The treatment, called Paxlovid, is likely to work against Omicron and could be available in the United States before the end of the year.

By Carl Zimmer

Pfizer announced on Tuesday that its Covid pill was found to stave off severe disease in a key clinical trial and that it is likely to work against the highly mutated Omicron variant of the virus. The results underscore the promise of the treatment, which health officials and doctors are counting on, to ease the burden on hospitals as the United States braces for a mounting fourth wave of the pandemic.

If the Food and Drug Administration authorizes the drug, which could happen within days, then patients might begin receiving it by the end of the year. Although supply will be limited at first, public health experts are hopeful that the pills might curb the worst outcomes from the disease, no matter the variant.

Pfizer said its antiviral pill was found to reduce the risk of hospitalization and death by 88 percent when given to unvaccinated people at high risk of severe Covid within five days of the onset of symptoms. The company also said that laboratory experiments indicated that the drug will attack a key protein in the Omicron variant, which is surging in South Africa and Europe and is expected to dominate U.S. cases in the weeks ahead.

“This is quite amazing and potentially transformative,” said Sara Cherry, a virologist at the Perelman School of Medicine at the University of Pennsylvania who was not involved in the study. “If we could keep people out of hospitals, that would have a huge impact on health care.”

The promising Pfizer study was good news on a day when the Centers for Disease Control and Prevention warned that a tidal wave of infections, from both the Omicron and Delta variants, could arrive as soon as next month, just as influenza and other winter respiratory infections peak.

“The early signals say there are going to be waves coming,” said Scott Becker, chief executive of the Association of Public Health Laboratories, who was on the call when the C.D.C. issued its alert during a briefing.

Some U.S. states are already seeing record hospitalizations from the Delta variant spread, mostly among the unvaccinated. Researchers are warning that Omicron may spread even more rapidly and seems to evade some of the immune defenses provided by vaccines or previous infection.

In a study released on Tuesday, South African researchers found that two doses of the Pfizer-BioNTech vaccine provide much less protection against infection with Omicron than against other variants.

Although the shots still provide strong protection against severe disease and hospitalization, it’s possible that Omicron’s drastic rate of transmissibility will create a surge of severe infections, particularly in unvaccinated people. Those seriously ill people could swamp hospitals in the next few months. A highly effective antiviral pill like Pfizer’s could be crucial to easing that surge, Dr. Cherry said.

Last month, Pfizer asked the Food and Drug Administration to authorize the treatment, known as Paxlovid, for high-risk adults, based on a preliminary batch of data. The new results will undoubtedly strengthen the company’s application for the drug, which is meant to be prescribed by a health care provider after a positive virus test and taken at home.

The results, based on an analysis of more than 2,200 unvaccinated volunteers at high risk of severe disease, largely match the company’s initial, smaller analysis of the clinical trial, released last month.

Pfizer said that in its final analysis, 0.7 percent of patients who received Paxlovid were hospitalized within 28 days of entering the trial, and none died. By contrast, 6.5 percent of patients who received a placebo were hospitalized or had died.

Pfizer also released preliminary data from a separate trial looking at people with a lower risk. These volunteers included vaccinated people who carried a risk factor for severe disease, as well as unvaccinated patients with no risk factors.

Among this group of 662 volunteers, Paxlovid reduced the risk of hospitalization and death by 70 percent, the company said.

Several public health experts said they thought it was unlikely that the F.D.A. would immediately authorize Paxlovid for people at standard risk of becoming severely ill from Covid based on the preliminary results, though the agency may do so eventually.

“Maybe it’s something that your physician would think about if you had severe underlying conditions,” Seema Lakdawala, a virologist at the University of Pittsburgh, said.

Dr. Lakdawala said that regulators might consider expanding the drug’s use if the benefits outweighed any potential risks. Paxlovid might shorten the amount of time that people shed the coronavirus, for example, which could, in turn, reduce how long people have to spend in quarantine. It might even cut down the chances that infected people pass on the virus to others. “All of those would be hugely beneficial,” she said.

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