“The protection from hospitalization is obviously fantastic,” said Andrew Pekosz, vice chair of microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health. “That number was a great number. The fact that it was conserved in that final analysis really points to this being a really important weapon in our arsenal to fight Covid-19 particularly as we see more variants that are going to be chipping at that efficacy number.”

Pfizer also reported results from a second study in adults with Covid at normal risk of developing severe disease, a group that included vaccinated people. That study failed to meet its main goal, of increasing the sustained alleviation of self-reported symptoms, at an interim analysis; the study is continuing. But Pfizer said that there was a decrease in hospitalization in that group, too, although numbers were small.

In the study of high-risk patients, called EPIC-HR, 5 of 697 patients who received a five-day course of Paxlovid were hospitalized or died, compared to 44 of 682 who received a placebo. There were no deaths in the Paxlovid group and 9 in the placebo group. Adverse events occurred at similar rates between the placebo and Paxlovid groups, and patients on Paxlovid were less likely to have a severe problem or to stop taking the drug due to a perceived side effect.