Short-term effects of meteorological factors on the incidence of mumps: A nationwide time-series analysis in Japan.

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Mumps (infectious parotitis) is an acute respiratory illness caused by the mumps virus in humans. While numerous studies have explored the impact of climate variability on mumps incidence in specific cities or regions, few have analyzed nationwide associations across multiple locations. This study aims to systematically assess the short-term effects of meteorological factors on mumps incidence across Japan over a 15-year period. Weekly time-series of mumps cases and meteorological factors in the period 2005-2019 were collated from the 47 Japanese prefectures. A two-stage time-series design was employed. The exposure-response relationships between incidence of mumps, mean temperature, and relative humidity were quantified using a quasi-Poisson regression with a distributed lag non-linear model for each prefecture and the estimates from all the prefectures were then pooled using a multivariate mete-regression model to derive nationwide average associations. Between 2005 and 2019, 1,455,583 mumps cases were reported. Our results indicate that increased mean temperature and relative humidity are positively associated with increased mumps incidence in Japan. Specifically, using -1.3°C as the reference temperature, the relative risk (RR) of mumps peaked at 19.4°C, with an RR of 1.16 (95% confidence interval [CI]: 1.08-1.24). For humidity, using 45.5% as the reference, the RR peaked at 68.3%, with an RR of 1.05 (95% CI: 1.02-1.09). Our findings demonstrate the influence of heat and humidity on mumps incidence in Japan and underscore the need for preventive measures to mitigate the impact of climate variability on mumps transmission.