Ensitrelvir Demonstrates Significant COVID-19 Post-Exposure Prophylaxis Efficacy in Phase 3 Trial

A landmark phase 3 study published in the New England Journal of Medicine has demonstrated that ensitrelvir (Xocova; Shionogi & Co.), an investigational oral SARS-CoV-2 main protease inhibitor, significantly reduces the risk of symptomatic COVID-19 when initiated within 72 hours of household exposure to an infected individual. Results from the SCORPIO-PEP (NCT05897541) trial, which is the first and only phase 3 study of an oral antiviral to meet the primary end point of preventing COVID-19 following exposure, signal a potential paradigm shift in how pharmacists and clinicians approach COVID-19 prevention for high-risk patients.^1,2,3

Study Design and Primary Findings

SCORPIO-PEP was a global, double-blind, randomized, placebo-controlled trial that enrolled 2387 participants aged 12 years and older who had been exposed to a household member with symptomatic COVID-19. To be eligible, participants had to test negative for SARS-CoV-2 at enrollment and be free of symptoms at the time of study entry. The primary analysis included 2041 household contacts confirmed negative by central laboratory testing.¹

Participants were randomly assigned 1:1 to receive a 5-day course of ensitrelvir (375 mg on day 1, followed by 125 mg once daily on days 2 to 5) or placebo, initiated within 72 hours of the index patient's symptom onset. Notably, more than 98% of household contacts tested positive for SARS-CoV-2 antibodies, reflecting near-universal prior exposure or vaccination.^1,2

In the primary analysis population, only 2.9% of participants who received ensitrelvir developed symptomatic COVID-19 through day 10, compared with 9.0% in the placebo group, representing a 67% reduction in relative risk (risk ratio: 0.33; 95% CI: 0.22–0.49; P < 0.0001). These results establish ensitrelvir as the first oral antiviral to demonstrate such efficacy as a post-exposure prophylactic agent for COVID-19.^1,2

Vulnerable Populations: A 76% Risk Reduction

Perhaps most consequential for pharmacy practice is a prespecified subgroup analysis examining participants with one or more risk factors for severe disease. Among this cohort, ensitrelvir reduced the relative risk of symptomatic COVID-19 by 76% compared with placebo (2.4% vs 9.9%; risk ratio: 0.24; 95% CI: 0.12–0.49).^1,2

These data carry meaningful implications for how pharmacists approach preventive care for elderly and immunocompromised patients.

"These data suggest we should move beyond simply waiting for infection to develop before treating high-risk patients. For older adults and immunocompromised persons, COVID-19 prevention after a close household exposure should be strongly considered as a complement to vaccination and non-pharmaceutical prevention efforts,” Paul E. Sax, MD, clinical director of the division of infectious diseases at Brigham and Women's Hospital, said in an interview with Pharmacy Times.

The need for an effective post-exposure tool is underscored by the evolving SARS-CoV-2 landscape. The heavily mutated Omicron subvariant BA.3.2 is now circulating in at least 25 US states, and COVID-19 surges remain difficult to predict. Additionally, while vaccines remain a cornerstone of prevention, there are currently no FDA-approved therapies specifically indicated to prevent COVID-19 following exposure, a gap ensitrelvir may soon fill, given its Prescription Drug User Fee Act (PDUFA) action date of June 16, 2026.^1,2

Safety and Tolerability Profile

Ensitrelvir was generally well tolerated in SCORPIO-PEP, with adverse event rates nearly identical between the active treatment and placebo groups (15.1% vs 15.5%, respectively). The most commonly reported treatment-emergent adverse events occurring in at least 1% of participants included headache, diarrhea, nasopharyngitis, cough, fatigue, and influenza. Importantly, there were no reports of altered taste attributed to ensitrelvir, which is a notable distinction from other oral antivirals used in COVID-19 management.¹

The 72-Hour Window: Triage Strategies for Pharmacists

A critical operational challenge for pharmacy teams will be identifying and acting on eligible patients within the 72-hour treatment window. Drawing on established post-exposure prophylaxis models from other infectious diseases, rapid pharmacist intervention is essential.

"Analogous to post-exposure prophylaxis for other infections, the sooner the treatment is initiated, the better. Upon hearing that a patient has had a household exposure to an active case, the next step would be expediting dispensing once drug interactions have been excluded,” Sax said.

This approach suggests pharmacists should establish standing workflows for rapid eligibility screening when patients report COVID-19 household exposure, particularly in community pharmacy settings, which serve as accessible front-line healthcare touchpoints. A streamlined triage process that prioritizes drug interaction screening and prompt dispensing could optimize outcomes within this narrow prophylactic window.^1,2

Counseling Considerations: Dosing Regimen and Adherence

From a pharmacy counseling standpoint, ensitrelvir's once-daily regimen offers a relatively straightforward treatment course.

"This is a relatively straightforward treatment course involving seven pills over five days, with once-daily dosing. Pharmacy counseling would still need to emphasize rapid initiation, completion of the full course, and screening for drug interactions,” Sax noted.

This positions ensitrelvir favorably in terms of adherence relative to more complex dosing schedules. However, the counseling emphasis must remain on the time-sensitive nature of the therapy: any delay in initiation risks compromising its prophylactic benefit.¹

These results represent a meaningful advance in the antiviral armamentarium, one that pharmacists will be well-positioned to deploy in clinical practice should FDA approval follow.

REFERENCES

1. New England Journal of Medicine publishes Shionogi study demonstrating ensitrelvir prevents COVID-19 following exposure. News release. Shionogi Inc. May 13, 2026. Accessed May 15, 2026. https://www.shionogi.com/us/en/news/2026/05/new-england-journal-of-medicine-publishes-shionogi-study-demonstrating-ensitrelvir-prevents-covid-19-following-exposure.html

2. Hayden FG, Shinkai M, Clark TW, et al. Ensitrelvir COVID-19 post-exposure prophylaxis in household contacts. N Engl J Med. 2026;383(19):1905-1915. doi: 10.1056/NEJMoa2509306

3. Phase 3 Study of S-217622 in Prevention of Symptomatic SARS-CoV-2 Infection (SCORPIO-PEP). Updated August 6, 2025. Accessed May 15, 2026. https://clinicaltrials.gov/study/NCT05897541