Difficult to define scientifically, the long COVID is better and better understood. A recent burst of studies provides new elements on the causes and frequency of this pathology, although none can claim definitive conclusions.
What causes?
Long COVID is characterized by the persistence of symptoms – fatigue, shortness of breath, loss of smell… – several months after a COVID 19 infection.
This definition is very vague, which lends itself to lively scientific polemics. Is there a single long Covid or different pathologies of various origins? Should we see more psychological or physiological causes?
Several recent studies argue for the second track. One, published at the end of January in the journal Cell and carried out on several hundred patients diagnosed with Covid, shows several commonalities in those ultimately suffering from lasting symptoms.
They frequently had a high level of autoantibodies, antibodies that turn against the body itself. Another element often noted, the prior presence in the blood of the Epstein-Barr virus, in particular at the origin of mononucleosis.
Another study, published in the journal Gut and carried out in Hong Kong with around 100 patients, shows that the symptoms of long-term COVID are associated with a long-lasting disturbance of the microbial balance in the intestine.
This work gives promising leads, but must be taken with caution: the number of people examined remains low and further studies will be needed to confirm these conclusions. Even then, a direct causal mechanism will still have to be established.
In the Hong Kong study, it is thus possible that the victims of long COVID have changed their diet because of their weakening, in turn affecting their microbiota rather than the reverse.
Vaccines better protected?
Do anti-Covid vaccines protect against long COVID? The answer is not evident.
Admittedly, vaccines largely avoid serious COVIDs, a priori more likely to cause sequelae. But many patients also report lasting symptoms after mild forms, against which vaccines largely lose their effectiveness over time.
However, several works are reassuring, including, the latest, an Israeli study made public in January. She followed a few thousand patients who tested positive for COVID in 2020 and 2021.
Previously vaccinated patients reported no more lasting symptoms than members of a control group, never diagnosed with COVID. On the other hand, the frequency was much higher in unvaccinated patients.
This work has not yet been independently reviewed and therefore remains questionable. But it is in line with a previous study of the same nature, carried out in the United Kingdom and published at the end of 2021 in the Lancet Infectious Disease.
However, these two studies have a limitation. They were carried out before Omicron took off, at the end of 2021, and therefore do not allow a conclusion to be drawn on the effectiveness of vaccines against possible long-lasting COVIDs developed after infection with this variant, which is now largely dominant in many countries.
Limited risks in children?
Of all the debates related to the long Covid, this is the most sensitive.
Children have, in the short term, little risk of developing a severe form of COVID. But a high risk of a long form would be a game-changer, for example as to the urgency of vaccinating them.
However, a study, published in January in the journal European Journal Pediatrics, is reassuring, on the basis of elements collected from several tens of thousands of Danish homes.
The researchers compared the symptoms reported over a period of several months in two groups of children, some tested positive for COVID, the others not. The aim was to prove the existence and frequency of manifestations specifically linked to sequelae of COVID.
Ultimately, “long COVID is rare in children and typically does not last,” concludes this work, which found that less than 1% of children tested for COVID develop lasting symptoms.
The methodology of this study, like other similar work, is however criticized by several researchers. According to them, it is not appropriate for a syndrome of this type and probably misses the authors many cases of long COVID.