ACTG Announces Publication of STOMP Results in the New England Journal of Medicine

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Study findings do not support routine use of tecovirimat in people with mpox

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CHAPEL HILL, N.C., Feb. 25, 2026 (GLOBE NEWSWIRE) — ACTG, a global clinical trials network focused on HIV and other infectious diseases, today announced the publication of “Tecovirimat for the Treatment of Mpox (STOMP)” in the New England Journal of Medicine (NEJM). The publication reports on results from STOMP (Study of Tecovirimat for Mpox, also known as A5418) that demonstrated that tecovirimat did not improve mpox resolution. STOMP stopped enrolling participants in December 2024 after an interim analysis showed that the treatment did not reduce the time to lesion resolution or have an effect on pain among outpatient adults with clade II mpox.

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“This publication reminds us of the importance of randomized clinical trials – tecovirimat was widely expected by the scientific community to be an effective treatment for mpox and it was only through STOMP that we were able to conclusively demonstrate that it is not,” said ACTG Chair Joseph J. Eron, M.D., University of North Carolina. “Given that mpox continues to be a global health issue, these findings highlight the urgent need to conduct randomized controlled trials to identify safe and effective therapeutics for mpox and other orthopoxviruses.”

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STOMP was a phase 3, randomized, placebo-controlled, double-blind trial evaluating the safety and efficacy of tecovirimat for the treatment of mpox. The U.S. Food and Drug Administration (FDA) has approved tecovirimat (SIGA Technologies, Inc.) to treat smallpox, based on its activity in animal models (since smallpox has been eradicated), but prior to STOMP it was not known if it could effectively or safely treat mpox in humans. ACTG launched STOMP in September 2022 in response to a global outbreak of mpox that was characterized by increased person-to-person transmission. STOMP enrolled participants who had symptoms of mpox for less than 14 days and randomized them to receive either tecovirimat (600 mg) or a placebo twice daily for 14 days. Participants with or at risk for severe disease, pregnant women, and children were enrolled in an open-label arm in which everyone received tecovirimat.

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STOMP randomized 412 eligible participants to tecovirimat (275) or placebo (137) at 49 sites in seven countries (United States, Peru, Mexico, Thailand, Argentina, Japan, and Brazil). A total of 344 participants had laboratory-confirmed mpox and 336 had at least one active skin or mucosal lesion and were included in the primary analysis. Those participating in STOMP broadly represented the population of people who were most affected by mpox during the 2022-2023 outbreak; the median age was 34 years, 99 percent were male, 52 percent were White, 13 percent were Black, 44 percent were Hispanic, and 23 percent had received at least one dose of an mpox vaccine. Of 337 participants whose HIV status was known, 35 percent were living with HIV.

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By day 29, 79 percent of participants in the tecovirimat group had experienced clinical resolution of their symptoms (defined as the first day all skin lesions were scabbed or healed and visible mucosal lesions were healed), compared to 81 percent in the placebo group. There were no differences in pain reduction among those with severe pain, complete lesion healing, or clearance of viral DNA. Adverse events were similar between arms.