Intensive surveillance of Zika virus infection conducted on Yap Island has provided crucial information on the epidemiological characteristics of the virus, but the rate of infection and medical attendance stratified by age and geographical location of the epidemic have yet to be fully clarified. In the present study, we reanalyzed surveillance data reported in a previous study. Likelihood-based Bayesian inference was used to gauge the age and geographically dependent force of infection and age-dependent reporting rate, with unobservable variables imputed by the data augmentation method. The inferred age-dependent component of the force of infection was suggested to be up to 3-4 times higher among older adults than among children. The age-dependent reporting rate ranged from 0.7% (5-9 years old) to 3.3% (50-54 years old). The proportion of serologically confirmed cases among total probable or confirmed cases was estimated to be 44.9%. The cumulative incidence of infection varied by municipality: Median values were over 80% in multiple locations (Gagil, Tomil, and Weloy), but relatively low values (below 50%) were derived in other locations. However, the possibility of a comparably high incidence of infection was not excluded even in municipalities with the lowest estimates. The results suggested a high degree of heterogeneity in the Yap epidemic. The force of infection and reporting rate were higher among older age groups, and this discrepancy implied that the demographic patterns were remarkably different between all infected and medically attended individuals. A higher reporting rate may have reflected more severe clinical presentation among adults. The symptomatic ratio in dengue cases is known to correlate with age, and our findings presumably indicate a similar tendency in Zika virus disease.
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