Notes from the Field: Mpox Cluster Caused by Tecovirimat-Resistant Monkeypox Virus - Five States, October 2023-February 2024.

The antiviral drug tecovirimat* has been used extensively to treat U.S. mpox cases since the start of a global outbreak in 2022. Mutations in the mpox viral protein target (F13 or VP37) that occur during treatment can result in resistance to tecovirimat† (1,2). CDC and public health partners have conducted genetic surveillance of monkeypox virus (MPXV) for F13 mutations through sequencing and monitoring of public databases. MPXV F13 mutations associated with resistance have been reported since 2022, typically among severely immunocompromised mpox patients who required prolonged courses of tecovirimat (3-5). A majority of patients with infections caused by MPXV with resistant mutations had a history of tecovirimat treatment; however, spread of tecovirimat-resistant MPXV was reported in California during late 2022 to early 2023 among persons with no previous tecovirimat treatment (3). This report describes a second, unrelated cluster of tecovirimat-resistant MPXV among 18 persons with no previous history of tecovirimat treatment in multiple states.