Why are the flu and bronchiolitis epidemics so strong and early this year?
Two factors help explain the seasonality of infectious disease epidemics and their annual waves. On the one hand, there is the sensitivity of each individual, that is to say to what extent you are immune or not: if you catch the flu, for example, you are partly protected once morest it for the year next. And on the other hand, there is the question of your exposure to respiratory infections which answers the way in which certain viruses are transmitted or survive more or less well in the environment. These two factors vary depending on the disease.
Even if we lack certainty, the arrival of bronchiolitis and of influenza earlier this year might be related to the fact that for two years there were only very mild outbreaks of these two diseases, resulting in quite low pre-existing immunities once morest them. As a result, the circulation of these viruses is now facilitated, especially since the vaccination coverage once morest influenza is not very high. Epidemics then break out as soon as environmental conditions allow: they no longer wait for the heart of winter, when everyone lives in closed rooms, to take off.
And by what mechanism did covid-19 come to add to it?
For covid-19, the reasoning is different. The current wave is linked to the emergence of a new variant, BQ.1.1, which greatly eludes immunity. Without its appearance, we would potentially not have had a wave of covid synchronized with those of influenza and bronchiolitis.
Why have the flu and bronchiolitis epidemics been weaker in the past two years?
A priori, it is the use of the mask, ventilation, barrier gestures that have limited their circulation. The influenza virus has an airborne mode of transmission quite similar to SARS-CoV-2 (the virus responsible for covid). Its reproduction number, that is to say the number of people that an infected person infects, is around 1.5. For covid, it was 3 at the beginning and we can therefore say, by caricaturing, that it was then transmitted twice as much as the flu. Effective barrier measures to prevent the circulation of covid were therefore all the more so for the flu.
Do we suffer from an “immune debt”, a form of weakening of our immune system, a concept that is much debated?
You have to be careful. I’m not saying that our immune system has weakened entirely, only the specific immunities to influenza and RSV (the virus responsible for the majority of bronchiolitis). Where the immune debt debate is groundless is that it sticks to a holistic view of immunity when the issue being discussed is specific immunity once morest one type of infection.
For influenza and RSV, we might expect a lower peak next year
Can these different viruses compete and one end up causing the other two to disappear?
No, there is no competition between them at all. On the other hand, we know that there can be co-infections: people infected with several respiratory viruses at the same time. We discover them in particular when we carry out screenings, especially in children.
Is there a greater risk of serious forms in the event of co-infection?
At the moment, we do not have a lot of data. Some studies have been carried out in a very piecemeal manner. But to demonstrate that a co-infection is more virulent than a single infection, you need both people infected with flu, covid and both, and population sizes such that you can correct all the biases.
Can a flu infection partly protect us once morest covid and vice versa?
We see rather that an infection by influenza leads to a weakening, the classic example being bacterial superinfections. They were one of the factors that explain the enormous mortality during the Spanish flu epidemic following the First World War, in the absence of antibiotics.
This triple epidemic leads to very strong tensions on the hospital. Is there a risk of suffering from it every year?
By definition, nothing is known regarding the future because it depends entirely on public health policies. If investments are made to improve indoor air quality, for example, everything will change. But let’s assume that the situation remains as it is, that is to say that we let everything circulate: we expect there to be more circulation of covid-19 in winter. And even if the current explosion is linked to a new variant, there is no reason to think that the evolution of variants will stop. For influenza and RSV, one might expect a lower peak next year as a greater proportion of the population will have immunity…unless a new influenza variant emerges, as in 2009.
* Samuel Alizon is director of the Ecology and Evolution of Health team at the Collège de France in Paris and author of the book “Evolution, ecology and pandemics, bringing Pasteur and Darwin into dialogue”, published by Points.
