Mpox clade Ib

Mpox clade Ib is a lineage within clade I of the monkeypox virus, distinct from the clade IIb strain that drove the global 2022 outbreak. As of 13 June 2026, European health authorities confirmed local transmission of Mpox clade Ib among men who have sex with men with no African travel history, the first confirmed local clade I transmission on the European continent. In the United States, all 23 confirmed clade I cases as of the same date remained travel-linked, though CDC projected numbers would rise with World Cup travel.

Clade I (previously called the Congo Basin clade) has historically been associated with higher clinical severity and higher case-fatality rates than clade II, which caused the 2022 global outbreak. Clade Ib specifically refers to a lineage within clade I; this is distinct from a separately tracked recombinant strain combining clade Ib and IIb genetic material, which was identified circulating across four countries in May 2026. The European cases confirmed in June are clade Ib, not the recombinant strain. European public health agencies including ECDC and UKHSA are monitoring whether Jynneos vaccine stocks, validated against earlier mpox strains, provide sufficient protection against clade Ib.

The significance of local European clade Ib transmission is that the lineage is now establishing itself in a new geography. All prior clade I mpox had been geographically confined to central and eastern Africa. The World Cup mass-movement context elevates the practical concern: thousands of people travelling between clade-Ib-affected regions and the rest of the world over the coming weeks creates a larger opportunity window for Onward seeding in new populations.