The idea had emerged from the first days of the COVID-19 epidemic in Europe: what if the SARS-CoV-2 virus disappeared with the arrival of fine weather, like a common seasonal flu?
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Three months later, the general decline observed on the old continent at the end of spring revives the hypothesis of a “seasonality” for the new coronavirus.
The idea is not far-fetched for a “respiratory virus” and has even been the subject of several studies among the avalanche of scientific publications which accompanied the pandemic wave.
“Many respiratory viruses are seasonal, such as that of the flu or RSV (Respiratory syncytial virus, responsible for bronchiolitis in infants, note),” observes epidemiologist Antoine Flahault.
SARS-CoV-2 could therefore also be subject to the influence of the seasons: temperature, humidity, sunshine or human behavior. What arguments support this assertion?
“A pronounced decline”
First, it is “in winter” and “in mainland China” that this virus emerged at the end of 2019. Then “it caused strong epidemics in the temperate zones of the northern hemisphere between January and May” lists Professor Flahault, while “its activity was less in the temperate zones of the southern hemisphere”.
For the past few weeks, “we have been experiencing a pronounced decline almost everywhere, except in certain regions of the northern hemisphere such as Sweden, Poland and certain states of the United States,” continues the expert who heads the Institute of Global Health. from the University of Geneva.
Conversely, “with the approach of the southern winter, Argentina, Chile, southern Brazil and South Africa are experiencing strong epidemic growth which reminds us of ours a few months ago”, he adds.
“The impression is, in total, that there is a summer brake, but it may be partial and will not necessarily succeed in preventing a circulation, perhaps moderate, throughout the summer in our hemisphere”, according to Antoine Flahault.
The president of the Scientific Council COVID-19 who advises the French government on the epidemic, Jean-François Delfraissy, also evokes this hypothesis.
The “number one scenario” expected for the summer is that of “controlling the epidemic” in the country, thanks to “the consequences of confinement”, but also “the fact that this virus may be sensitive to temperature, ”he said on France Inter radio.
The seasonality of SARS-CoV-2 remains a difficult hypothesis to verify, tempers the infectiologist Pierre Tattevin. At a time when temperatures and sunshine were increasing in France and in Europe, “we were completely confined”, he underlines. It is therefore difficult to distinguish the seasonal influence of the effect of containment on the current slowdown of the epidemic.
“There are so many parameters that come into play, that one cannot know what is linked to the climate, what is linked to the season or to the fact that people are paying attention”, underlines this CHU practitioner from Rennes (west).
“Whatever the weather conditions”
A study by researchers from the American University of Princeton, published in May in the journal Science, concluded that a side effect of temperature and humidity on the spread of the virus, at least in the early days of the pandemic.
“The virus will spread quickly, whatever the weather,” predicted the study’s first author, Rachel Baker. Because it is a much more preponderant factor for the circulation of SARS-CoV-2: the current weak collective immunity of the population.
Antoine Flahault recalls, however, that the seasonality of viruses like those of the flu is not limited to temperature and humidity. Sunshine (destructive role of ultraviolet rays on the envelope of viruses) and behavior linked to the seasons (more time spent outside when the weather is nice) are also taken into account.
The flu never causes an epidemic in summer in Europe, but produces it throughout the year in the intertropical zones.
A seasonal coronavirus would augur a more peaceful summer in the northern hemisphere, but would open the door to a more threatening autumn / winter with a “high risk of resurgence” when the cold returns.
“It’s a hypothesis that holds water if we accept the idea of a seasonal component. The influenza pandemics have all had a second wave, always winter in the temperate zones of the northern hemisphere, “notes Professor Flahault.