Does summer slow spread of Covid-19?

AT FACE value, it seems 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, hospital admissions 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 can boost our immune systems. There’s also evidence that the ultraviolet (UV) radiation in sunlight reduces how long the virus can survive on surfaces. It’s also possible that humidity and temperature may influence transmission. When combined, the factors will probably have an effect on the virus’s spread.

But how significant is the effect? And what are the implications for controlling Covid-19 as warmer months approach, as well as for the potential of another winter resurgence? As research had produced inconclusive results on whether and how the seasons affect Sars-CoV-2, my colleagues and I set out to see if we could find some more conclusive answers to these questions.

Epidemiologists use something called the reproductive number, or R, to describe the growth of an epidemic – the higher the R number, the faster the spread. At the start of an epidemic, its growth won’t be affected by anyone having been exposed to the disease and developing immunity and so, it will spread exponentially. At this point the R number that describes this spread is referred to as R0.

Using data from outbreaks across the world, our new research determined R0 for Covid-19 in 359 large cities. Each city included in our study had more than 500 000 inhabitants and had experienced a significant Covid-19 outbreak last year.

As UV radiation levels are higher in summer, our findings suggest there is some seasonal effect on transmission. However, it’s important to note that the correlation doesn’t necessarily mean that UV radiation is the cause of the decrease in transmission, since UV radiation may correlate with other causal factors.

For example, the higher the UV radiation in a city, the hotter it tends to be. We didn’t find a separate statistically significant link between R0 and temperature or humidity on a global level, but we can’t rule out such relationships. Association between viral spread and temperature or humidity may have been masked by many other factors that affect R0, as well as the strong correlation between UV radiation and temperature. There’s some weak evidence of an association between viral spread and temperature in other studies.

But predicting this behaviour for such a complex system as the world is difficult, and as we move out of the initial epidemic phase, the longer-term behaviour of Covid-19 transmission will probably depend on many other factors.

These will include the level and duration of immunity acquired by infected individuals, as well as the efficacy and length of protection provided by current and future vaccines, and the evolution of new variants of the virus.