Ensitrelvir for Covid-19 Postexposure Prophylaxis in Household Contacts
In household contacts, ensitrelvir started within 72 hours reduced symptomatic Covid-19 by day 10 versus placebo.
*Double-blind randomized placebo-controlled trial; Level 1b (OCEBM).
*Double-blind randomized placebo-controlled trial; Level 1b (OCEBM).
Citation
Hayden FG, Shinkai M, Clark TW, et al. Ensitrelvir for Covid-19 Postexposure Prophylaxis in Household Contacts. N Engl J Med. 2026;394:1905-1915. doi:10.1056/NEJMoa2509306.
Background
Households are a common setting for Covid-19 spread, and prior oral antiviral prevention studies in household contacts have not shown clear benefit. This trial tested whether a 5-day course of ensitrelvir prevents Covid-19 after a household exposure.
Patients
Household contacts aged 12 years or older, locally negative for the virus that causes Covid-19, enrolled within 72 hours after symptom onset in the household “index” patient. Key exclusions: fever or Covid-19 symptoms, known recent infection, vaccination within 6 months, pregnancy, and certain interacting medicines.
Intervention
Ensitrelvir by mouth: 375 mg day 1, then 125 mg daily days 2–5.
Control
Placebo for 5 days.
Outcome
(Primary) Symptomatic Covid-19 by day 10: central laboratory nasal swab positive plus at least 1 prespecified symptom lasting at least 48 hours.
Follow-up Period
10 days for primary outcome.
Results
| Outcome (day 10) | Ensitrelvir | Placebo | Effect | NNT |
|---|---|---|---|---|
| Symptomatic Covid-19 (primary) | 30/1030 (2.9%) | 91/1011 (9.0%) | Risk ratio 0.33 (95% CI, 0.22 to 0.49) | 17 |
| Infection with or without symptoms | 14.0% | 21.5% | Risk ratio 0.66 (95% CI, 0.55 to 0.79) | 14 |
Adverse events and serious adverse events were similar between groups; no Covid-19–related hospitalizations or deaths occurred.
Limitations
Primary follow-up was short (10 days), so later infections may be missed. Many participants had prior immunity, and results may differ in less-immune groups. Household masking/ventilation behaviors were not measured. Pregnancy and some patients on interacting medicines were excluded.
Funding
Shionogi (sponsor involved in design and analysis).
Clinical Application
Consider ensitrelvir (where available) for rapid postexposure prevention in household contacts, especially higher-risk patients, when started within 72 hours of the index patient’s symptoms.
Discussion
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In this double-blind RCT of postexposure ensitrelvir in household contacts, symptomatic PCR-confirmed Covid-19 by day 10 fell from 9.0% to 2.9% (RR 0.33). What threats to applicability (timing <72h, highly immune cohort, household clustering, drug interactions) might limit practice change? Using the trial’s day-10 symptomatic Covid-19 risks (9.0% placebo vs 2.9% ensitrelvir), which is the closest number needed to treat (NNT) to prevent one case over 10 days?