Your questions, our answers | More mRNA in third dose?

In late spring, people who had received the AstraZeneca viral vector vaccine in the first dose were recommended to receive an mRNA vaccine in the second dose. The interchangeability of vaccines, experts assured, was safe and as effective as two doses of the same type of vaccine.

Judith Lachapelle
Press

The time for the booster dose, the third dose, has now arrived for many vaccinees. But so far, experts recommend only one vaccine, the mRNA vaccine (Pfizer or Moderna), regardless of which one received for the first two doses.

However, several other types of vaccines are now available. Why not offer them as a booster dose?

“Because we know that with the variants that are currently circulating, mRNA vaccines are more antigenic, that is to say, they stimulate a higher production of antibodies,” says Dr.^r Donald Vinh, microbiologist-infectious disease specialist at the McGill University Health Center. The efficacy of Delta variant mRNA vaccines in preventing severe forms of COVID-19 has been assessed to be better than that of AstraZeneca’s viral vector vaccine. For the new Omicron variant, evaluations are ongoing.

Supporting study

A study published Friday in the scientific journal The Lancet has also reaffirmed the recommendation of a booster dose mRNA vaccine.

The study, carried out in June in the UK on some 3,000 people, compared many combinations of vaccine doses. Like the vast majority of Canadian vaccinees, the patients studied had received either the AstraZeneca vaccine or that of Pfizer-BioNTech. After at least two months (for those who received the Pfizer vaccine) or three (AstraZeneca), they received a third dose.

This third dose could again be given by means of an mRNA vaccine (Pfizer) or a viral vector (AstraZeneca). Or with one of the following vaccines:

• CureVac: an mRNA vaccine developed by a German company, which however ceased production in October after disappointing results;
• Moderna: the American mRNA vaccine, little used in Europe, but well known in Canada;
• Novavax: this vaccine developed by an American company is a so-called “subunit” vaccine, based on proteins that trigger an immune response, without virus. A license application was filed in November with Health Canada;
• Valneva: the Franco-Austrian laboratory used a more classic technology, the “inactivated virus” vaccine, as in the case of influenza vaccines;
• Janssen: the American viral vector vaccine developed by Johnson & Johnson, licensed in Canada, but very little used (less than 15,000 Canadians have received it).

For control purposes, participants were also given a placebo vaccine (in this case, a meningitis vaccine).

In almost all of the settings other than the placebo, the participants generated an increased amount of antibodies to the coronavirus. The only exception is that vaccinees who received two doses of Pfizer did not derive any significant benefit from one dose of the “inactivated virus” vaccine from Valneva (which is not licensed in Canada).

While the other combinations worked fairly well, the third dose mRNA vaccines from Pfizer and Moderna showed the greatest increase in antibody production.

In addition, study participants received their booster dose much faster (three months after the second dose) than what most public health authorities currently recommend (six months after the second dose). Studies generally show that the longer the interval between two doses, the better the immune response.

“The good news is that people don’t have to worry about choosing their third-dose vaccine,” says Dr.^r Vinh. “Pfizer or Moderna, there is no difference. If it’s your turn for the booster dose, you have to go get it now, ”he said, adding that if pharmaceutical companies have to adapt their vaccine to fight the Omicron variant, new versions will not be available for several months. .

With Agence France-Presse

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