How will the flu coexist with the Covid during the winter? –

The fight against the coronavirus will have an influence on the flu epidemic. This influenza season could be delayed, or even absent, according to the example of the countries of the southern hemisphere. A consolation in this tense context, where a double infection would be an aggravating factor.

When the coronavirus pandemic took off in March, the flu season had just started in the southern hemisphere. Countries like Australia and South Africa, however, did not see the usual wave of influenza arrive and influenza cases remained at a historically low rate during the months of July and August, in the heart of the southern winter.

Experts expect a similar effect in Switzerland. The precautionary measures taken against Covid-19, such as wearing a mask, washing hands and sanitary distances, make it possible to reduce droplet transmission, the vector of the virus responsible for influenza.

“If we manage to control the incidence of Covid-19, this means that at the same time, we will be able to control the incidence of the flu”, says Didier Trono, virologist and expert of the national Covid task force -19, Tuesday in the 7:30 p.m. Note that influenza causes between 400 and 1000 deaths each year in Switzerland.

Competition between viruses

Scientists are also wondering about the interactions between the two viruses. A preliminary study carried out in England between January and the end of April 2020 suggests that an influenza infection would decrease the risk of infection with Covid-19.

Based on tests carried out on nearly 20,000 individuals for influenza and SARS-CoV-2, researchers say the risk of testing positive for Covid-19 drops by 58% when people have been tested positive for influenza.

A study to be taken with a grain of salt, as it has yet to be peer reviewed. But it nevertheless refers to a phenomenon well known in virology, competition between viruses, which is driven by molecules called interferons.

“When a virus enters a cell, it causes it to make substances which are released and which prevent other viruses from entering all the neighboring cells. These interferons therefore establish a kind of firewall which makes the organism is nonspecifically protected against other viral infections, ”explains Didier Trono.

Interferons can stay in the body for days or even weeks. Early results show that the flu vaccine could also trigger this non-specific immunity against the coronavirus. But knowledge is still limited on this subject.

Risk of double infection

As for the possibility of infection at the same time by the two viruses, it remains rare, all the more so if the influenza virus is not very present. But the phenomenon is possible and leads to an aggravating factor. “Two recently published studies suggest that the death rate is doubled when there are two infections”, notes Blaise Genton, Chief physician of the Infectious Diseases Department of Unisanté in Lausanne.

More than ever, the doctor recommends that the population be vaccinated. “The problem is that flu patients have the same symptoms as those of Covid. These people will want to be tested and this will create a greater influx of patients. The sectors are already tight, it does not they shouldn’t be overwhelmed. “

Due to strong demand this year, vaccine doses are being distributed as a priority to vulnerable people and health personnel. The rest of the population will be able to be vaccinated from the beginning of December. “In general, influenza epidemics occur between mid-December and mid-March. Knowing that it takes about two weeks to build up immunity, that leaves a reasonable time,” concludes Blaise Genton

Aurélie Coulon and Feriel Mestiri

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