Legacy immunity from prior smallpox vaccination and serological evidence of asymptomatic mpox transmission in a West African population

The 2022 multi-country mpox outbreak, driven by MPXV Clade IIb has spread globally to over 100 countries, posing a new threat to global public health. Smallpox vaccination was discontinued in 1980 following global elimination with recent reports demonstrating evidence of asymptomatic infections particularly in individuals unlikely to have received a smallpox vaccine. To determine whether residual vaccination-derived immunity still shapes infection risk and to characterise population level burden of covert and asymptomatic transmission in a West African population, we combined serological analysis with phylogenetic analysis. We first quantified IgG binding to six MPXV antigens (A29L, A35R, B6R, D6L, H3L and M1R) in 176 Nigerian adults comprising of 75 healthcare workers sampled in 2021 and 101 community volunteers sampled in 2023 using a six-plex Luminex assay. Using a stringent ≥4-antigen reactivity threshold, at baseline, 24/176 (13.6%) participants met definition of MPXV seropositivity likely due to previous vaccination. Most participants with MPXV seropositivity were born before 1980, suggestive of residual smallpox immunity. Magnitude-breadth analysis scores were 2-fold higher in pre-1980 cohort relative to post-1980 cohort. In a subset of individuals (n=153) with follow-up samples over a median of 9 months, 5/153 (3%) showed evidence of silent asymptomatic exposure determined by antibody boosting and characterised by ≥2-fold increases in both total magnitude and breadth scores antibodies against ≥ 4/6 tested antigens all of which were MPXV seronegative individuals at baseline and mostly males. Participant level kinetics showed B6-specific titres boosted most strongly (median 11-folds), followed by M1R (6.2-folds) and A35R (5.2-folds) highlighting their potential utility as sero-surveillance markers. Complementary phylogenetic reconstruction of 105 Nigerian MPXV genomes showed two epidemic phases with over-dispersion (k≈0.3), indicating transmission sustained by sporadic superspreading despite high degree of dead-end infections. Serological and genomic evidence points to sustained asymptomatic transmission of MPXV in healthy individuals in the African setting, thereby informing public health interventions.