Since the first days of the COVID-19 epidemic in Europe, the idea of whether the virus would disappear with the arrival of summer has not stopped coming up.
Will the new coronavirus act like the seasonal flu?
It is not a crazy idea for a “respiratory virus”, so it has been studied and discussed in multiple scientific publications.
“Many respiratory viruses are seasonal, such as influenza or RSV [respiratory syncytial virus responsible for bronchiolitis in newborns],” explains epidemiologist Antoine Flahault, who heads the Institute of Global Health at the University of Geneva.
Thus, SARS-CoV-2 could also be subject to the influence of the seasons: temperature, humidity, exposure to the sun, or human behavior.
First, the virus emerged “in winter” in “Mainland China” in late 2019. Then, “it caused strong epidemics in the temperate zones of the northern hemisphere between January and May,” Flahault notes, while “its activity was lower in temperate zones of the southern hemisphere ».
For a few weeks, “we have registered a pronounced decline almost everywhere, except in some regions of the northern hemisphere such as Sweden, Poland and some States of the United States,” added the expert.
Instead, “as the southern winter approaches, Argentina, Chile, southern Brazil, South Africa are witnessing strong epidemic growth, reminiscent of Europeans a few months ago, ” he points out.
“It gives the impression that there is a summer break, but it may be partial and it may not necessarily prevent circulation, perhaps moderate, throughout the summer in our hemisphere,” adds Antoine Flahault.
In France, the president of the COVID-19 scientific council that advises the government on the epidemic, Jean-François Delfraissy, has also hinted at this hypothesis.
How does the coronavirus change depending on the season of the year?
The “number one scenario” expected for the summer is “control of the epidemic” in France, thanks “to the consequences of confinement” but also “to the fact that this virus may be sensitive to temperature,” he said in France Inter radio station.
However, the seasonality of SARS-CoV-2 remains a difficult hypothesis to verify, says infectious disease specialist Pierre Tastevin.
Just as sun exposure and temperatures increased in Europe and France, “we confine ourselves to the maximum,” he stresses.
Likewise, it is difficult to differentiate the influence that the change of season had and the effect of confinement on the current slowdown of the epidemic.
“There are so many parameters that come into play that we cannot know what is linked to the weather, what is linked to the season or to the fact that people pay more attention,” says the expert at the CHU hospital in Rennes (western France).
A study from American Princeton University, published in the journal Science in May, concluded that humidity and temperature had a side effect on the spread of the virus, at least in the early stages of the pandemic.
“The virus will spread quickly, whatever the weather conditions, ” said study lead author Rachel Baker. “Well, there is another much more important factor that facilitates the circulation of SARS-CoV-2: the weak collective immunity of the population.”
However, Antoine Flahault recalls that the seasonality of viruses like the flu is not limited to temperature and humidity alone.
Sun exposure and season-related behaviors also influence (people spend more time outside in hot weather).
Furthermore, the flu never causes epidemics in the summer in Europe, whereas in the inter-tropical areas there are them throughout the year.
With a seasonal coronavirus, the northern hemisphere could enjoy a calmer summer, but in fall/winter there would be a “high risk of regrowth. ”
“It is a hypothesis that holds if we accept the idea of a seasonal component. All influenza pandemics have a second wave, always wintry in the temperate zones of the northern hemisphere, ” concludes Flahault.