It is well known that influenza is seasonal, peaking in the winter in temperate regions and during the wet season in tropical locations. So does Covid-19 follow a similar seasonal pattern?
To answer this question scientists compared coronavirus rates between March 2020 and March 2021 from five countries – Canada, Germany, India, Ethiopia and Chile – with daily mean temperature, humidity, ultraviolet radiation and air-drying capacity (a measure of how quickly droplets evaporate).
In countries with temperate climates they found Covid-19 peaked during the winter months, when temperature and humidity were low. Meanwhile, in countries with tropical climates the cases peaked during the summer monsoons, when humidity was high.
The results, published in the journal GeoHealth, reveal that there were two variables – ultraviolet levels and air-drying capacity – that consistently correlated with Covid-19 levels in all countries. Bright sunshine and fast evaporation were associated with falling rates of coronavirus, while cloudy skies and slow evaporation appear to aid the spread of the virus.
Anticipating the seasonality of Covid-19 will be important in combating its spread, ensuring booster vaccines are given before the peak season takes-off, for example. But seasonality is only one factor, we shouldn’t drop our guard just because it is sunny and dry.
