Coronavirus: does cold weather have an impact on Covid-19?

As Covid-19 spread through Europe last spring, there were hopes that warm summer temperatures would see off the virus - and fears about what winter might bring.

But in fact, the relationship between the weather and infection rates appears to be more complex: chilly Norway and Finland fared well in February, and Spain’s second wave took hold in August.

So what should we expect as the weather turns?

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A cold-loving virus

The bad news is that the Sars-CoV-2 virus that causes Covid seems to be better adapted to wintry conditions than we are.

“All viruses survive outside the body better when it is cold,” says the BBC. “The UK’s Scientific Advisory Group for Emergencies (Sage) says a temperature of 4C is a particular sweet spot for coronavirus.”

According to the Centre of Evidence-Based Medicine (CEBM) at Oxford University, a 1C drop in temperature raises the R number - a measure of the spread of the epidemic - by about 0.04.

So if an outbreak was stable at 27C, with ten infectious people infecting ten others, lowering the temperature to 5C would mean those ten people would infect about 20 more, and the epidemic would grow exponentially.

The virus is also hindered by humidity and UV light - both in short supply on winter days.

“When the weather turns cold, air gets drier,” says the medical news site STAT. “Turning on the heat dries both the air and the tissues lining the airways, impairing how well mucus removes debris and invaders like Sars-CoV-2.”

The human factor

Our own responses to cold weather also makes life easier for the virus. “We gather indoors once the weather turns and beer gardens and BBQs are less appealing,” says the BBC. “We also slam the windows shut so there is little ventilation.”

The human immune system performs less well during the winter too, which may mean people who catch Covid-19 get more seriously ill.

During the spring, scientists recorded “a roughly 15% drop in mortality for every one degree Celsius rise in temperature”, the Covid Symptom Study reports. But as temperatures drop this autumn, the trend may be reversed.

However, says the CEBM, “weather alone cannot explain the variability” in the spread or seriousness of the virus. “Confounding” factors such as social distancing and other public health measures may be far more significant.