Pfizer announced on Friday that its pill to treat Covid-19 had been found in a key clinical trial to be highly effective at preventing severe illness among at-risk people who received the drug soon after they exhibited symptoms.

The antiviral pill is the second of its kind to demonstrate efficacy against Covid. It appears to be more effective than a similar offering from Merck, which is awaiting federal authorization.

Pfizer’s pill, which will be sold under the brand name Paxlovid, cut the risk of hospitalization or death by 89 percent when given within three days of the start of symptoms.

Pfizer said an independent board of experts monitoring its clinical trial had recommended that the study be stopped early because the drug’s benefit to patients had proved so convincing. The company said that it planned to submit the data as soon as possible to the Food and Drug Administration to seek authorization for the pill to be used in the United States.

“The results are really beyond our wildest dreams,” said Annaliesa Anderson, a Pfizer executive who led the drug’s development. She expressed hope that Paxlovid “can have a big impact on helping all our lives go back to normal again and seeing the end of the pandemic.”

The treatment could become available in the next few months, though supplies are likely to be limited at first. The Pfizer and Merck pills are both geared toward patients regarded as high-risk, such as those above the age of 60 or with conditions like obesity that make them more susceptible to severe consequences from Covid.

President Biden said on Friday that the pill would be “another tool in our toolbox to protect people from the worst outcomes of Covid,” but he emphasized that the best approach was to prevent infections through vaccination.

Posted in: USA