A more virulent strain of mpox is expanding its footprint in the United States. As of late May 2026, 23 cases of clade I mpox have been confirmed in the U.S. — a jump from fewer than 15 cases reported earlier in the spring — all diagnosed in individuals who had recently traveled to areas of Central or Eastern Africa with known outbreaks, or who were linked to people with such travel histories. The CDC has explicitly stated that it expects additional cases as summer international travel peaks and the World Cup brings tens of millions of visitors through American airports and host cities.
The distinction between clade I and clade II matters. The 2022 global outbreak that spread to over 100 countries — including the United States, where it caused approximately 32,000 cases — was caused by clade IIb, a version of mpox with a case fatality rate under 0.1 percent in high-income countries. Clade I historically caused mortality rates between 1 and 10 percent in Central African outbreaks, where it has been endemic for decades, though more recent data suggest the current clade Ib strain may cause less severe illness outside endemic settings.
