Three doses, three different vaccines: what are the risks?

AstraZeneca, Pfizer-BioNTech and Moderna. Thousands of Quebeckers aged 45 and older have received three doses of different vaccines to protect against COVID-19. Can they suffer the repercussions of this immune cocktail?

“The combination of the three vaccine formulas that are more or less different should not be alarmed,” says Benoit Barbeau, a professor in the department of biological sciences at the University of Quebec in Montreal (UQAM). He himself received these three COVID-19 vaccines that were authorized by Health Canada.

Different technologies for the same purpose

The vaccines designed by Moderna and Pfizer-BioNTech are based on the same technology: messenger RNA. Each carries RNA in a lipid capsule and delivers it to cells to make a spike protein and mount an immune response.

What distinguishes these two vaccine products? Lipid capsule composition and RNA assay. “There is not a big difference between Moderna and Pfizer-BioNTech. That is why we are more accepting of their interchangeability,” says Professor Barbeau.

Instead, the vaccine designed by the pharmaceutical company AstraZeneca is made from an adenovirus, that is, a harmless virus that contains a DNA molecule. Its composition has been modified to prevent it from reproducing.

“By injecting the genetic information into the cell, your body will produce the spike protein and trigger an immune response, just like RNA vaccines,” explains the virologist.

So all three vaccines work in a similar way: they provide an important ingredient to activate your immune system and prepare it to fight the COVID-19 virus.

“And receiving different vaccines can generate a stronger immune response, adds Benoit Barbeau. There is a kind of complementarity between the vaccines due to their differences.”

Studies carried out in particular in the United Kingdom, Germany and Spain conclude that a varied vaccination stimulates the immune system more.

“Fully vaccinated?”

If mixed vaccination does not cause any adverse health effects – at least according to the studies carried out so far – what happens to the vaccination status? Canada considers that a person is fully vaccinated if he or she has obtained two doses of a single vaccine, a separate injection of a Janssen/Johnson & Johnson vaccine, or “a mixture of two vaccines” recognized by public health authorities after a while. 15 days.

Thus, a person who received a dose of AstraZeneca and then a dose of a messenger RNA vaccine (Pfizer-BioNTech or Moderna) is fully vaccinated. Ditto if you get a combination of Pfizer-BioNTech and Moderna vaccines. The booster dose, which is now offered to the entire population, is not currently considered in the vaccination status.

And to travel?

But if you go on a trip, will the countries you visit show the same openness?

In the United States, you must have received two doses of a COVID-19 vaccine, or just one of the Janssen/Johnson & Johnson product, for at least 14 days to be fully vaccinated. Although the Centers for Infection Control and Prevention (CDC) do not recommend mixing vaccines, combinations between vaccines validated by the World Health Organization (WHO) are accepted.

Further south, in the Caribbean Sea, Trinidad and Tobago, which until last summer refused to recognize AstraZeneca’s product, now approves all WHO-approved vaccines. A traveler is considered fully vaccinated if they have received two doses within 14 days. So you don’t have to quarantine for 14 days.

In the European Union, each country has its own health requirements. The European Medicines Agency and the European Center for Disease Prevention and Control (ECDC) advocated last December for a “heterogeneous vaccination against COVID-19”. France agrees with this position. It states that a fully vaccinated person must have received a second dose of vaccine for at least seven days and that the combinations “AstraZeneca/Pfizer, AstraZeneca/Moderna and Pfizer/Moderna” are authorized.

Germany, where a dose of AstraZeneca’s vaccine has been given to virtually every adult, is also in favor of diversifying vaccines, to the extent that they are licensed by the Paul-Ehrlich Institute.

Australia, for its part, has drawn up a list of vaccines recognized by its health authorities – which includes, among others, those of Pfizer-BioNTech, Moderna and AstraZeneca/Covishield – mentioning that combinations are accepted.

In other words, the mixed vaccination idea is gaining ground.

>> Read also: 6 things you need to know about the third dose of the COVID-19 vaccine and COVID-19: reliable and useful resources


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