Anti-Covid vaccines: why the HAS recommends choosing the right time for boosters

Last year, many thought we should re-vaccinate every six months to maintain a high level of protection against severe forms of Covid-19. And then finally no. In its opinion published this Friday, February 24, the High Authority for Health does not recommend new reminders in the general population. Even for populations at risk (those over 65, pregnant women, patients with comorbidities such as diabetes or obesity), there’s no need to rush, experts say: the next booster will be in the fall, in same time as the influenza vaccine. Only people really at high risk of serious forms and a rapid reduction in their defences, namely the over 80s and the immunocompromised, are advised to have a new injection in the spring – and again, provided that their last dose, or their last infection, more than six months ago.

The favorable epidemic context, the strong circulation of the virus in recent months, which has given a high level of immunity to the population, or even the arrival of fine weather, do not on their own explain the caution of the experts of HAS with regard to recalls. “Specialists have alerted us a lot to the fact that with doses too close together with the same vaccine, the body may gradually not respond as well to these repeated stimulations”, explains Professor Elisabeth Bouvet, president of the Technical Committee for vaccinations to the High Authority for Health (HAS).

In question, various immunological phenomena, which the experts monitor like milk on the fire. The first is called the “immune fingerprint”. A risk that appeared theoretical until now, but which has finally begun to manifest itself. “The immune response will narrow to certain targets on the virus as we stimulate it with very similar vaccines. This could disrupt our ability to respond if Sars-CoV-2 evolves a lot, which could be “prove dangerous in the future,” explains Professor Bouvet.

Finding the “fair balance”

Added to this is an evolution in the quality of the defenses induced by multiple reminders. “Our body manufactures different types of antibodies, some proving to be more efficient than others. However, a German team recently showed that as the injections progressed, there was a risk of increasing the fraction of the least useful antibodies. , and therefore to lose vaccine effectiveness”, summarizes Professor Jean-Daniel Lelièvre, head of the immunology and infectious diseases department at the Henri-Mondor hospital (AP-HP), in Créteil. It is therefore better, according to this expert, to avoid giving injections at a time when the virus is circulating less, to preserve their effectiveness for the periods when we will really need them.

For those at high risk, however, the benefit of revaccination would outweigh these considerations. “It is a question of finding the right balance between the fact of still wanting to protect them by raising their antibody titers, and the risk of having a less good response”, specifies Elisabeth Bouvet. But for the general population, the HAS is therefore once again pushing back a broad recall campaign, whereas in the fall, the last injection in the general population will date back more than two years. Health authorities rely on our longer-term cellular immunity to protect us from serious forms. They consider, in fact, that there is – for the moment, with the variants in circulation – no greater risk of becoming reinfected, which will not fail to happen, at least next fall, may -be even before. It is therefore a strategy similar to that of the flu that has been adopted: everyone can decide whether or not to be vaccinated, the recommendations only concerning certain audiences.

In the fall, the two vaccines can also once again be injected at the same time – pending, in the near future, according to the president of the CTV, the arrival of bivalent influenza-Covid vaccines. As for the risk of developing a long Covid in the event of reinfection, frequently mentioned, it was not included in the HAS analysis, for lack of conclusive scientific studies on this subject. But, of course, this strategy is as scalable as the virus itself, warns Professor Bouvet: “If a new dangerous variant were to arrive, we would not hesitate to recommend a new campaign of injections for the whole population. “.