Covid-19: can we do our booster dose with a different vaccine?


Most vaccination centers offer Moderna, which worries people vaccinated with Pfizer. But specialists want to be reassuring.

“There will be enough vaccine doses” recall against Covid-19 “If we use all the vaccines” to messenger ARN, that is to say from Pfizer but also from Moderna: this was hammered out, this Wednesday, December 8, the President of the Scientific Council Jean-François Delfraissy, during a hearing in the Senate. A message intended to be reassuring, after the concern about the lack of stocks of doses of Pfizer.

“If we only use Pfizer, we will not have enough doses because the orders are gone”, indeed warned the doctor. Pfizer’s vaccine was indeed the most used during the previous vaccination campaign. Almost 80 million doses have been injected so far against just over 11 million Moderna.

Sell ​​Moderna stocks

According to the Directorate General of Health, France currently has 5 million doses of Pfizer, for 20 million doses of Moderna. It is therefore a question of disposing of Moderna’s stocks for “rebalance»Supplies. Now, according to the Ministry of Health, “People over 30 will be able to benefit this week mainly from the Moderna vaccine in vaccination centers“.

This new vaccine strategy can be explained by another logistical argument: each vial of Moderna contains 24 doses, compared to six for Pfizer. The first is therefore preferred in vaccination centers, where enough people will parade to avoid waste, while the second is sent to pharmacies, where the flow of people remains less important.

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Half-dose of Moderna or full-dose of Pfizer, more of the same?

But receiving a third dose of Moderna worries those who have been previously vaccinated with Pfizer. However, there is no risk, either way, explain the health authorities.

It can be Moderna or Pfizer, indiscriminately“, Had already answered the Minister of Health Olivier Véran, Friday, December 3 on franceinfo. “I say to French people who have had for example two doses of Pfizer and who are worried about having Moderna: no problem. Likewise, the French who have had Moderna twice and to whom we are going to offer Pfizer: no worries, it’s the same“Wanted to reassure the minister.

The only difference is that if you are under 30, we prefer to give you Pfizer rather than Moderna., had continued the Minister of Health.

The Moderna more “dose»And slightly more efficient

A recent study, conducted by researchers from Harvard and Boston, also confirms a slightly increased effectiveness of Moderna compared to Pfizer. The gap remains small: 1.2 per 1,000 contaminations, and 0.02 per 1,000 deaths due to Covid-19.

Moderna would be more efficient because more “doseThan Pfizer. This is what explains to Figaro Pierre Parneix, hygienist and public health doctor at Bordeaux University Hospital, and member of the assessment mission for the management of the Covid crisis: “We have 100 micrograms of messenger RNA in one dose of Moderna, or 50 in a half dose. However, in a dose of Pfizer, we have 30 micrograms, which is less than in a half-dose of Moderna. The Pfizer multiplies the antibody rate by 10, while the Moderna multiplies it by 20, or even 40 “.

For those under 30, the preferred Pfizer

As part of the vaccine booster, it is therefore a “half doseOf Moderna that is being administered – 50 micrograms of messenger RNA. Logically, this slight “overdose»Compared to Pfizer “Increases the risk of side effects”, continues Pierre Parneix.

During the vaccination campaign, a risk of hospitalization for myocarditis thus appeared within seven days of the injection, in particular with the Moderna vaccine, more often in young men under 30 and after the second dose. This finding, amounted to 131.6 cases in excess for a million doses of Moderna injected (against 26.7 with Pfizer), was drawn up by the Epi-Phare investigation on November 8. “This risk of vaccine myocarditis is still less frequent and less serious than the risk of myocarditis induced by the virus., emphasizes on Franceinfo Mathieu Molimard, head of the medical pharmacology department at the Bordeaux University Hospital. “In those over 30, there is no significant excess risk.He adds.

SEE ALSO – Moderna is working on an annual booster dose “which will include influenza and Covid”

It is on the basis of these observations that the High Authority for Health (HAS) recommended, on November 8, to favor Pfizer for people under 30 years of age.

Janssen, AstraZeneca and messenger RNA vaccines: a “cocktail»Effective doses

For the population aged over 30, boostering with Moderna does not present any particular risk, whether one has already been vaccinated with Moderna or Pfizer.

“There is nothing to worry about. Between a half-dose of Moderna or a dose of Pfizer, it does not change anything ”, assures Pierre Parneix. And conversely, having been vaccinated at Moderna and receiving Pfizer is not a problem: “The Pfizer is a little less dosed, but remains effective. These vaccines are interchangeable in vaccine strategies around the world, it is the same mechanism, the same level of efficacy and safety ”.

“These vaccines (Pfizer and Moderna) are interchangeable in vaccine strategies around the world, it is the same mechanism, the same level of efficacy and safety”.

Pierre Parneix, hygienist and public health doctor at Bordeaux University Hospital, and member of the assessment mission for the management of the Covid crisis.

What about those who have been vaccinated previously with Janssen or AstraZeneca, vaccines not based on the technique of messenger RNA? Getting a booster dose at Pfizer or Moderna wouldn’t be a problem either. “The reminders are for the RNA vaccine, so either Pfizer or Moderna. We have seen that vaccinations of different concepts, such as Janssen with RNA vaccines, were effective strategies “, assures Pierre Parneix.

This approach has already been tested. In the United Kingdom, the AstraZeneca vaccine was used extensively during the first vaccination campaign, and then messenger RNA vaccines succeeded it for the booster campaign. Same vaccination schedule, in France, for caregivers.

Not only is there no problem, but it is not impossible that making a messenger RNA vaccine after doing a Janssen or AstraZeneca-type vaccine could further improve the immune response.», Adds on Franceinfo Dominique Deplanque, president of the French Society of Pharmacology and Therapeutics. “The immune response will be based on the ‘foreign substance’ presented to the immune system. If this ‘foreign substance’ is only partially identical to that of the first injection, this expands the repertoire of the immune response“, He argues again.


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