The COVID-19 is sensitive to the seasons?

La COVID-19 est-elle sensible aux saisons?

The idea had emerged from the first days of the epidemic COVID-19-in Europe : and if the virus SARS-CoV-2 disappeared with the arrival of beautiful days, like a common flu (influenza)?

Three months later, the fall in general observed on the old continent at the end of the spring revives the hypothesis of a “seasonality” to 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 has accompanied the pandemic wave.

“Many respiratory viruses are seasonal, like the flu or RSV (respiratory syncytial Virus, responsible for bronchiolitis in infants, editor’s NOTE),” notes the epidemiologist Antoine Flahault.

The SARS-CoV-2 could, therefore, be subject to the influence of the seasons : temperatures, humidity, sun or human behaviour. What are the arguments in support of such an assertion?

“A recession is pronounced”

First of all, it is “winter” and “mainland China” that this virus emerged in late 2019. Then “it has led to large epidemics in the temperate zones of the northern hemisphere between January and may,” lists the professor Flahault, so that his “activity was lower in the temperate zones of the southern hemisphere”.

Since a few weeks, “we know a little bit everywhere a recession is pronounced, except in some regions of the northern hemisphere such as Sweden, Poland and some States of the United States,” continues the expert, who is director of the Institute of Global Health from the University of Geneva.

Conversely, “at the approach of winter in the southern hemisphere, Argentina, Chile, southern Brazil, South Africa are aware of the growing epidemic strong, and remind us of ours a few months ago”, he adds.

“The impression is, in total, that there is a brake in the summer, but it may be partial, and will not necessarily prevent a movement, may be moderate throughout the summer in our hemisphere”, according to Antoine Flahault.

The chairman of the scientific Council COVID-19, which advises the French government on the epidemic, Jean-François Delfraissy, also evokes this hypothesis.

“Scenario number one” waited for the summer is that it is “a control of the epidemic” in the country, “the consequences of containment”, but also “the fact that this virus may be temperature-sensitive”, has he said on radio France Inter.

The seasonality of the SARS-CoV-2 remains a hypothesis difficult to verify, says infectious diseases physician Pierre Tattevin. At a time when temperatures and insolation increased in France and in Europe, “it was confined to the bottom”, he says. Thus, it is difficult to distinguish the influence of seasonal effect of the confinement on the current slowing of the epidemic.

“There are so many parameters that come into account, that we may not know what is linked to the climate, which is linked to the season or the fact that people do care,” emphasizes the practitioner at the CHU of Rennes (west).

“Regardless of the weather conditions”

A study by researchers from the american university of Princeton, published in may in the journal Science, concluded that it was a secondary effect of temperature and moisture on the spread of the virus, at least in the early days of the pandemic.

“The virus spreads quickly, regardless of the weather conditions”, anticipated by the first author of the study, Rachel Baker. Because it is a factor far more predominant for the circulation of SARS-CoV-2 : the current low herd immunity of the population.

Antoine Flahault reminded, however, that the seasonality of the virus such as influenza is not limited to the temperature and humidity. Also take into account the sunshine (destructive role of ultraviolet radiation on the envelope of the virus) and the behaviors related to the seasons (more time spent outside when the weather is nice).

The flu does not cause, moreover, never of the epidemic in the summer in Europe, but in the product throughout the year in tropical zones.

A coronavirus seasonal would hope for a been more peaceful in the northern hemisphere, but entrouvrirait the door to an autumn/winter most threatening with a “high risk of resurgence” for the return of the cold.

“It is a hypothesis that holds up if one accepts the idea of a seasonal component. Pandemics of influenza have all had a second wave, still winter in the temperate zones of the northern hemisphere,” said professor Flahault.

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