Objective To delineate the genetic and environmental determinants of COVID-19 spreading. Design Retrospective case series. Setting Spain, Italy, Sweden, Finland, Norway. Participants All laboratory-confirmed infection cases (n=168,089) collected from February 21st to April 14th 2020. Main outcome measures Infection spreading velocity according to viral mutation load and to climate region. Results The mean doubling time of COVID-19 was 6.63 days in northern Italy, 5.87 days in central areas, and 5.38 days in southern Italy, with shorter COVID-19 doubling time in warmer regions. Spain extended this trend, with a mean COVID-19 doubling time of 4.2 days. At the other end of the spectrum, slower diffusion across progressively colder regions was observed in Scandinavia, with 9.4 days COVID-19 doubling time in Sweden, 10.8 days in Finland and 12.95 days in Norway. Mutations and mutation rates of SARS-CoV-2 versus COVID-19 spreading were analyzed worldwide. Models of increased aggressiveness of SARS-CoV-2 upon progressive acquisition of genetic changes were not supported by regional mutation data. Conclusion Current propagation models suggest dependence of COVID-19 pandemic spreading on wintertime conditions, with expected waning over the summer. Our findings indicate association of COVID-19 to a sharp North/South climate gradient, with faster spreading in southern regions. Thus, warmer climate conditions may not limit SARS-CoV-2 diffusion. Very cold regions may be better spared by recurrent courses of infection.
Genetic drift and environmental spreading dynamics of COVID-19