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 a seasonal flu?
Three months later, the general decline of the epidemic observed in the Old Continent during the last days of spring has revived the hypothesis.
This is not a crazy idea for a Respiratory virus, reason why it has been studied and treated in multiple scientific publications.
“Many respiratory viruses are seasonal, such as influenza or RSV [virus respiratorio sincitial, responsable de la bronquiolitis en los recién nacidos]», explains the epidemiologist Antoine Flahaultwho runs the Institute of Global Health, University of Geneva.
Thus, the SARS-CoV-2 It could also be subject to the influence of the seasons: temperature, humidity, sun exposure or human behavior. But what arguments support this claim?
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»points out Flahault, while “Its activity was less in the temperate zones of the southern hemisphere”.
For a few weeks now, “We 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”, adds the expert.
Instead, «As the southern winter approaches, Argentina, Chile, southern Brazil, South Africa witness strong epidemic growth, reminiscent of Europeans from a few months ago», aim.
“It seems that there is a summer brake, but perhaps it is partial and does not necessarily prevent a circulation, perhaps moderate, throughout the summer in our hemisphere”add Antoine Flahault.
In France, the president of the scientific council COVID-19 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?
He “Scenario number one” expected for the summer is that of “A control of the epidemic” in France thanks “To the consequences of confinement” but also “The fact that this virus may be temperature sensitive”, he indicated on the radio station France Inter.
All in all, the seasonality of SARS-CoV-2 remains a difficult hypothesis to verify, qualifies the infectious disease specialist Pierre Tattevin.
Just as sun exposure and temperatures increased in Europe and France, “We confine ourselves to the maximum”, 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”, warns the expert from the CHU hospital in Rennes (western France).
A study from Princeton American University, published in the May magazine Science, concluded that humidity and temperature had a secondary effect on the spread of the virus, at least in the first moments of the pandemic.
“The virus will spread quickly, whatever the weather conditions”, predicted the study’s 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.”
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However, Antoine Flahault remember that the seasonality of viruses like the flu is not just a matter of temperature and humidity.
Sun exposure and season-related behaviors also influence (people spend more time outside in hot weather).
Furthermore, the flu never causes epidemics in summer in Europe, whereas in the inter-tropical areas there are them throughout the year.
With a coronavirus Seasonally, the Northern Hemisphere could enjoy a calmer summer, but in Autumn / 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 »conclude Flahault.