Winter in a pandemic: How cold air could make the coronavirus spread more easily

The summer months are officially behind us, and the dip in temperature has been met with a spike in coronavirus cases across Canada.

Some parts of Canada are already experiencing a second wave, Prime Minister Justin Trudeau said on Wednesday. But will the coronavirus grow even more rampant during the winter months, just as other respiratory illnesses do?

“As the weather cools down, this means the coronavirus may actually stay in the air longer,” said Colin Furness, an infection control epidemiologist and assistant professor at the University of Toronto.

As the virus leaves your body, the cold air helps preserve it and keep it alive longer, Furness explained.

“That’s one of the reasons we have the flu season in the colder months.”

Respiratory infections, such as coronaviruses, are spread by droplets that are released when a person coughs or sneezes, according to the Public Health Agency of Canada.

Timothy Sly, an epidemiologist with Ryerson University’s school of public health, explained that we don’t exhale the virus particles. The virus is inside droplets of various sizes.

When the air is dry, which is typical during the winter months, the moisture in the droplets evaporates, “possibly within seconds,” leaving the nucleus of the droplet — containing the virus — to float in the air, he said.

This means the droplets are smaller and lighter than they would be with humid weather, meaning they could spread farther.

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Another reason the spread of the coronavirus could be worse in cold temperature is that our mucous membrane dries out in cold weather, making it much harder for the nose to filter pathogens, Furness said. When airways dry out, it allows the virus to have easier access to the body.

“All of these contribute to ideal conditions for respiratory viruses in the winter,” Furness said.

But this does not mean it will happen with COVID-19 specifically, Sly warned.

Because the Northern Hemisphere hasn’t been through a full winter with the coronavirus pandemic yet, it is really “speculation” at the moment, he said.

But he added it’s still important that people practise safe distancing and wearing masks, even when outside during the winter months.

“It’s still the best way to prevent infection,” Sly said.

Of course, there are other reasons for viruses to spread during the winter months.

“Winter is coming,” Catherine Smallwood, senior emergency officer at WHO Europe, said last month, according to CNBC.

“People are travelling more, they are going back to work, schools are reopening — these are all factors that are going to increase the risk of community transmission and further transmission.

“As we approach the flu season and the winter months, there are additional factors that will conflate and add even more to that level of risk,” she said, saying more people are likely to congregate indoors and in more crowded settings.

Studies look into COVID-19 and cold temperature

It’s still not completely known if COVID-19 can spread more easily in cold air, but studies have started looking into it.

A study published in August in the journal Transboundary and Emerging Diseases found that the number of COVID-19 cases in Sydney, Australia, increased as the air became drier and the humidity level dropped.

“There is growing evidence that climatic factors could influence the evolution of the current COVID‐19 pandemic,” the authors stated. “Overall, a decrease in relative humidity of one per cent was associated with an increase in cases of seven to eight per cent.”

Temperature and relative humidity can affect coronavirus transmission as the virus can survive longer at lower temperatures, the authors said. The virus can also stay “suspended in the air” longer at lower humidity, they claimed.

Another study published in August in the Science Direct journal examined the link between daily average temperature and relative humidity and daily counts of COVID-19 cases in 30 Chinese provinces.

It found that every one-degree increase in the average temperature led to a decrease in the daily confirmed coronavirus cases by 36 per cent to 57 per cent when the humidity level was in the range of 67 per cent to 85.5 per cent.

“Environmental factors such as temperature and relative humidity may influence the transmissions of coronavirus by affecting the survival of the virus in its transmission routes,” the authors stated.

However, the authors noted there were limitations to the study — the study period was only around a month-long and the rate of COVID-19 cases decreasing could have also been influenced by government health interventions.

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