Colder and drier winter conditions are associated with greater SARS-CoV-2 transmission: a regional study of the first epidemic wave in north-west hemisphere countries.

Landier, J.; Paireau, J.; Rebaudet, S.; Legendre, E.; Le Hot, L.; Fontanet, A.; CAUCHEMEZ, S.; Gaudart, J.

Higher transmissibility of SARS-CoV-2 in cold and dry weather conditions has been hypothesized since the onset of the COVID-19 pandemic but the level of epidemiological evidence remains low. During the first wave of the pandemic for Spain, Italy, France, Portugal, Canada and USA presented an early spread, a heavy COVID-19 burden, and low initial public health response until lockdowns. We used regional death counts as a proxy for infections while diagnostic tests remained limited and calculated a basic reproduction number (R0) in 63 regions. After adjusting for population density, early spread of the epidemic, and aged population, temperature and humidity were negatively associated to SARS-CoV-2 transmissibility. A -1g/m3 lower mean absolute humidity was associated with a 0.15-unit higher R0. Below 10{degrees}C, a 1{degrees}C-higher temperature was associated with a 0.16-unit lower R0. Our results confirm a strong dependency of SARS-CoV-2 transmissibility to weather conditions in the absence of control measures. The initiation of the second wave in north-west hemisphere countries was likely triggered by the transition from summer- to winter-like conditions. High levels of restrictions to social activities should be maintained until spring to avoid or limit a third wave.