Oral antiviral ensitrelvir cut Covid infections in household contacts in one trial

No antiviral drug has been approved to stop Covid-19 before it takes hold in someone who has merely been exposed. A large randomised trial now reports that the oral antiviral ensitrelvir, given to household contacts of an infected person, lowered their chance of developing symptomatic Covid-19 — from 9.0% on placebo to 2.9% on the drug [s1]. The result is a genuine first for post-exposure prophylaxis against Covid, but it comes with the caveats that matter for any single, industry-funded trial: it was funded by Shionogi, the drug's maker, and it was conducted largely in one setting [s1].

Ensitrelvir is an oral inhibitor of the SARS-CoV-2 3C-like protease, an enzyme the virus needs to replicate. It is already approved in Japan to treat mild-to-moderate Covid-19 [s1]. The new trial, SCORPIO-PEP, tested a different use: not treating an established infection, but heading one off in a person who has been exposed but has not yet tested positive [s1].

How the trial was run

The trial was double-blind, randomised and placebo-controlled [s1]. Investigators enrolled people who lived with someone newly diagnosed with Covid-19 — the "index patient" — and who themselves tested negative for SARS-CoV-2 at the outset [s1]. Contacts were randomly assigned, within 72 hours of the index patient's symptoms beginning, to receive either ensitrelvir (375 mg on day 1, then 125 mg daily on days 2 through 5) or a matching placebo [s1].

The primary endpoint was carefully specified: laboratory-confirmed Covid-19, defined as a positive central-laboratory PCR test plus at least one of 14 prespecified symptoms lasting at least 48 hours, occurring by day 10 [s1]. That combined definition matters, because it counts symptomatic disease rather than any positive swab, and it is the outcome a household most cares about.

The modified intention-to-treat population included 1030 participants assigned to ensitrelvir and 1011 to placebo [s1]. Their mean age was 42.4 years; 71.1% had been randomised within 48 hours of the index patient's symptom onset, and 37.0% had at least one risk factor for severe Covid-19 [s1].

What it found

Symptomatic Covid-19 by day 10 occurred in 2.9% of the ensitrelvir group versus 9.0% of the placebo group — a risk ratio of 0.33 (95% CI 0.22 to 0.49; p<0.001) [s1]. In relative terms that is about a two-thirds reduction; in absolute terms it is roughly six fewer symptomatic infections per hundred contacts treated, a framing worth keeping in view because relative reductions can sound larger than the real-world difference [s1].

Safety looked reassuring in this trial. Adverse events occurred at similar rates in the two groups — 15.1% with ensitrelvir and 15.5% with placebo — and serious adverse events were rare and equal, at 0.2% in each group [s1]. No Covid-19-related hospitalisations or deaths were reported in either group [s1]. That last point cuts two ways: it is reassuring, but it also means the trial could not show whether the drug prevents severe outcomes, only that it prevented symptomatic infection in a population that was not, on the whole, getting severely ill.

The limits

The strongest caveat is structural: this is one trial, funded by the manufacturer, and it enrolled a population with a mean age in the early forties in which severe Covid was essentially absent [s1]. Whether post-exposure ensitrelvir would help the older, higher-risk contacts who most need protection against severe disease is exactly what this study cannot answer. An accompanying exchange of correspondence in the same journal reflects the scrutiny such a first-in-class prophylaxis result invites [s2].

There is also the practical question the trial does not settle: post-exposure prophylaxis only works if contacts can be identified and treated within a narrow window — here, within 72 hours of the index case's symptoms — and most households do not move that fast [s1]. In the trial, 71.1% of contacts were dosed within 48 hours, a speed made possible by a study infrastructure that ordinary care rarely matches [s1]. A further open question is durability: the endpoint was measured at day 10, so the trial shows only that a five-day course suppressed infection over a short window, not that contacts were protected weeks later once the drug had cleared [s1].

What it means

SCORPIO-PEP establishes, for the first time in a rigorous trial, that an oral antiviral can reduce symptomatic Covid-19 in exposed household members [s1]. The honest reading is that the effect on symptomatic infection is real and the short-term safety was clean, but the trial says nothing about preventing severe disease, rests on a single sponsor's study, and describes a relatively young population [s1][s2]. What to watch next is whether regulators outside Japan act on it, and whether any independent trial tests the same question in the older adults for whom the stakes are highest.

Sources

• [s1] Ensitrelvir for Covid-19 Postexposure Prophylaxis in Household Contacts. New England Journal of Medicine, 14 May 2026. https://doi.org/10.1056/nejmoa2509306
• [s2] Ensitrelvir for Covid-19 Postexposure Prophylaxis (Correspondence). New England Journal of Medicine, 1 September 2026. https://doi.org/10.1056/nejmc2608575