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

•Indoor relative humidity (RH) displays a seasonal pattern.•Outdoor RH remains high around the year in subtropical regions.•Absolute humidity has stronger an indoor-to-outdoor correlation than temperature and RH.•Indoor RH is a better indicator of influenza seasonality than outdoor RH. 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. 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 (Rt) of influenza and then used time-series analyses to explore the relationship between indoor/outdoor RH/absolute humidity and the Rt 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 Rt of influenza. 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 Rt 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 Rt. In addition, indoor RH may be a better indicator for Rt of influenza than indoor absolute humidity. The findings indicated that indoor RH may be the driver of influenza seasonality in both temperate and subtropical locations in China. [Display omitted]