely to send people to the hospital. Out of more than 52,000 Omicron patients identified from electronic medical records of Kaiser Permanente of Southern California, a large health system, the researchers found that not a single patient went on a ventilator during that time.
“It’s truly a viral factor that accounts for reduced severity,” said Dr. Lewnard, an epidemiologist at the University of California, Berkeley who was an author of the study, which was posted online on Tuesday and has not yet been published in a scientific journal.
Despite the less severe virulence of Omicron, U.S. hospitals are buckling under an influx of coronavirus cases. Dr. Lewnard said that this was the result of the variant spreading like wildfire. On average, more than 730,000 people are testing positive every day in the United States, almost three times the previous peak last winter.
“Since it’s more transmissible, there will just at some point be a lot of hospitalizations that inevitably occur,” Dr. Lewnard said.
In recent weeks, Britain and several other countries have reported that Omicron has a lower risk of hospitalizations. When the variant hit the United States last month, Dr. Lewnard and his colleagues began analyzing electronic health care records maintained by Kaiser Permanente of Southern California, which serves 4.7 million people.
They analyzed 69,279 symptomatic patients who tested positive for the coronavirus from Nov. 30 to Jan. 1. Three-quarters of the positive samples contained the Omicron variant, and the rest were Delta.
The researchers then followed the people who tested positive to see whether they ended up in the hospital. They excluded the so-called incidental Covid patients who showed up at hospitals for other complaints and tested positive for the coronavirus after arriving.
Compared with Delta, Omicron cut the risk of hospitalization in half, the study found, and the people who came to the hospital with Omicron stayed for a shorter period. The variant cut hospital stays by more than three days, a reduction of 70 percent compared with Delta.
Fourteen of the Delta-infected patients died, while only one Omicron patient did.
As scientists have gathered evidence that Omicron is less severe, they have struggled to understand why. One reason is that the people infected with Omicron have more immune defenses than in previous waves.
In other countries, researchers have found that earlier infections with other variants reduce the chances that people end up severely ill with Omicron. Vaccination also offers protection.
“Vaccines are quite helpful,” Dr. Lewnard said. He and his colleagues found that Californians who were vaccinated were 64 to 73 percent less likely to be hospitalized than unvaccinated people.
Even among unvaccinated people, however, Omicron was less likely to lead to hospitalizations than Delta.
Dr. Lewnard said that this extra analysis showed that Omicron was fundamentally less severe. Studies on animals suggest that Omicron readily infects cells in the upper airway but works poorly in the lungs, which could explain its milder effects.
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Top federal health officials on Tuesday defended the Biden administration’s efforts to protect Americans from the highly contagious Omicron variant, facing withering accusations from senators about the scarcity of coronavirus tests and confusing guidance on how soon people who test positive for the virus can return to normal life.
In a nearly four-hour hearing, lawmakers charged that the administration remained woefully unable to meet the demand for at-home tests, noting that the White House would fulfill its pledge to send 500 million of them to American households for free only after the current surge had peaked.
The health officials testified before the Senate Committee on Health, Education, Labor and Pensions during one of the most trying weeks yet in the administration’s struggle with the pandemic. Infection rates are skyrocketing nationwide, and hospitals set a single-day record on Sunday for the number of patients with the virus, surpassing last winter’s peak.
While Democratic senators offered only gentle criticism, Republicans were unsparing, claiming that President Biden and his pandemic response team had mangled public health strategy and messaging.
“Most Americans can’t make heads or tails of anything coming out of this administration,” said Senator Tommy Tuberville, Republican of Alabama.
The officials who testified said they had mounted an all-out effort to test, treat and vaccinate Americans in the middle of a shape-shifting pandemic.
“It’s hard to process what’s actually happening right now, which is, most people are going to get Covid,” said Dr. Janet Woodcock, the acting commissioner of the Food and Drug Administration, offering one of the federal government’s most pointed acknowledgments of Omicron’s effect since the variant arrived in the United States.
Dr. Anthony S. Fauci, Mr. Biden’s chief medical adviser, said the virus had “fooled everybody all the time, from the time it first came in, to Delta, to now Omicron,” adding, “We’re doing the best we possibly can.”
The hearing came as the Omicron variant, coupled with the Delta variant, has strained hospital systems and caused businesses to struggle to stay open because of staff shortages. An average of more than 735,000 infections are being reported in the United States each day, according to a New York Times database.
On average over the past seven days, more than 135,000 people were hospitalized with the virus, an 83 percent increase from two weeks ago.
Modeling scenarios cited in an internal government document dated Jan. 5 and obtained by The New York Times suggested there would be more than a million confirmed infections per day by the end of the month. That number is widely viewed as a vast undercount because of the scarcity of tests and the widespread failure of people to report positive results from at-home tests to government authorities.
More than half of people in Europe could be infected with the Omicron variant of the coronavirus in the next six to eight weeks, the World Health Organization warned on Tuesday, amid “a new west-to-east tidal wave sweeping across the region.”
“The region saw over seven million cases of Covid-19 in the first week of 2022, more than doubling over a two-week period,” Dr. Hans Kluge, the agency’s regional director for Europe, said at a news conference.
While coronavirus vaccines remain remarkably effective at preventing severe illness and death, the agency cautioned against treating the virus like the seasonal flu, since much remains unknown — particularly regarding the severity of the disease in areas with lower vaccination rates, such as Eastern Europe.
The W.H.O. has cautioned for months that booster shots could worsen vaccine inequality around the world, but Dr. Kluge said on Tuesday that they would play an essential role in protecting the most vulnerable people from severe disease and should also be used to protect health workers and other essential employees, including teachers.
Since Omicron was first detected in late November, it has torn across the planet at a pace unseen during two years of the pandemic. As friends, co-workers and family members test positive, the reality that the virus is moving quickly and widely has been a defining feature of this wave of infection.
But the steep rise that Dr. Kluge cited, based on forecasts by the Institute for Health Metrics and Evaluation, is a stark paradigm shift. Although the institute’s models have frequently been criticized by experts, it is clear that the virus is spreading quickly. Even if many people avoid severe illness, the virus promises to cause societal disruption across the continent.
While much of the public discussion has revolved around whether this was the moment when governments should shift policies and restrictions to treat the coronavirus as an endemic disease — removing most restrictions and allowing people to manage risk in a way similar to the way they do with influenza — the W.H.O. said it was too early to call this virus endemic.
Catherine Smallwood, a W.H.O. senior emergencies officer, said that one of the key factors in declaring the virus to be endemic was some sense of predictability.
“We are still ways off,” she said. “We still have a huge amount of uncertainty.”
Dr. Kluge added that there were simply too many unknown factors, including exactly how severe Omicron is for unvaccinated people and how high the risk is of infection leading to “long Covid” symptoms.
“I am also deeply concerned that as the variant moves east, we have yet to see its full impact in countries where levels of vaccination uptake are lower, and where we will see more severe disease in the unvaccinated,” he said.
Nations in the Balkans and Eastern Europe, where Omicron is just starting to spread widely, have much lower rates of vaccination than nations in Western Europe.
“For countries not yet hit with the omicron surge, there is a closing window to act now and plan for contingencies,” he said.
One of the central struggles of governments across Europe has been trying to keep schools open, and Dr. Kluge described those efforts as essential.
“Schools should be the last places to close and the first to reopen,” he said, although he added that “the numbers of infected people are going to be so high in many places that schools in many countries are going to be unable to keep all classes open” because of illness and staff shortages.
MIAMI — The Food and Drug Administration has extended the shelf life of up to one million rapid coronavirus tests that had expired in Florida, the state’s Division of Emergency Management said on Tuesday.
The extension will allow the tests to be used over the next three months, the division said in a statement. It provided a copy of a letter from the F.D.A. dated Jan. 7 that granted Florida’s request to lengthen the tests’ shelf life to 15 months, meaning March of this year.
Floridians have experienced long lines at testing sites and a shortage of at-home tests amid a surge of the Omicron variant. The confirmation by state officials last week that so many tests were sitting unused prompted harsh criticism of the administration of Gov. Ron DeSantis, a Republican.