Association between meteorological factors and varicella incidence in Japan: a nationwide time-series study.

Varicella, commonly referred to as chickenpox, is an airborne infectious disease that continues to pose an increasing threat to public health. Despite previous investigations, the global accumulation of epidemiological studies exploring the association between varicella epidemics and meteorological factors remains insufficient. This study aimed to quantify the short-term effect of meteorological factors, such as mean temperature and relative humidity, on the incidence of varicella across all 47 prefectures of Japan over a decade (2010-2019). Using a two-stage time-series modelling design, we first applied distributed lag non-linear models to estimate the exposure-response relationships for each prefecture, followed by a meta-regression to pool these results for a nationwide estimate. The analysis covered 1,315,616 varicella cases. Our findings indicate a significant increase in varicella risk associated with lower temperatures and reduced humidity. Specifically, the cumulative relative risk (RR) at the 5th temperature percentile, compared to the 99th, was 2.45 (95% confidence interval [CI]: 2.15, 2.79). The cumulative RR at the 5th humidity percentile, compared to the 90th, was 1.11 (95% CI: 1.03, 1.09). These results suggest that public health strategies targeting temperature- and humidity-related varicella morbidity could be more effective if adapted to local climatic conditions. Understanding the influence of meteorological factors on varicella transmission may further elucidate the mechanisms driving the disease's seasonal trends.