APOBEC3-Driven Mutations Attenuated 2022 Mpox Clade IIB Strain, Cutting Virulence While Aiding Silent Spread

The team compared the 2022 B.1 isolate with A.1 in cell culture and murine models, measuring dissemination, cytokine profiles, and lethality. B.1 produced elevated interferon-beta, weaker suppression of inflammatory signals, and reduced cell-to-cell spread, resulting in lower overall mortality. Genetic analysis attributed the 46 changes to human APOBEC3 editing rather than polymerase error, indicating the virus had circulated long enough in people for host enzymes to accumulate attenuating mutations.

These findings explain why the 2022 outbreak produced milder disease than historical Clade I cases yet achieved rapid international transmission. Patients remained ambulatory, facilitating undetected exportation. The result contrasts with earlier reports that treated all Clade II viruses as uniformly mild and underscores that even small nucleotide shifts in a large orthopoxvirus genome can alter fitness and immune evasion.

Ongoing surveillance of Clade I viruses in Africa, which lack these APOBEC3 signatures, remains essential. Vaccine supply constraints and the possibility of further adaptation mean the current attenuation is not guaranteed to persist. Next studies must track whether new lineages regain virulence or acquire transmissibility advantages under immune pressure.