This Pill Can Prevent You From Getting Covid-19 After Exposure to the Disease-Causing Virus, According to a Clinical Trial
The drug showed promising results in an international study involving nearly 2,400 participants, and the U.S. Food and Drug Administration is expected to make an approval decision for it in June
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After more than six years of humans living with Covid-19, we might finally have a pill that can prevent the disease after exposure to the SARS-CoV-2 virus.
The oral drug, called ensitrelvir, significantly reduced the risk of getting sick among individuals who lived with someone ill with Covid-19, according to clinical trial results published May 13 in the New England Journal of Medicine. The experimental pill could be especially useful for older adults and people with weakened immune systems.
“It’s wonderful to have another preventative modality available to people who are exposed to Covid, particularly those in high-risk circumstances,” says William Schaffner, an infectious-diseases specialist at the Vanderbilt University School of Medicine who was not involved in the study, to Korin Miller at Prevention.
Ensitrelvir was developed by the Japanese pharmaceutical company Shionogi. It works by blocking an enzyme that the virus needs to make copies of itself, just like Pfizer’s drug Paxlovid, also called nirmatrelvir-ritonavir. While Paxlovid can dramatically reduce the risk of hospitalization and death from Covid-19, it failed to offer protection against getting sick after contact with an infected person in a clinical trial published in NEJM in 2024.
The recent ensitrelvir trial included nearly 2,400 people ages 12 and older in the United States, Argentina, Japan, South Africa and Vietnam, and it took place between June 2023 and mid-September 2024. Participants had a household contact with Covid-19, confirmed via positive test and the presence of at least one symptom.
Half of the participants started a five-day course of ensitrelvir within 72 hours of their household contact showing signs of illness, while the other half took a placebo. The study was double-blinded, meaning neither the participants nor the researchers knew who got what kind of pill.
Within ten days of starting their pills, around 9 percent of the placebo group showed symptoms of Covid-19 and tested positive for it, while less than 3 percent of those who received ensitrelvir developed symptoms and tested positive. Regardless of symptoms, 21.5 percent of the placebo group had lab-confirmed Covid-19, compared with 14 percent of the ensitrelvir group, the researchers found.
What’s more, those who developed Covid-19 while taking the drug had lower amounts of the virus in their noses and throats than those who got sick while taking the placebo.
Did you know? Some people have never come down with Covid-19
A small group of people has managed to avoid getting sick with the respiratory illness. They’re sometimes referred to as “novids” or “super-dodgers.” Having high levels of a specific gene might help them clear the virus so efficiently that a prolonged infection cannot occur, researchers reported in 2024.
“This is really the first clear demonstration in a well-performed phase III placebo-controlled, double-blind trial that we actually have an agent that is easily administered orally and effective if taken in a timely fashion for protecting individuals who are exposed to Covid-19 in the household setting,” says study co-author Frederick Hayden, a clinical virologist at the University of Virginia School of Medicine, to Terrence Rudd at MedPage Today.
Most people have gained some immunity to Covid-19 through past infection or vaccination. So, older individuals and those with underlying health conditions will probably benefit the most from the experimental drug, says David Boulware, an infectious-diseases specialist at the University of Minnesota Medical School who was not involved in the study, to Nature’s Elie Dolgin.
“There’s a role for occupational exposure as well,” he adds, such as health care workers who see patients with Covid-19.
That doesn’t mean you should stop taking other precautions, like getting Covid-19 booster shots, experts say. Most of the study participants had some prior immunity to SARS-CoV-2, says study co-author Paul E. Sax, an infectious-diseases specialist at Harvard Medical School, to Prevention. Ensitrelvir therapy is “complementary to vaccination rather than replacing it,” he adds.
The drug was recently approved in Japan, under the brand name Xocova, as a post-exposure protection measure against Covid-19. It’s currently under consideration for approval by the Food and Drug Administration for the same use in the United States, with a decision expected in June.
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