Influenza, gastroenteritis, bronchiolitis, chickenpox… Viral infections often present some form of seasonality. Due to the infectious nature of the SARS-CoV-2 responsible for Covid-19, many were counting on the summer to see it disappear. “We were indeed expecting the establishment of a seasonality which never happened”, explains Professor Vincent Maréchal, virologist at the Saint-Antoine Research Center (Inserm / Sorbonne University). “We are now in the seventh wave. And over time, there hasn’t really been a break. The last waves almost fit into each other. »
Thus, from the beginning of the summer of 2020, the virus returned. Ditto in 2021. And 2022 is already marked by a massive upsurge in the number of cases “which, given the levels of circulation in wastewater, seems greater than during the second wave of September – November 2020” , continues Vincent Maréchal.
A cocktail of factors
For this specialist, the factors that determine the seasonality of respiratory viral infections and their impact on health are not yet well understood. According to him, “a combination of several elements can explain the persistence of Covid. »
“The virus is transmitted by air and this transmissibility is influenced by temperature and humidity of the air”, he recalls. “Thus, SARS-CoV-2 prefers dry and cold conditions, between 5 and 15°C. Winter is therefore more favorable there, because we live more indoors, we ventilate less… But that does not mean that traffic stops in summer. »
Another factor is atmospheric pollution. It seems that there is a “correlation between the severity of the cases and the levels of pollution, such as the levels of fine particles, the levels of ozone and nitrogen dioxide, the health impact of which is more visible l ‘summer. »
Third explanatory factor, “changes in population behavior. Which move more in summer, promoting the circulation of the virus. Summer is also a time of conviviality, where social contacts, especially among young people, are frequent. Without forgetting that when it is hot, barrier gestures fall, like wearing a mask, which is more difficult to bear when the temperatures are high. »
And immunity?
There remains the question of immunity. Which is provided either by vaccination or after contamination (this is natural immunity). But this does not protect 100% from infection and does not prevent transmission. Because more than the rhythm of the seasons, it is that of the variants which seems to dictate its law.
The recent arrival of BA.4 and BA.5, which are particularly contagious and capable of escaping the immune response, is reviving the epidemic. “They overcome the seasonal barrier and circulate more easily. »
“Populational” immunity is struggling to establish itself, as is possible seasonality.