More than two years after the mpox outbreak in the U.S. was declared over, a new crop of cases in California has infectious disease experts on edge.

The mpox outbreak that spread through men who have sex with men was declared over at the start of 2023, though low-level transmission has persisted since then.

A collection of three unrelated mpox cases recently detected in California is raising concerns, as they were caused by a more infectious, more dangerous strain of the virus called clade I mpox.

The strain that affected the U.S. during the 2022 outbreak was from clade II, which has a relatively low mortality rate. Of the more than 32,000 cases that were recorded during the public health emergency for the 2022 U.S. outbreak, 58 deaths were confirmed, marking a mortality rate of 0.2 percent.

While both strains are present in Central Africa, clade I mpox is more likely to cause severe illness and historically has a higher mortality rate and higher transmissibility.

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