Illustration of a grass pollen allergy @BelgaImage
This was the big question when the Covid landed: are people with allergies more vulnerable to Covid-19 or not? More than two years after the start of the pandemic, the first responses are emerging. This is what follows in particular from a study in the scientific journal The Lancet Respiratory Medicine, and another from PNAS, published by the American National Academy of Sciences. If we had to sum up the answer, it would be: it depends.
Cells that barricade themselves better
That of PNAS is the most reassuring. It turns out that only about 5% of asthmatics, in this case those with severe asthma, are more likely to be serious Covid-19 patients. The others could on the contrary be better equipped to face the disease. A real paradox that has intrigued researchers. “We knew there had to be a biomechanistic reason that might explain why some asthmatics seemed protected against severe Covid.“, Explain in Science Daily the author of the study, Camille Ehre.
His team therefore set out to find out what it was. To do this, they studied the mechanisms of the respiratory system, both affected by Covid-19 and by asthma. In the case of the coronavirus, it is its Spike proteins that allow it to attach to human cells via the ACE2 receptor. To protect themselves, these cells have a defense mechanism, known as mucin MUC5AC. Problem: when the invasion is too great, the proteins responsible for producing it are overwhelmed. The amount of MUC5AC mucin becomes insufficient and the coronavirus thrives.
In people with asthma, however, there is an overproduction of mucin MUC5AC due to the presence of interleukin 13 (IL-13), ie the mediator of allergic inflammation. Logically, thanks to this anti-Covid barrier, they are therefore better prepared to face the coronavirus. Even better, IL-13 also reduced the production of the ACE2 receptor, which further limits the possibilities of infection and then contagion to surrounding cells. In summary, the respiratory system is barricaded. Laboratory experiments with the use of IL-13 on human lung cells confirmed this hypothesis. A track to develop new treatments against Covid-19. “This study shows the importance of certain specific mechanisms, involving the expression of IL-13, and how this could be used to protect patients and prevent them from developing severe forms of Covid“, says Camille Ehre.
Some asthmatics much more vulnerable than others
However, the researcher admits that there are still gray areas on the exact mechanism of IL-13. Which is more protective: MUC5AC mucin production or ACE2 receptor reduction, or a mixture of both? Then using IL-13 as a drug as such is excluded, since it would cause inflammation in people who already suffer from this problem. Finally, there are still people with asthma who react very badly to the coronavirus.
Among those particularly sensitive to Covid-19, there are at least some children. This is the subject of The Lancet study. To show this, the researchers saw large, with a cohort of 750,000 children aged 5 to 17. 5.8% of them contracted Covid and 0.9% were hospitalized. Among the 63,000 asthmatics taken into account, these rates rose to 6.8% and 1.5% respectively. But one child with asthma is not another. Again, those with severe asthma were most likely to end up in hospital, with 548 cases per 100,000, compared with 94 per 100,000 among other asthmatics.
“This study partly explains why asthma in general did not emerge as a risk factor for the severe form of COVID-19 in some studies, because it is a risk factor that is only really significant. for a proportion of asthmatics, [ceux dont la maladie n’est pas contrôlée] and not necessarily for all asthmatics“, explains to the Quebec newspaper Le Devoir Dr. Jesse Papenburg, pediatric microbiologist-infectiologist at the Montreal Children’s Hospital. According to her, we must therefore remain cautious about the a priori similar number of hospitalized Covid asthmatics and non-asthmatics. In addition, confinement may have encouraged people to protect themselves from allergens as well, which can skew the figures. The rest of the pandemic will show whether subsequently, in the absence of quarantine, the representativeness of allergic people changes among patients with Covid-19.