On Nov. 16, 2024, the California Department of Public Health (CDPH) announced it had identified through laboratory testing the first known case of clade I mpox in the United States. This case was confirmed in an individual who recently traveled from Africa and is related to the ongoing outbreak of clade I mpox in Central and Eastern Africa.

Historically, clade I has caused more severe illness than clade II, however, recent infections from clade I mpox may not be as clinically severe as in previous outbreaks, especially when cases have access to quality medical care.

The affected individual received health care in San Mateo County based on their travel history and symptoms. The individual was isolating at home and recovering. People who had close contact with this individual are being contacted by public health workers, but there is no concern or evidence that mpox clade I is currently spreading between individuals in California or the United States.  The mpox specimens from the trave​ler were sent to the Centers for Disease Control and Prevention (CDC) for further laboratory testing.