Suppression of COVID-19 death incidence on open west coasts in the USA.

The drivers behind the large-scale patterns of COVID-19 infections are largely unknown. Earlier studies have shown a connection between continentality, a measure for oceanic air influence over land, where lowest continentality implies highest oceanic influence, and COVID-19. In Europe, open west coasts with lowest continentality had the lowest COVID-19 incidence. We test if this applies to the US. We use a combination of geographical information systems and statistics, and data for every US county, to assess the connection between the COVID-19 death incidence and continentality. We normalize for known factors that influence COVID-19 local scale death incidence, namely the socio-economic status, population aged over 65, and the index of urbanization (crowding). We find that open west-coasts in the US, where continentality index values are low, had the lowest COVID-19 death incidence, rising non-linearly with rising continentality values, with highest death incidence in areas with the highest continentality, in north-central USA. The influence of oceanic air was associated with lower COVID-19 death incidence on the west coast of the US. These findings suggest that oceanic influence may be an important environmental determinant of spatial variations in COVID-19 death incidence and provide a contribution to studies on the relationship between oceans and health.