ArticleJournal of epidemiology and global health2025
Association between Short-Term Exposure to Humidex and Influenza Incidence in Japan: A Nationwide Time-Stratified Case-Crossover Study.
Article in Journal of epidemiology and global health, 2025. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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Abstract
backgroundSeasonal influenza is a highly contagious acute respiratory infection that imposes a considerable global health burden. Although numerous investigations have examined the short-term effects of ambient temperature or humidity on influenza incidence, relatively few have addressed their combined effects. To bridge this gap, the present study sought to conduct a time-series analysis to examine the combined effects of temperature and relative humidity on influenza incidence in Japan, while accounting for potential confounders.
methodsWeekly time-series data on influenza incidence and meteorological factors (i.e., mean temperature (°C), relative humidity (%), precipitation (mm), wind speed (m/s) and sunshine duration (hours)) were collected from all 47 prefectures in Japan from 2010 to 2019. A composite exposure metric, humidex, was calculated to capture the combined effects of temperature and relative humidity. We employed an extended two-stage time-series design. In the first stage, a time-stratified case-crossover design was implemented using conditional quasi-Poisson regression integrated with a distributed lag non-linear model to characterize the humidex-influenza associations at the prefectural level. In the second stage, a multivariate meta-analysis was conducted to derive pooled national estimates.
resultsA total of 14,526,346 influenza cases were analyzed. Overall, an inverted J-shaped association between short-term exposure to humidex and influenza incidence was observed. Decreases in humidex were associated with an increased risk of influenza, with the maximum relative risk (RR) reaching 10.75 (95% confidence interval [CI]: 8.35-3.82). The elevated risk corresponding to low humidex (5th percentile) became apparent at week 0, persisted until week 4, and peaked at approximately week 1 (RR = 3.31, 95% CI: 2.89-4.31). Furthermore, significant geographical heterogeneity in influenza incidence was detected across prefectures (Q = 343.4, p-value < 0.001; I² = 46.4%).
conclusionsOur findings demonstrates that the combined effects of ambient temperature and relative humidity are significantly associated with an elevated risk of influenza in Japan. These findings underscore the urgent need for bespoke public health interventions tailored to mitigate adverse health outcomes in regions characterized by persistently low humidex values.
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