Could the very strong bronchiolitis epidemic be linked to the anti-Covid measures observed over the past two years? On November 13 on BFM TV, Olivier Véran affirmed it, declaring: “The hospital is under tension, very strong in pediatrics, with an epidemic of bronchiolitis which was early, which is very intense. Why is it so intense? Precisely because for two years, we wore masks, and since it’s a respiratory virus, we had protection, so we had an epidemic that was very weak, we had less immunization of the little ones, and suddenly, as there is a collective relaxation, the virus circulates very strongly, it circulates earlier, and therefore there are many children who are sick.
This statement can be interpreted in several ways. The first consists in considering that many children who could have been infected by the viruses responsible for bronchiolitis during a given period were not, and are now overtaken by the infection. Another interpretation, defended by some members of the medical community since mid-2021, would have it that less exposure to many viruses (due to wearing a mask) has somehow unaccustomed the body to reacting to infections, making it more susceptible to disease. This second interpretation, which some refer to as “immune debt”is only a hypothesis that is not, for the time being, supported by concrete scientific data.
A low circulation of the viruses responsible for bronchiolitis for two years?
First of all, it is necessary to question the first element of Olivier Véran’s argument: “the virus” responsible for bronchiolitis would not have circulated for two years.
First, it should be noted that bronchiolitis is not linked to the respiratory syncytial virus (RSV) alone, often presented as the “bronchiolitis virus”. According to Public Health France, a minority of samples taken from patients are currently positive for RSV. It is therefore not excluded that this epidemic of bronchiolitis is simply linked to the increased circulation of multiple viruses in the fall. Note that several studies published since 2021 observe that SARS-CoV-2 infection can, in infants, manifest as bronchiolitis.
Second, the idea that the current bronchiolitis epidemic is occurring after two years of epidemic respite is simply false, as illustrated by hospital data collected by Public Health France. While few cases were recorded at the end of 2020, a (moderate) wave of hospitalizations for bronchiolitis took place at the start of 2021. A second wave, earlier and longer than that of 2019, occurred in the fall 2021. Part of the “catch-up” of infections therefore seems to have already taken place at two points in the year 2021. The fact that the epidemic then “was particularly extended in time” has also led “to a greater number of infected children”, as observed by the Opecst in a report published in early 2022.
This idea that the current epidemic includes, in part, the catch-up of infections that did not occur at the end of 2020, was spontaneously raised by several interlocutors approached by CheckNewssuch as Mauro Gaya, researcher at the Marseille-Luminy Immunology Center (CIML), for whom the trail of an effect of “immune debt” (understood as does “that our innate immune system would not have been challenged enough, trained enough, and that we would now be more susceptible to other diseases”) would be, for the time being, as superfluous as it is speculative.
“Immune debt”: an ambiguous concept, and still very speculative
It should be noted that the expression “immune debt”, which appeared in 2021 under the pen of French authors, is brandished by various actors in the public debate to designate very distinct concepts. For some, it designates in a very prosaic way the risk linked to the delay in inoculation of current vaccines or their reminders, since the start of the epidemic.
But others defend, as we have said, a more radical thesis: by avoiding exposure to certain seasonal viruses, our bodies would have missed an opportunity to maintain significant levels of defense against these viruses (or, for the most young people from being exposed to these viruses). “[Au cours d’une année normale]we can be exposed to a little bit of virus and your body fights it”thus speculates, in a popular article published in Naturean immunologist at Imperial College London. “This kind of asymptomatic reinforcement may not have happened in recent years.”
Brigitte Autran, Emeritus Professor of Immunology at the Sorbonne Faculty of Medicine and President of Covars (Committee for monitoring and anticipating health risks), considers that “this concept is very vague”and that he “does not have a solid immunological basis”. “This is only a hypothesis put forward to try to explain the precocity and intensity of the current epidemic of bronchiolitis or influenza.”
Brigitte Autran tells us “that there is no precise immunological definition” to the term proposed by the French researchers. “It is a concept of global immunity suggesting that the immune system of the child needs permanent stimulations [de l’immunité innée comme de l’immunité adaptative] to maintain a good balance in its maturation and defend itself against a much wider range of pathogens”. But does reducing exposure to many pathogens actually promote more infections after the crisis? “This is what is still difficult to quantify.” Nevertheless, if this hypothesis were verified, it would militate “for the rapid development of the vaccine against bronchiolitis, the implementation of the vaccine against gastroenteritis and especially for an intensification of the vaccination campaign against influenza”. And, even if the phenomenon were at work, “it would be difficult to speak of debt, because there is no book debt here and it is impossible to quantify the quantity of stimulations which the immune system would need”.
Mauro Gaya notes for his part that the idea of ”immune debt” seems to be based on the concept “trained innate immunity”, recently highlighted phenomenon according to which certain rapid reactions of the organism vis-a-vis the infections can be more rapid following frequent meetings with pathogenic agents. “The immune debt would then be the increased susceptibility to infections due to the absence of trained innate immunity.” But he points out that the argument of trained innate immunity has already been invoked, wrongly, to explain a lower number of cases in the countries of the South at the start of the Covid crisis. “The BCG vaccine being identified as promoting this trained innate immunity, researchers have postulated that the populations of the South, where this vaccine is used, were better protected. In reality, it was just the effect of sanitary measures (masks, distancing). Where they have been relaxed, the epidemic has started to rise again.” In the case of bronchiolitis, “although the hypothesis may be true, there is as yet no scientific proof that this may be the case”.
An “easy and attractive” but “dangerous” hypothesis
On social media, various other commentators have also taken issue with these interpretations. This is the case for epidemiologist Antoine Flahault, for whom the concept is above all “the illustration of our great ignorance in the field of immunology. Instead of saying “I don’t know why we have this level of bronchiolitis”, some scientists, especially doctors [et] especially paediatricians, rush into it without restraint. We have significantly cleaned up – on a microbiological level – our lifestyles for a century. [Or] instead of paying our digestive, cutaneous, genital and respiratory immune debt, we have seen infectious diseases melt away and life expectancy more than double”.
Epidemiologist Mahmoud Zureik also recalled, on Twitter, that the idea that the virulence of the bronchiolitis epidemic would be linked to such a definition of “immune debt” did not rest “on no scientific argument at this stage”.
“This concept, adopted too quickly by some (minority) scientists and politicians, could have dangerous consequences in terms of public health and on people’s lives”, judge Zureik. And to denounce the argument according to which “If we don’t get sick now thanks to prevention, we will get sick later (and perhaps more seriously) and so what are barrier gestures and prevention for?” For the epidemiologist, even if this hypothesis were to be verified in the case of bronchiolitis, “it doesn’t change [rait] in no way the course of action to be taken now and in the future: preventing and limiting the spread and viral transmission by applying barrier gestures adapted to the intensity of viral circulation”.
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