DECRYPTION – According to Public Health France (SPF), the epidemic is “the longest” since at least 2010. The week of March 20 to 26 was the eighteenth week of the flu this year.
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Louis Valleau
Posted update
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It’s a epidemic that lasts. In its latest epidemiological bulletin published on March 29, Public Health France (SPF) notes a “constant circulation” from flu In France. The flu epidemic was in its eighteenth week in week 12 (that running from March 20 to 26, 2023). According to SPF, the average duration of flu epidemics from 2010 to 2012 is 11 weeks. This year it is also regardingthe longest» flu epidemic for at least twelve years.
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In France, eight regions remain in the epidemic phase, as many as last week but three less than two weeks ago. “We are certainly at start of decline of the epidemic», analyzes Bruno Lina, virologist at the Hospices Civils de Lyon and member of the committee for monitoring and anticipating health risks (Covars).
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In one week the incidence rate of consultations with the doctor for flu-like illness per 100,000 inhabitants increased by 8%, once morest an 11% increase the previous week. The increase was 40% two weeks ago. The flu epidemic therefore slows down but still persists. So how do you explain such longevity?
Two strains of the virus
From the sixteenth week of the epidemic – that going from March 6 to 12 – Public Health France described “longest flusince at least 2010. A longevity to which we must now add two weeks. “We can look for duration records», believes Mircea Sofonea, epidemiologist and lecturer in the PCCEI unit of the University of Montpellier, for whom the level of the epidemic peak – now largely passed – “nothing exceptional». It was lower than in 2017-2018 and almost half that in 2018-2019.
This season, the start of the flu epidemic was “earliest in recent yearswith an incidence exceeding 100 ILI doctor visits as early as week 46 (November 14-20, 2022), three weeks earlier than in 2017, the second-earliest start in the last five years. For 2022-2023, the start of the epidemic was carried by an influenza A strain, a certain “variant” of the flu. “Strain B would have taken precedence over A between weeks 4 and 6 (end of January mid-February 2023, Ed), analyzes Mircea Sofonea. They are not the same viruses and there is no cross-immunity between the two». Clearly, an infection with influenza A does not protect once morest reinfection with influenza B.
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According to virologist Bruno Lina, “the succession of a strain of influenza A then another of influenza B is not unusual and this is each time responsible for a longer epidemic». «There was little influenza B last year, so immunity this year is probably lower, especially among younger people», he completes. However, according to him,impossible to really know why strain B appeared following strain A This year”.
Another hypothesis put forward, the effect of travel restrictions during the health crisis linked to the Covid-19. «During the confinements, in 2020 then in 2021, the flu circulated little in France. But around the world, especially in the tropics, the flu has never gone away.he assures. During this period, only the most resistant strains of influenza survived. The epidemic may therefore have started once more with the most successful type of virus.».
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The vaccine in question?
However, the vaccines administered this season protect many influenza sub-lineages having mainly circulated – H3N2 for influenza A and Victoria for influenza B. Each year, the composition of the vaccine once morest influenza evolves according to the recommendations of the World Health Organization (WHO) and which strains are most likely to circulate.
The lower vaccination coverage than last year may also have had a slight effect. In its latest weekly epidemiological bulletin, SPF estimates – that as of December 31, 2022 – the vaccination coverage among people “at risk of severe flu” was 1.4 points lower than that estimated for the 2021-2022 season.
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More “the viruses currently in circulation are indeed present in the vaccine», insists Bruno Lina for whom this is probably not the main explanation. “The use of barrier gestures in the general population has returned to its pre-Covid level», he completes. This promotes the circulation of a virus, which affects between two and six million people in France each year, according to Health Insurance.