‘An antiviral pill has, for the first time, been shown to prevent COVID-19 in people exposed to the SARS-CoV-2 virus at home, according to trial results published today in the New England Journal of Medicine.

The drug could be a lifeline for those who still face real danger from the virus, such as care-home residents or transplant recipients on immune-suppressing medication.

The advance arrives years after the peak of the COVID-19 pandemic, so the real-world impact might be felt by only a narrow band of individuals. Still, “as a 78-year-old with comorbidities, I certainly would use it if I had a known exposure”, says study co-author Frederick Hayden, a clinical virologist at the University of Virginia School of Medicine in Charlottesville.

The drug, called ensitrelvir, is made by the Japanese pharmaceutical company Shionogi. It blocks an enzyme that coronaviruses need to make new copies of themselves, hitting the same target as one of the two active ingredients in Pfizer’s antiviral Paxlovid. But whereas that ingredient, nirmatrelvir, failed to prevent household infections in trials2, ensitrelvir has come through.

In an international study of more than 2,000 household contacts conducted from June 2023 to September 2024, about 9% of people who received a placebo within 72 hours of a housemate developing symptoms became symptomatic themselves, compared with only about 3% of those who got a five-day course of ensitrelvir. Rates of viral transmission were lower in the ensitrelvir group, too: confirmed infections, symptomatic or not, turned up in only 14.0% of those who received the drug, compared with 21.5% of those who got a placebo.

The therapy generally proved tolerable, without the taste-related side effects often seen with Paxlovid and with similar toxicity, overall, to that of the placebo.

On the strength of these trial data, Japan’s health ministry approved ensitrelvir, marketed as Xocova, in March for post-exposure use. Regulators in the United States, Europe and elsewhere are considering whether to follow suit, with a US decision expected in the next month or so….’ (Elie Dolgin via Nature)