Objective: The aim of this study was to explore whether indoor or outdoor relative humidity (RH) modulates the influenza epidemics transmission in temperate and subtropical climates.

Methods: In this study, 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 was 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 (AH) and Rt of influenza. Furthermore, we expanded the measured one-year indoor temperature and RH data into five years, and used same method to examine the relationship between indoor/outdoor RH and Rt of influenza.

Results: Indoor RH displayed a seasonal pattern with highs during the summer months and lows during the winter months, while outdoor RH fluctuated with no consistent pattern in subtropical regions. Rt of influenza followed a U-shaped relationship with indoor RH in both temperate and subtropical regions, while 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 AH.

Conclusion: The findings indicated that indoor RH may be the driver of influenza seasonality in both temperate and subtropical locations in China.