Indoor relative humidity shapes influenza seasonality in temperate and subtropical climates in China

Objectives: The aim of this study was to explore whether indoor or outdoor relative humidity (RH) modulates the influenza epidemic transmission in temperate and subtropical climates. Methods: In this study, the daily temperature and RH in 1558 households from March 2017 to January 2019 in five cities across both temperate and subtropical regions in China were collected. City-level outdoor temperature and RH from 2013 to 2019 were collected from the weather stations. We first estimated the effective reproduction number ( R t ) of influenza and then used time-series analyses to explore the relationship between indoor/outdoor RH/absolute humidity and the R t of influenza. Furthermore, we expanded the measured 1-year indoor temperature and the RH data into 5 years and used the same method to examine the relationship between indoor/outdoor RH and the R t of influenza. Results: Indoor RH displayed a seasonal pattern, with highs during the summer months and lows during the winter months, whereas outdoor RH fluctuated with no consistent pattern in subtropical regions. The R t of influenza followed a U-shaped relationship with indoor RH in both temperate and subtropical regions, whereas a U-shaped relationship was not observed between outdoor RH and R t . In addition, indoor RH may be a better indicator for R t of influenza than indoor absolute humidity. Conclusion: The findings indicated that indoor RH may be the driver of influenza seasonality in both temperate and subtropical locations in China.& COPY; 2022 The Author(s). Published by Elsevier Ltd on behalf of International Society for Infectious Diseases. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

This study aimed to investigate the impact of indoor and outdoor relative humidity (RH) on influenza transmission in temperate and subtropical climates. The findings showed a U-shaped relationship between indoor RH and the effective reproduction number (Rt) of influenza in both regions, while no correlation was observed between outdoor RH and Rt. These results suggest that indoor RH may play a key role in the seasonality of influenza in temperate and subtropical locations in China.