A quiet start, then an acceleration foot to the floor. With the announcement by Emmanuel Macron of the upcoming integration of the booster dose of the Covid-19 vaccine into the health pass, the number of daily injections has risen sharply. On average, 400,000 people flock to vaccination centers, pharmacies and doctors’ offices every day for a third dose (or a second after contracting the disease). This is four times more than a month ago.
What do we know about the effects of this booster dose on our immune system? Why did the government choose to generalize it? Does having your first vaccination schedule with AstraZeneca or Janssen provide the same protection? Franceinfo interviewed specialists.
What is the benefit of the booster dose?
It aims to increase the quantity of neutralizing antibodies against Covid-19, because their number drops a few months after the end of the first vaccination schedule. This fall becomes “significant” between five and eight months after the administration of the second dose, explains Sandrine Sarrazin, Inserm researcher at the Marseille-Luminy immunology center. This situation poses a problem in France, where the Delta variant, against which the vaccine efficacy is lower in terms of transmission, is actively circulating. “The first dose of the vaccine triggers the immune response, but antibody production remains moderate. The second dose strengthens this reaction and increases the antibody level.”, summarizes Sandrine Sarrazin.
By presenting our immune system once again with a pathogen it has already encountered, the booster dose allows the antibody level to jump to a level “ten to twelve times higher” to the one following the second injection, according to this researcher. With spectacular results: a study carried out in Israel (article in English), where the administration of the third dose was opened to all adults on August 29, shows that the booster is 93% effective against hospitalization, 92% against severe forms of the disease, and 81% against the death.
How to explain the strong increase in the level of antibodies with this third dose?
“It’s really a classic pattern”, says Sandrine Sarrazin, who recalls that vaccines against hepatitis B, whooping cough or tetanus are administered in three doses spaced a few months.
This significant increase is explained by the functioning of our immune system. Vaccination involves two types of B lymphocytes, white blood cells responsible for producing antibodies. “The first will immediately produce antibodies. The second, called memory B lymphocytes, will keep in mind the plan for making these antibodies and be stored in the bone marrow for later. cellar to spend the quiet winter “, she illustrates.
After having been requested twice by the first injections (or by a Covid-19 infection), our body is well aware of the antigen presented to it by the booster dose. “It will directly solicit these memory cells. These B lymphocytes, sensitized by the first injections, will mobilize very quickly and produce very large quantities of antibodies”, developed for Franceinfo Professor Yves Buisson, epidemiologist and president of the Covid-19 cell of the National Academy of Medicine.
Is the immunity provided by the booster dose effective against transmission of the virus?
This is what the first surveys on the subject show. Israeli study published on November 30 (article in English) in the Journal of the American Medical Association compared the frequency of Covid-19 infections in more than 300,000 people over the age of 40 who have or have not received a booster dose of the Pfizer vaccine.
The results are very encouraging: between 28 and 65 days after the injection, the proportion of positive PCR tests was reduced by 86% among the subjects who benefited from the booster. “A vaccinated person will acquire the virus less often, and when they are infected, their viral load will be lower: statistically, they will be less transmitting”, summarizes Olivier Schwartz, head of the Virus and Immunity Unit at the Institut Pasteur.
“With the third dose, we will again be able to affirm that the vaccines protect against the transmission of the virus. The Delta variant and the gradual decline in immunity after the second dose had made us lose this advantage, but we should regain it thanks to to the reminder “, enthuses Sandrine Sarrazin.
Is the booster dose the same as the previous ones?
Yes and no. Unlike the first stage of the vaccination campaign, the AstraZeneca and Janssen products are not administered for boosters, unlike the messenger RNA vaccines from Pfizer-BioNTech and Moderna. A purely pragmatic question, explains Professor Yves Buisson.
“Messenger RNA vaccines have been shown to confer longer lasting immunity and fewer side effects than adenovirus vector vaccines [comme ceux produits par AstraZeneca et Janssen]. “
Yves Buisson, president of the Covid-19 cell of the National Academy of Medicineto franceinfo
The dosage of the Moderna vaccine was also changed a bit for the booster. Unlike previous injections, Moderna’s booster only contains 50 micrograms of messenger RNA, compared to 100 micrograms previously. The dosage of Pfizer’s vaccine remains unchanged at 30 micrograms. The administration of the Moderna vaccine has also been ruled out for those under 30 years of age because of a higher risk of mild heart inflammation in this age group.
For the rest, the Moderna and Pfizer-BioNTech vaccines have so far not changed since the first vaccination campaign: the formula used is still that created to counter the original strain of Sars-CoV-2 identified in Wuhan.
Does having received a vaccine other than that of my booster dose give less good immunity?
“Not only is there no problem, but it is not impossible that making a messenger RNA vaccine after making a vaccine of the Janssen or AstraZeneca type could further improve the immune response”, recently explained to franceinfo Dominique Deplanque, president of the French Society of Pharmacology and Therapeutics.
“The immune response is going to be based on the ‘foreign substance’ presented to the immune system. If that ‘foreign substance’ is only partially identical to that of the first injection, it expands the repertoire of the immune response.”
Dominique Deplanque, President of the French Society of Pharmacology and Therapeuticsto franceinfo
Ditto in case you switch from a Pfizer-BioNTech vaccine to Moderna, or vice versa. “Pfizer and Moderna are based on the same principle, their level of efficacy is comparable and the profile of side effects is generally the same, apart from this small increased risk of rare and non-serious myocarditis in young adult males.”, assures Dominique Deplanque.
Would I be less well protected if, after contracting Covid-19, I only received two doses of the vaccine?
This can offer even stronger protection against the virus than a simple vaccination. “The immune system of a person infected with Covid-19 will not only produce antibodies against the spike [que ciblent les vaccins], but also against other components of the virus “, Sandrine Sarrazin analysis. In this case, vaccination can act as an enhancer of this wider range of antibodies.
Be careful though: to wait voluntarily to contract Covid-19 before getting vaccinated to obtain more complete immunity would be a bad calculation. “The paradox is that the immune response is better when the disease is severe”, remarks Claude-Agnès Reynaud, research director and immunologist at the Necker-Enfants Malades Institute. Contracting Covid-19 can also leave persistent sequelae (we speak of “long Covid”), drive to intensive care, or even kill. On average, 112 deaths linked to Covid-19 are recorded every day by French hospital services.
How long will the protection generated by the booster dose last?
Hard to say at the moment. “The immediate protection generated by the booster dose is enormous, but will slowly decrease. It will now be necessary to observe at which level the antibodies will stabilize. If we follow the classic immune response patterns, the level of the antibodies could reach a threshold. high enough to provide good protection “, analysis Claude-Agnès Reynaud.
“Quite logically, the booster dose should be able to provide much longer lasting immunity than the initial two-dose regimen.”
Claude-Agnès Reynaud, research director and immunologist at the Necker-Enfants Malades Instituteto franceinfo
“In my laboratory, we are following a cohort which had its third dose two months ago. The antibody level has decreased but remains very satisfactory because it started from higher thanks to the booster”, adds Olivier Schwartz, head of the Virus and Immunity Unit at the Institut Pasteur.
Will we need a fourth dose?
If the epidemic situation remains comparable to that which France is going through, not necessarily. It will all depend on the level at which the antibodies produced by our body after the booster dose stabilize.
However, if a variant escaping the antibodies were to supplant the Delta variant in the epidemic dynamic, a suitable booster could be required. This is the fear aroused by the Omicron variant: its many mutations in the Spike protein make it partly more resistant to current vaccines, which target the version of this protein from the original virus.
Omicron is “probably not sufficiently neutralized after two doses”, admitted Wednesday the Pfizer and BioNTech groups, whose vaccine against Covid-19 is initially delivered in two injections. Corn “the vaccine is still effective against Covid-19 (…) if it has been administered three times”, they tempered in a joint statement, on the basis of studies not yet published. The two groups specified that they are working on “development of a specific vaccine” to the Omicron variant, which could be available as soon as “March 2022”.