On Oct. 10, 2024, health officials from the U.S. Centers for Disease Control and Prevention announced they had identified a cluster of cases caused by an mpox variant that are resistant to Siga Technologies’ (SIGA.O), antiviral tecovirimat, branded as TPOXX, in five U.S. states.

The drug-resistant mpox cases followed an initial, unrelated cluster reported in California in late 2022-early 2023, and were the first involving multiple states. The cases involved a total of 18 individuals infected with clade II mpox between Oct. 6, 2023, and Feb. 15, 2024, who had never taken the treatment before, according to the report published in the CDC’s Morbidity and Mortality Weekly Report.