It’s hot outside. Does warm weather mean you’re less likely to catch Covid?

Meeting outdoors helps minimise infection risks, but heat has a negligible effect on the virus

Summer Covid risk: ‘Transmission outside is minimal,’ says Prof John Edmunds. Photograph: Nick Bradshaw

Summer Covid risk: ‘Transmission outside is minimal,’ says Prof John Edmunds. Photograph: Nick Bradshaw

 

With the recent rising temperatures and more people now mixing outdoors as restrictions have eased, experts explain how much of an effect the weather has on Covid-19.

Does the hot weather have an impact on Covid?

Studies show that the season, and the temperature, do have an impact on the spread of coronavirus but that impact is not significant.

Prof John Edmunds, a member of the UK’s Scientific Advisory Group for Emergencies, or Sage, says the evidence shows that warmer temperatures affect transmission but that the impact is small, as most transmission occurs indoors.

“Transmission outside is minimal,” says Edmunds, an epidemiologist at the London School of Hygiene & Tropical Medicine. “Most transmission happens in enclosed settings. If it’s sunny outside viruses will die quicker, but they wouldn’t have lasted very long in any case.”

A recent study of US data led by Imperial College London found that higher temperatures were associated with lower transmission rates, if no other measures, such as lockdowns and mask wearing, were in place. The study found that a 20 degree difference – typical between summer and winter – would probably see a change in the reproduction number, or R, of 0.8.

“From an epidemiological perspective this change can be significant,” says one of the study’s authors, Dr Ilaria Dorigatti. But in the “presence of interventions climatic factors play a negligible role.”

So why do other viruses such as the flu flourish in winter?

Because even a small increase in a relatively low R number can make a difference, according to Edmunds.

“Vaccination or [natural] immunity will bring the reproduction number right down... So a small change can allow the reproduction number to go slightly above one, or slightly below one. In the winter if it is slightly above one you get winter epidemics, and if in summer it is slightly below one, less so.”

Do ultraviolet rays kill coronavirus?

Yes, but as most transmission of Covid happens indoors, again the difference is slight, says Edmunds. “UV does kill viruses. So higher UV is bad for the virus, but that only really matters outside. And given that almost no transmission occurs outside anyway, it doesn’t make a lot of difference.”

Dorigatti says that the Imperial study focuses on temperature and does not specifically look at UV but that other studies, by the Yale professor Yiqun Ma and others, have found that higher UV rates could have a role to play in reducing transmission rates.

Does the type of heat impact transmissibility?

One study by scientists in China, yet to be peer reviewed, suggested mortality rates were lower on days when humidity and temperatures were higher. Dorigatti says other studies by Mohammad M Sajadi and Yiqun Ma had found a role for humidity, with lower humidity associated with higher transmission rates, implying that humid heat may reduce transmission more than dry heat.

Can we worry less about Covid in warmer weather?

In short, no. Dr Will Pearse, another researcher on the Imperial College London study, says the critical message is that the weather is only a small factor in the spread of Covid.

“I would really strongly encourage people to not think, Okay, it’s warm weather outside, so I don’t need to worry about Covid,” he says. “We’ve seen what the situation looks like in places warmer than the UK and in places colder than the UK. Warm weather is no substitute for mitigation.”

Kathleen O’Reilly, an epidemiologist with the London School of Hygiene & Tropical Medicine, says: “Hopefully, most people will be sensible and pragmatic; meeting outdoors will minimise Covid-19 infection risk, but staying out of direct sunlight, especially at midday, is still important. Meeting indoors but ensuring good ventilation will be good to minimise infection risk and also to keep cool.” – Guardian

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