Towards a seasonal virus?

“Plan now the necessary doses for next fall” : February 2, Emmanuel Macron suggests, with a sentence on TF1, that it will be necessary to deal with the virus for a long time. For a year, studies have been multiplying on the seasonal nature or not of Covid-19, with results that remain to be consolidated, if only because they lack the hindsight of a cycle of complete seasons in the two hemispheres .

Among them, the one led by Predict, a subsidiary of Météo France, sought to measure the impact of the climate on the transmission of the virus for the year 2020, by creating an index that models the capacity of the droplets emitted when we breathe or cough to stay in suspension and increase the circulation of the virus.

This capacity is favored by temperatures between – 3 and 13 ° C, even more in the range of 2 to 6 ° C, and when the humidity level is high, between 60% and 90%. These results would explain in particular why the slaughterhouses, cold and humid, were more affected by the virus. On the other hand, the droplets are less dangerous when it is very cold, since they then fall quickly to the ground: the current temperatures, particularly low, could “Bring a lull, especially in the East”, underlines Alix Roumagnac, president of Predict.

As for the resumption of the epidemic in October 2020, it could be explained by the passage of storm Alex, which blew cold and humid air over France and led to a significant increase in the index. Hence the increase, ten days later, in hospitalizations and deaths in regions with a high index.

“The index is an additional element of explanation, to be crossed with human behavior”, however specifies Alix Roumagnac, who recalls that the non-respect of barrier gestures can generate outbreaks of infection in hotter areas, and therefore a priori less favorable to the transmission of the virus. The World Health Organization recalls, moreover, in its column “Ending conventional wisdom”, that “Exposure to the sun or to temperatures above 25 degrees does not prevent contracting the disease” and “You can contract Covid-19 in any climate, even in hot or sunny weather.”

“It remains difficult to provide firm and definitive proof of the role of summer as such”, tempers Arielle Rosenberg, virologist at Cochin hospital, “But it at least allows the population to live more easily outside”, which mitigates the risks associated with insufficiently ventilated interiors in winter.

Predict, who made his data available to health authorities, indicates “Not to have received any denial of his results”. And works with the Faculty of Medicine of Paris on the development of an application of “Covid weather forecast”, which would make it possible to imagine different public health recommendations, depending on the region, or even depending on the time of day. Whose curfew, “Which contributed to the stabilization of the number of cases”According to Alix Roumagnac.In a study from the University of Nicosia published on February 2, researchers Talib Dbouk and Dimitris Drikakis also pleaded for better consideration of the climate parameter. They are campaigning for a new index, that of the rate of airborne infections (AIR, Airborne Infection Rate). From the analysis of the transmission of the virus in New York, Paris and Rio de Janeiro at the time of the second epidemic, the results suggest that “Two waves of epidemics per year are inevitable because they are directly linked to meteorological seasonality”. With, for Paris, a wave from this month of February 2021.

“Alongside seasonal parameters, the global vaccination campaign is changing the configuration”, adds Romulus Breban, researcher in the epidemiology of emerging diseases unit at the Institut Pasteur. According to him, if we manage to vaccinate a majority of adults by the end of the summer, collective immunity “ could be sufficient “ before the fall and the return of conditions favorable to the virus.


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