At face value, it seems highly plausible that SARS-CoV-2 – the virus that causes COVID-19 – could behave seasonally, being more prevalent in winter and less so in summer. The four other coronaviruses that commonly circulate in humans behave in this way. We’ve also seen COVID-19 cases, hospitalisations and deaths spike over winter in the UK and other countries, which is suggestive of a seasonal effect.
Some association between viral transmission and the seasons is to be expected. Many human behaviours are seasonal. In summer, we spend more time outdoors, where risk of infection is much lower, and we are likely to lead more active lifestyles, which can raise the body’s ability to resist infection. We’re also likely to benefit from increased exposure to sunlight, which raises vitamin D levels and so can boost our immune systems.