Tpoxx doesn’t improve on placebo in achieving key mpox outcomes, phase 3 trial concludes
February 26, 2026
A rigorous trial published in The New England Journal of Medicine delivers a sobering conclusion: tecovirimat offers no measurable benefit over placebo for clade II mpox. The STOMP study found no significant differences in time to clinical resolution, pain reduction, or viral DNA clearance. For a drug widely deployed under emergency conditions and approved for smallpox under the Animal Rule, the findings underscore a critical lesson. Assumptions must yield to randomized evidence. The search for safe and effective mpox therapeutics must continue with scientific discipline and urgency.
The expectation that tecovirimat would meaningfully alter the clinical course of clade II mpox has now been tested and, decisively, tempered. In a phase 3 randomized controlled trial reported in The New England Journal of Medicine, investigators from the STOMP study found no significant difference between tecovirimat and placebo in achieving faster clinical resolution, reducing severe pain, or accelerating viral DNA clearance.
The trial enrolled more than 400 participants across multiple countries during the global mpox outbreak. At 29 days, clinical resolution rates were nearly identical between groups. Viral clearance by day 15 was similarly comparable. Adverse events were infrequent and balanced. Even among participants with severe pain or immunosuppression, outcomes did not demonstrate a therapeutic advantage.
Tecovirimat had been widely expected to confer benefit, particularly given its approval for smallpox under the FDA Animal Rule based on nonhuman primate data. In the urgency of a public health emergency, it became the primary antiviral option available. As Joseph Eron of University of North Carolina noted, only through randomized evidence could the field conclusively determine its lack of clinical efficacy for clade II mpox.
This finding does not invalidate the drug’s safety profile or its regulatory rationale in the context of smallpox preparedness. It does, however, clarify that extrapolation from animal models to human orthopoxvirus infection cannot substitute for controlled human trials. For immunocompromised patients, who face higher morbidity and case fatality risk, the absence of proven antiviral benefit is particularly consequential.
The STOMP trial reinforces a broader principle. Public health emergencies demand speed, but they also demand rigor. Rapid therapeutic deployment must be paired with randomized evaluation. The outcome may disappoint, but it advances knowledge. Mpox remains a global health issue. Effective therapeutics will require continued investment, transparent data, and the willingness to test even our most hopeful assumptions.