Flu epidemic: why is it so severe this year ?
To deal with the flu epidemic, 87 hospitals had launched the white plan on Friday, January 10. This system allows for the relocation of certain non-urgent operations and the recall of staff on leave. The epidemic is currently affecting all age groups and seems particularly severe, especially among the most vulnerable.
Is the 2024-2025 flu virus more virulent than that of previous seasons? ? Can we already say that this epidemic will be particularly deadly? ? Answers with Sibylle Bernard-Stoecklin, epidemiologist and seasonal flu surveillance coordinator at Santé publique France.
In its bulletin of Wednesday, January 8, Santé publique France mentions an “exceptionally high” level of flu-related hospitalizations in the first week of January. Is the flu more severe than in previous seasons? ?
We are still at the beginning of the epidemic, only the 5th week (usually the flu lasts 10 to 12 weeks, editor's note). It has exceptionally high levels, levels that have never been observed in the past. And this exceptional nature is true in all age groups. However, it is more significant in those under 5 and those aged 65 and over, which is normal because it is at the extremes of life that we are most at risk of developing severe forms of flu. The disease also has a very strong impact on nursing homes and establishments that accommodate senior citizens.
Another sign is that electronic certificates that mention flu as the cause of death have increased sharply over the last two weeks and reached a value of 6% in week 1. This is already a higher value than those reached at the peaks of the last three seasons.
How can we explain this exceptional nature ?
The flu is very unpredictable and presents different specificities from one epidemic to another, variations that are often difficult to explain. We can imagine that this tension on the hospital, concomitant with the school holidays, is partly linked to the holidays that doctors take during this period and to the fact that patients go directly to the emergency room rather than to the general practitioner.
Furthermore, the intensification of the circulation of the flu occurred during the school holidays. As we know, the end-of-year holidays are a good time for family gatherings, especially with grandparents. People most at risk of developing severe forms of the flu are very exposed during this period. And children are really the driving forces behind the epidemic, which began just before the holidays.
Another explanation. Currently, we are seeing the circulation of three influenza viruses, H1N1, H3N2 and B-Victoria. All influenza viruses can infect a person regardless of their age and are capable of causing a serious form. But on a population scale, the impact of each virus is different depending on the age group. Thus, the B-Victoria virus is well described to have an impact rather on children. The H1N1 virus is capable of causing large epidemics, particularly among young adults. And the H3N2 virus is well described to be particularly severe in the elderly. And there, the three viruses are circulating, which may explain why all age groups are particularly affected at the moment.
200% Deposit Bonus up to €3,000 180% First Deposit Bonus up to $20,000Do we know if we have reached the epidemic peak? ?
For the moment, the peak has not been reached. But school holidays are known to slow the circulation of the virus within the population. Since children are the drivers of transmission, this may be enough to break the epidemic. But the circulation was so intense during these holidays that it could lead to a rebound at the start of the school year. In addition, we are currently seeing that H3N2 is circulating more than at the beginning of the season. If it continues to intensify, it could accentuate the impact of the epidemic in the coming weeks. We are monitoring it closely.
“This epidemic is expected to be severe. It is already severe in the hospital and will probably have an effect on mortality.”
Flu is responsible for an average of 9,000 to 10,000 deaths each season. Will we be above that for this 2024-2025 episode? ?
It is once again too early to tell. In general, we observe the following lag: first we have the effects of the flu epidemic in the city, then a lag of one to two weeks for hospitalizations and there is another lag of one to two weeks for mortality. We are in the fifth week and for the moment, we only have the figures for all-cause mortality for week 52, for which we have not yet observed excess mortality. This data will be consolidated over time.
The electronic certificates I was talking about earlier give a trend and are a snapshot at time T; they do not allow us to say that there will be excess mortality linked to the flu. But these certificates show, in week 1, a higher level of mortality linked to the flu than at the peaks of the two previous seasons. This is what allows us to say that this epidemic is set to be severe. It is already severe in hospitals and it will probably have an effect on mortality. We can already anticipate this, but we will need more time to measure the mortality linked to the current epidemic.
We are talking about more virulent symptoms, particularly among young people. Can this be explained? ?
At present, we do not have data to confirm that the viruses currently circulating are more virulent than usual. No virological factor explains, at the individual level, an increased virulence of symptoms. At the general population level, we observe a severity of the flu epidemic on hospitalizations but nothing can intrinsically explain the severity of symptoms compared to previous epidemics. However, virological explanations could emerge with more hindsight.
Is it still worth getting vaccinated? ?
The peak has not yet been reached, we do not know how the epidemic will evolve. The flu can still circulate intensely for several weeks. We must not delay, but it is not too late to get vaccinated, especially if we are at risk of severe illness. The first data on vaccine sales and dispensations show us that they are at lower levels than in previous years. However, the vaccine remains one of the ways to protect ourselves against the virus. Another tool is barrier gestures, which are very effective against all respiratory viruses. This has been proven and documented, they help limit the transmission of the virus on a collective and individual scale. When we have the flu, we stay at home, we do not visit vulnerable people, we air the rooms, we wash our hands and we wear a mask in crowded places.