Weather, mobility and the evolution of the Covid-19 pandemic

We estimate the effective reproduction number (Rt) of the current Covid-19 pandemic, with US daily infections data between February and September of 2020, at the county level. This is then used to estimate the effect of weather and mobility on the spread of the pandemic. We find a strong and significant effect of the weather: lower temperaturas are associated with a higher Rt, and this effect is bigger at temperatures below 0ºC. At low temperatures, precipitations are also associated with a higher Rt. We also find that mobility reductions related to certain types of locations (retail and recreation, transit stations, and workplaces) are effective at reducing Rt, but it is an increase of the time spent in parks that helps reduce the spread of the pandemic. The negative effect of increased general mobility is bigger in counties with higher population density, worse numeracy and literacy PIAAC scores, or a lower share of employment in the services sector. Quantitatively, our estimates imply that a 20ºC fall in temperatures from summer to winter would increase Rt by +0.35, which can be the difference between a wellcontrolled evolution and explosive behavior; and, if this can’t be neutralized through general improvements in the fight to stop the pandemic, the additional reduction in mobility that would be needed to compensate for this would be equivalent to returning, from the more relaxed levels observed in the summer, back to the strictest mobility reductions recorded in the US in April.