Prolonged low-level inter-human mpox transmission in Nigeria prior to the global outbreak in 2022: a mathematical modelling study

Background The 2022 global mpox outbreak was linked to clade IIb viruses that previously circulated in Nigeria. Genomic evidence suggests that, in Nigeria, sustained human-to-human transmission may have begun as early as 2016. Methods Using a stochastic branching process model, we investigated the conditions under which mpox could have persisted at low levels in Nigeria between 2016 and April 2022 without triggering a clearly recognized outbreak and quantified the potential hidden burden of infection and international exportation. Results We found that when R is close to 1 and secondary infections show low variability (low heterogeneity, i.e. no superspreading), sustained but slow transmission can persist with non-negligible probability. Under our baseline modelling assumptions (R = 1.0, Poisson offspring distribution), 9% of simulations resulted in cryptic epidemics that lasted throughout the study period. Conditional on a trajectory being classified as cryptic under this scenario, the average epidemic size was 8,500 infections (95% prediction interval: 2,900–17,000) per cryptic epidemic, compared with 241 confirmed cases reported. This implies substantial under ascertainment with a detection rate of 2.8% (95% prediction interval: 1.4–8.3%). Estimated exportations to Europe and North America were consistent with the small number of documented travel-associated cases before 2022. Conclusions These findings support the plausibility of prolonged, low-level mpox transmission in Nigeria prior to the recognized global outbreak.