An antiviral drug that was used to fight the 2022-23 global mpox outbreak does not work against a more severe variant of the virus that is now spreading rapidly in Africa, according to researchers. 

The drug, tecovirimat, was not shown to reduce the duration of lesions in children and adults with clade I mpox, a more dangerous strain of the disease. The data are the results of a placebo test by researchers from the Democratic Republic of the Congo and the United States, reported Politico.

"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, the director of the U.S. National Institute of Allergy and Infectious Diseases (NIAID), commented.

The clade II strain of mpox, formerly known as monkeypox, was the version that caused the 2022 global outbreak. The new variant, however, is being associated with more severe illness and higher death rates.

Clade I is also affecting more children, which was not seen in the previous outbreak, leading the World Health Organization to declare an mpox global emergency Wednesday. 

Despite the findings about tecovirimat, the NIAID-backed study showed that deaths from mpox can be avoided if patients are treated in hospitals. The researchers studied 597 mpox patients who were hospitalized for 14 days, determining that their mortality rate was 1.7%, compared to 3.6% in all of the DRC, even among patients who received a placebo.

SIGA, the drug's manufacturer, pointed out that those who were treated early, or were the sickest, showed "meaningful improvement" and said the results support holding more trials in those groups. 

Tecovirimat has been approved in the EU to treat mpox and smallpox, but that was based only on animal studies.

The authorization had been granted under "exceptional circumstances" without requiring clinical efficacy data.

The United States made the drug available in 2022 to some patients while studying its effectiveness, and the European Commission bought supplies of it during the last outbreak. Some U.K. authorities also recommended it as an optional use for patients who were hospitalized.

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