There is no evidence that the strain is spreading in California or the U.S., and the risk of contracting Clade I mpox in the U.S. remains low.

San Mateo County health officials have announced the nation’s first known case of a severe form of mpox that has fueled an outbreak of the disease across Eastern and Central Africa.

The first case in the U.S. of a more severe strain of mpox was reported in San Mateo County, the California Department of Public Health said.