Prolonged low-level inter-human mpox transmission in Nigeria prior to the global outbreak in 2022: a mathematical modelling study
Paolo Bosetti, Dimie Ogoina, Antoine Gessain, Vittoria Colizza, Simon CauchemezAbstract
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.