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That’s the potential promise of a new study on a drug made by Japanese pharmaceutical company Shionogi. At a scientific conference in San Francisco, researchers reported that their drug, ensitrelvir, helped prevent people who were exposed to SARS-CoV-2 from testing positive for the disease.
There is currently no drug approved to prevent COVID-19, but ensitrelvir is already approved in Japan as a treatment for COVID-19. It reduces hospitalizations for COVID-19 among people at the highest risk of complications; for the less vulnerable, it cuts down on the number of days they're sick with symptoms.
The U.S. Food and Drug Administration is considering the drug for fast-track approval as a way to prevent COVID-19, based on this latest study presented at the Conference of Retroviruses and Opportunistic Infections. (The study has not yet been published in a peer-reviewed journal.)
Researchers studied more than 2,300 people age 12 and older who didn't have COVID-19 but lived with someone who had tested positive at the time of the study. They were then randomly assigned to receive either ensitrelvir or placebo pills for five days. Everyone in the study began taking their pills once a day within three days of when their housemate first reported symptoms of COVID-19.
Among those who took ensitrelvir, about 3% ended up developing COVID-19, compared to 9% of those taking placebo. It turned out that about 10% of the household members of the person who initially tested positive also were positive, even if they didn't experience symptoms and didn't realize they were positive—which highlights how transmissible the virus can be, and how important it is to protect people from getting the infection. The results mean that the drug lowered the risk of getting COVID-19 by 67%.
The idea of using an antiviral treatment to protect people at high risk of infection isn’t new. The popular flu treatment oseltamivir, or Tamiflu, is around 84% effective at protecting people from getting the flu when someone else in their house has it. But when scientists studied antiviral treatments for COVID-19, such as Paxlovid and molnupiravir, they didn’t find the same preventative benefits.
“This study is the first where this strategy [to prevent COVID-19] was documented to succeed,” says Dr. Frederick Hayden, professor emeritus of medicine at University of Virginia School of Medicine, who presented the data at the conference.
Finding a way to prevent COVID-19 is critical, especially for older adults, immunocompromised people, and others who are at high risk of developing complications. In the study, people in this category who were taking the drug reduced their risk of getting COVID-19 by 76%. Avoiding infection also allows people to sidestep complications such as Long COVID, for which there aren't yet many treatments.
Because ensitrelvir works by blocking the virus’ ability to make more copies of itself, it makes sense that it can both treat and prevent disease, depending on when people take it. The dose for treating COVID-19 is the same as the dose used in the study to prevent disease. If people take ensitrelvir early—within three days of being closely exposed to someone with the virus—then the drug can effectively hamper SARS-CoV-2 enough to prevent it from infecting too many cells. If people take it after they have been infected, the drug can help to reduce the amount of virus the immune system has to manage and can lower the chances of severe disease.
“This is really, to my knowledge, the first documentation that one could use an oral antiviral for the prevention of COVID-19 in higher risk transmission settings like households,” says Hayden.
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