The National Institutes of Health said Thursday that an antiviral often used to treat mpox did not resolve patients’ symptoms faster than placebo in a randomized trial.
The results are notable because the drug, tecovirimat, has rarely been studied clinically for mpox, despite its wide use during the 2022 and 2023 outbreaks in the U.S. and Europe.
And the results came a day after the World Health Organization once again declared mpox a public health emergency of international concern, after case numbers swelled in the Democratic Republic of Congo and multiple other African countries. Sweden on Thursday reported an infection in someone who had traveled to an affected region, the first reported case outside Africa in the current outbreak.
Tecovirimat, marketed as TPOXX, was approved by the Food and Drug Administration in 2018 for smallpox, a related virus that’s been eradicated but remains a bioterrorism concern. Doctors hoped it would also be effective for mpox, because of similarities between the two pathogens. But data were largely limited to animal studies and case reports.
In the NIH co-sponsored study, patients in the Democratic Republic of the Congo, where mpox is endemic, were randomized to receive either tecovirimat or placebo. All patients had to remain in the hospital for treatment. Both groups saw their symptoms resolve on a similar time scale, although the NIH noted mortality was 1.7% across both groups — below the 3.6% typically reported in the DRC — suggesting the broader, high-quality supportive care they received in the hospital can save lives, even if the drug itself didn’t have an impact.
“These findings are disappointing, but they give us essential information and reinforce the need to identify other therapeutic candidates for mpox while we continue research on tecovirimat use in other populations with mpox,” Jeanne Marrazzo, director of the National Institute of Allergy and Infectious Diseases, said in a statement.
There are two clades of mpox. The study focused on clade I, which is endemic in the DRC and is considered to cause more severe disease. The 2022 global mpox outbreak was driven by a variant of clade II, known as clade IIb. Both clades have contributed to the new global health emergency and can cause painful scarring rashes, along with fever, headaches, muscle aches and respiratory symptoms.
The 2022 outbreak spread largely via sexual contact. The WHO has said a new variant of clade I, called clade Ib, has also spread via sexual networks, although the virus can pass through other forms of contact.
Siga Technologies, the company that sells tecovirimat, said in a statement that two subsets of patients saw “meaningful improvement”: Those who enrolled in the study within seven days of symptom onset and those with severe disease.
The company did not share specific data or say whether those results were statistically significant. The NIH did not highlight either subset in its announcement, saying only that further analyses were planned. Siga’s stock was down 34% on Thursday morning.
Because smallpox is eradicated, tecovirimat was approved for that virus based on animal studies alone. In those studies, animals who were treated within four or five days showed the best results, with survival dropping to 50% for animals treated on day 6, suggesting early intervention can be essential.
Two trials are ongoing to study tecovirimat in clade II mpox.
Helen Branswell contributed reporting.