In many parts of the world, wintertime tends to be when people get sick from viruses that cause colds and influenza.
The illness COVID-19 broke out during China’s winter and spread in Iran, Italy and the rest of Europe as those countries endured their coldest months.
As Australia heads into winter, what could this mean for the spread of the coronavirus that causes COVID-19?
Photo: EPA-EFE
Experts have said that cooler temperatures can affect the behavior and spread of a virus in three interconnected ways:
First, temperatures can affect the virus directly. Some viruses might have a temperature range where they can survive for longer and this is partly dependent on their “envelope” — the outer layer that is made of lipids (which is why washing hands with soap is effective, because it disrupts the lipids).
Second, when temperatures drop, this changes the behavior of the human population. People tend to spend more time indoors and in closer contact, making it easier for the virus to spread.
Third, we already know that like in other parts of the world, winter in Australia is when other viruses circulate and this can affect the immunity of populations.
Associate professor Tom Kotsimbos of Monash University, who is also a respiratory physician at the Alfred Hospital in Melbourne, said that seasonality can cause an increase in the spread of viruses.
“That is well-established, but whether we understand that deeply is another thing. It does have an influence, but we don’t understand it specifically. This is a new virus, and that does not mean it will be like the others,” Kotsimbos said.
“We know it’s about the biology of the virus, the general and specific immunity of us and the environmental and behavioral conditions,” he said.
However, there is much less certainty about how important each factor is when it comes if a person gets sick, he added.
“It’s interesting that [COVID-19] has spread all over the world quite quickly — both north and south,” he said, adding that this suggested either the new virus was not dependent on temperature or that this dependency was not as important as the lack of immunity in the population that was helping it to spread.
Does the coronavirus have a preferred temperature?
Some studies that have not yet been peer-reviewed have tried to find out if there is a clear link between temperature and the rate of infection of COVID-19, but have been inconclusive or contradictory.
“There is no clear evidence to suggest a relationship between outdoor temperatures and spread of coronaviruses,” said Meru Sheel of the National Centre for Epidemiology and Population Health at Australian National University.
“We don’t know enough about SARS-CoV-2 yet to know how change in weather will affect outbreak and the disease epidemiology,” Sheel said.
“In some tropical countries, such as Pacific island countries, despite the warm weather, there is some seasonal influenza activity and it is related to increase in travelers from countries such as Australia and New Zealand, and to a much lesser extent related to change in weather,” she added.
Do viruses do well in winter because we all head indoors?
Sheel said that rather than the temperature itself, the change in the way COVID-19 spreads “is likely influenced by frequency at which people interact during cooler versus warmer weather.”
Chris Burrell, emeritus professor of virology of the University of Adelaide, said that there were other known coronaviruses that had been around for a half-century and these were also prevalent in winter.
“But that’s because of people congregating together,” he said.
He said one example of this effect was with respiratory syncytial virus (RSV) that causes a common respiratory disease in young children that tends to arrive in winter.
“If you think of children in preschool, in the summer they’re out in the sun and don’t have a lot of personal contact with each other. In winter, they’re inside and breathing the same air,” Burrell said.
“I think that any effect of temperature on coronavirus won’t be nearly as important as the effect of limiting its spread by isolating people,” he added.
What about COVID-19 spreading during flu season?
Ten years ago, Kotsimbos led a task force of respiratory specialists that looked at what impact a potential early winter arrival of swine flu could have on Australia’s population.
“Clearly this is a different virus — but we have been here before ... but we don’t know how COVID-19 will behave,” he said.
Kirsty Short, a virologist at the University of Queensland, said: “My concern is that [in winter] people tend to be in closer contact and it coincides with our flu season.”
“My main concern is that when you think of the healthcare system, that does get pushed in a bad flu season. If we combine a moderate flu season with a moderate coronavirus season, that could be problematic,” Short said.
“As Australia moves into cooler weather, we are likely to see an increase in respiratory diseases, such as common colds, influenza and RSV infections,” Sheel said.
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