Will warmer weather halt the march of coronavirus?

Speculation is growing over whether the coronavirus outbreak could be tempered by the warmer weather of the summer months after scientists said it has hallmarks of a seasonal virus.

The data for infection rates supports a climactic dimension to Covid-19. “Look at the World Health Organization (WHO) map of the coronavirus outbreak, and it is difficult to avoid spotting a pattern,” writes Tom Whipple, science editor of The Times. 

“This seems to be a disease of the temperate north. If you live somewhere that sees a frost, then coronavirus has found a way to infect you. If you don’t, generally it hasn’t.”

So, will the coronavirus disappear as the warmer weather of spring and summer reaches the UK?

Experts are resisting the temptation to offer a definite answer but there is some evidence to help planners address this question.

Harvard scientists who studied two previous coronaviruses, HCoV-OC43 and HCoV-HKU1, which were relatively benign compared to the current SARS-CoV-2, found that they were indeed seasonal.

“This is not surprising,” says Whipple. “Unlike parasites and bacteria that like warm and wet conditions, viruses generally prefer the cold. They need dry air behind shut windows to spread reliably. Like flu, it seemed that these viruses liked the cold.”

However, these two viruses appeared to only offer nine months of immunity once caught, meaning that annual outbreaks were possible.

A separate study by the Centre for Infectious Diseases at the University of Edinburgh, found that three types of coronavirus showed “marked winter seasonality”.

Mohammad Sajadi, an associate professor at the University of Maryland’s Institute of Virology, who has been studying the current outbreak, told the Financial Times: “Based on what we have documented so far, it appears that the virus has a harder time spreading between people in warmer climates.”

The FT notes that there are dissenting scientific voices. Marc Lipsitch, a Harvard University epidemiology professor, says there might be “modest declines” in the contagiousness of the new coronavirus in warmer, wetter weather but it was “not reasonable to expect these declines alone to slow transmission enough to make a big dent”.

In another study of Covid-19, researchers from Beihang University in China analysed the “reproduction number”, which tells how many new infections result from each previous one. Governments want to bring this number down below 1 because if each person infects fewer than one person, then the epidemic is defeated.

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In Britain, the reproduction number is currently between 2.5 and 3.5. The researchers found that for each degree change in average temperature, the number fell by 0.04. In Britain, this would mean it might fall to somewhere between 1.9 and 2.9 over the summer – far from the target figure of 1.

Neil Ferguson, the Imperial College professor whose modelling has shaped the government’s response to the outbreak, said: “transmissibility of the virus will be somewhat reduced in the summer, but not by more than 10-20 per cent. Our best guess is that you could still get a large epidemic in the summer”.

Also, any benefit of warmer weather is dependent on a sustained period of such conditions. Over the past decade, British summers have varied from the cool and wet to savage heatwaves. So there is yet to be any firm evidence that the summer months will be enough to save us from the pandemic.