The antiviral drug tecovirimat is frequently used to treat mpox. But is it really effective, especially against the new, more pathogenic strain of the virus that has been in circulation since 2024? A study by the Institut Pasteur, recently published in The Lancet Infectious Diseases (1), concludes that it is.
In early April 2025, a case of mpox was reported in the United Kingdom – and the patient had not been to Africa or come into contact with other infected individuals. This gave weight to the theory that the virus is circulating at a low level in Europe, including in France, where 48 cases were reported in the first quarter of 2025.
Given the global outbreaks of mpox observed since 2022, it is crucial that we improve our knowledge of the virus and treatments. There are four known forms (clades) of the virus in circulation: 2a, 2b, 1a, and clade 1b, the most transmissible and pathogenic, which emerged in 2024.
No resistance mutations in subtype 1b
In the event of mpox infection, the most common therapeutic option is to administer the antiviral drug tecovirimat (TPOXX) as soon as the first symptoms occur, to block the spread of viral particles through the patient's body. But the efficacy of tecovirimat had not previously been examined in detail.
An in vitro study conducted by the Virus and Immunity Unit at the Institut Pasteur sheds light on the question. Olivier Schwartz, who leads the unit, explains: "In cells in culture, we analyzed the sensitivity of the four clades to the introduction of tecovirimat." The results showed that "the treatment was effective for all the subtypes: it suppresses viral multiplication, with similar levels of inhibition."
The scientists also analyzed 310 genome sequences of clade 1b to see whether this subtype had resistance mutations – since tecovirimat-resistant mpox variants all have mutations on the F13 enzyme (2). But they did not find any.
Clinical trials to determine conditions for optimal treatment efficacy
The findings suggest that tecovirimat remains a valid therapeutic option. But "this needs to be confirmed by clinical trials," notes Olivier Schwartz. "Clinical trials will also enable us to determine the conditions for optimal treatment efficacy in patients."
One such factor is the timing of administration. Recent clinical trials performed on patients in several African and South American countries, before the emergence of clade 1b, did not reveal any clinical benefit in taking tecovirimat. But in these cases the treatment was generally administered more than five days after the first clinical signs, which may well have been too late.
(1) Antiviral activity of tecovirimat against monkeypox virus clades 1a, 1b, 2a, and 2b. The Lancet Infectious Diseases, March 2025.
(2) Nature Microbiology, February 2025.
