This is the scientific controversy of the moment. It starts from a simple question: how to explain the magnitude of the bronchiolitis epidemic? Early and violent, it took the health authorities of many countries in the northern hemisphere by surprise and again disrupted hospitals, overwhelmed by an unusual number of babies in respiratory distress. “It’s our spring 2020 to us”, summarizes a pediatric resuscitator. As is often the case with an unprecedented phenomenon, the explanations are not obvious. In this context of uncertainty, a group of French pediatricians has launched an attractive and easy-to-remember concept for the general public: “immune debt”.
According to them, this would be an unexpected consequence of health restrictions (confinements, masks and curfews) against Covid: “With these measures, the usual viruses and in particular the respiratory syncytial virus (RSV), responsible for a large part of bronchiolitis, circulated much less in 2020. The population was therefore not infected, and did not become reimmunized against these microbes”, explains Professor Robert Cohen, president of the National Professional Council for Pediatrics. He expected, he says, to see these diseases “come back in force at one time or another”. “As adults, adolescents and children are more likely to become infected, the viruses circulate more and the risk of them infecting newborns increases. It is possible that, for the same reasons, mothers transmit less antibodies to their babies”, adds Professor Emmanuel Grimprel, pediatrician at the Armand-Trousseau hospital (AP-HP) in Paris.
Apparently struck at the corner of common sense, the idea was however quickly challenged, other scientists, like Pr Antoine Flahault, director of the Institute of global health at the University of Geneva (Switzerland), getting annoyed at “a hypothesis which, once again, is in no way validated”. Since then, the debate has swelled in scientific journals, the media and on social networks. And it is probably no coincidence that we come across old acquaintances in each camp, who had already scrapped during the pandemic. With, on the one hand, the representatives of French pediatrics, long convinced – without proof – that children were infected and transmitted less, and therefore opposed to sanitary measures at school. On the other, epidemiologists who pleaded on the contrary to limit the circulation of the virus as much as possible, among the youngest as in the rest of the population. Since then, the role of schools in the epidemic and the interest in protecting children has been well demonstrated.
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Appearances of established scientific fact
This time, the former may not be completely wrong (anti-Covid measures have indeed disrupted the circulation of other respiratory viruses), but the latter are totally right (this idea of immune debt remains to be proven). The term, in any case, had never been used in the scientific literature before its appearance in an “opinion article” co-signed among others by Professors Cohen and Grimprel in May 2021 in the journal Infectious diseases now. Taken over by the Wall Street Journal, the concept quickly became viral in the international press. More recently, articles published in prestigious scientific journals (The Lancet, Nature…) referred to it. Government spokesman and former health minister Oliver Véran himself made reference to it on a television set. Problem, by dint of being repeated, this idea ended up taking on the appearance of an established scientific fact, while the reality turns out to be more complex. “It’s very tempting to sum up the current situation with a sexy but still poorly defined concept. The causes of this epidemic are more complex and probably multifactorial. It will take time and many studies to understand which factors turn out to be the most important”, slice Jean-Sébastien Casalegno, virologist at the Hospices Civils de Lyon (HCL) and member of the Lyon group on RSV.
It is certainly conceivable that a larger part of the population than usual is likely to be infected with RSV. In the absence of exposure to the virus, the antibodies conferred by a previous infection indeed decrease over time, even if the precise duration is not known. But while this protection may have declined after the lockdowns, no one has yet shown that population immunity today remains at lower levels than before the pandemic, or by how much. The question is all the more questionable as two waves of bronchiolitis took place last year. One moderate in spring and summer, then another, much stronger, which started in September and reached its peak in November. “In total, 2021 had already been a record year, with a higher number of emergency room visits for bronchiolitis than the previous ten years,” says epidemiologist Mircea Sofonea.
Study the impact of the two previous waves
Before asserting that there is still an “immune debt” vis-à-vis this virus, we should therefore start by studying the impact of last year’s waves on our immunity. “In France, we do not yet have the serological tools and the samples necessary for this study. Our Dutch colleagues are following a cohort of individuals representative of the population in which they will measure the level of antibodies against RSV repeatedly in time. This will be the most relevant study, but the results are not yet available”, continues Jean-Sébastien Casalegno. As if the subject were not complex enough already, it will also be necessary to take into account the fact that there are two types of virus (the experts speak of serotypes, A and B), which do not generally circulate at the same time. “However, we have little data on cross-immunity, that is to say protection against RSV type B after exposure to type A, for example”, adds the researcher.
In the meantime, one thing is certain: the Covid has turned the bronchiolitis calendar upside down. Traditionally, the epidemic started after the All Saints holidays and reached its peak around Christmas or early January. “Peaking in November last year, babies born after the holidays are less likely than usual to have been infected in their first weeks of life. Therefore, they may have joined the ranks of infections since then. back to school”, imagines Mircea Sofonea. The effect of this shift on hospitalizations would however remain marginal: “The risk of a serious form, very high for infants, decreases rapidly with age. Those born at the start of the year are therefore less concerned today, except for the 10% of children who suffer from heart or lung disease, or even asthma, which can be found at the moment in the hospital”, indicates Jean-Sébastien Casalegno.
Infographics
Dario Ingiusto / L’Express
Be that as it may, this hectic schedule may have taken the hospitalists by surprise, who did not expect to have an early epidemic for the second consecutive year. Usually, they schedule fewer interventions and recruit staff after All Saints to absorb the flow of young patients. This surprise effect, in a context of hospital crisis, makes the situation even more difficult to manage, even if it had already happened, before the covid, that the establishments were overwhelmed and forced to transfer babies. “City medicine may also have more difficulty responding to the influx of patients, which could also explain the overcrowding of emergencies”, wonders Professor Mahmoud Zureik, epidemiologist at the University Versailles Saint- Quentin.
Unusual virus co-circulation
RSV does not seem more severe than usual, because the proportion of children requiring hospitalization (compared to the total number of patients) remains the same as usual, according to Public Health France. For the moment, it is above all the other respiratory microbes that hold the attention of the experts: “In addition to RSV, the rhinovirus is circulating in the southern half of the country with a higher intensity than in previous years. The same goes for metapneumovirus in the northern half”, notes Vincent Enouf, from the National Reference Center for Respiratory Viruses at the Institut Pasteur. These pathogens also cause bronchiolitis in toddlers. “This co-circulation, which is quite unusual, also contributes to the current situation”, underlines Jean-Sébastien Casalegno. Is the Covid making its contribution? Mystery. “If it has been shown that it could cause bronchiolitis in babies, there is a lack of data at the national level to answer its weight in this epidemic”, regrets the epidemiologist Dominique Costagliola. Professor Grimprel at the Trousseau Hospital in Paris like Jean-Sébastien Casalegno at the HCL, however, claim to find very little for the moment in their little hospitalized patients.
On the other hand, let’s get rid of a misconception but much taken up on social networks, according to which barrier measures, by protecting us from germs, would have disrupted our immune system. “It’s nonsense, because it doesn’t work like a muscle that needs to be trained. And besides, no one lived in an aseptic environment in April 2020: viruses and bacteria surround us in permanently and our immunity remained active”, underlines Mircea Sofonea. Conversely, some research teams wonder whether an infection with Sars-CoV-2 could not have deteriorated our defenses and thereby increased individual but also population susceptibility to viral infections in general, and to RSV in particular. particular. Again, no certainty at this stage: “This effect has been documented after measles contamination, but there is no solid evidence for Covid”, replies immunologist Alain Fischer, former Chairman of the Council of orientation of the vaccine strategy (and columnist for L’Express).
As we can see, the hypotheses are not lacking. “But whatever the cause, the main thing is to reduce transmission. But this idea of immune debt risks having the opposite effect: if you have to get sick anyway, what’s the point of protecting yourself?” worries Professor Mahmoud Zureik. Behaviors could be permanently modified, as could political choices in terms of prevention. This is however essential. If RSV infection is inevitable, anything that can push it back to an age when it will no longer necessarily lead the baby to the hospital is good to take – avoiding crowded places, wearing a mask at the slightest cold, wash your hands… Messages that are visibly difficult to convey. Worrying, while the flu, always so unpredictable, is looming. While this almost never circulates at the same time as RSV, the United States is currently experiencing a triple epidemic – Covid, influenza and RSV. A scenario now dreaded by all French doctors.
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