Allisandra G. Kummer, Juanjuan Zhang, Maria Litvinova, Alessandro Vespignani, Hongjie Yu, Marco Ajelli

Considerable uncertainties surround the seasonality of respiratory infectious diseases. To which extent the observed seasonality is associated with biological reasons (e.g., virus survival rates, host immune dynamics) or human behavior remains unclear. Here, we investigate the association between temperature and human contact patterns using data collected through a contact diary-based survey between December 24, 2017 and May 30, 2018 in Shanghai, China. We found a significant inverse relationship between number of contacts and temperature seasonal trend (p=0.003) and temperature daily variation (p=0.009), with contacts increasing from 19.6 (95%CI: 14.9-22.2) in December to 24.4 (95%CI: 19.0-28.0) in January and declining to 10.9 (95%CI: 10.1-11.9) in May. This seasonal trend in number of contacts translates into a seasonal trend in the basic reproduction number – mean number of secondary cases generated by a typical infector in a fully susceptible population. By setting the basic reproduction number at 1.4 on December 24, weekly mean estimates showed a clear increasing trend during the fall, beginning at 1.14 (95%CI: 0.78-1.39) in October and reaching 2.02 (95%CI: 1.60-1.35) in February and then remaining below 1 in the summer. Epidemic dynamics comparable with those of seasonal influenza are obtained through model simulations when the infection is seeded during the fall; however, their dynamics become more complex when seeded after February (e.g., double peaks or no epidemic until after the summer). Our findings indicate a distinct seasonal trend among human contact patterns and highlight a behavioral mechanism contributing to the seasonality of respiratory infectious diseases.

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