Canada advises mixing and combining COVID-19 vaccines from AstraZeneca, Pfizer, and Moderna.

In some circumstances, Canada is altering its rules on mixing and matching second doses of COVID-19 vaccinations, urging Canadians to mix and match the AstraZeneca-Oxford, Pfizer-BioNTech, or Moderna shots interchangeably. The National Advisory Committee on Immunization (NACI) modified its recommendation to provinces and territories on Tuesday, recommending that an AstraZeneca vaccine be given first, followed by either Moderna or Pfizer.

If the same initial dose is unavailable or unknown, NACI recommends that Canadians who have received the first dosage of Moderna or Pfizer take either of the two shots as a second dosage because they both use a comparable mRNA technology. The amended NACI guideline is based on new studies from Spain and the United Kingdom, which demonstrated that combining AstraZeneca and Pfizer vaccinations to prevent COVID-19 was both safe and effective.

Dr. Theresa Tam, Canada's Chief Public Health Officer, said the choice to combine mRNA vaccinations interchangeably was "nothing new," and that the same approach had previously been applied to other types of vaccinations, such as influenza and Hepatitis A shots.

Previous NACI guidelines stated that a vaccination series that begins with AstraZeneca should be followed up with the same type of shot, and that mRNA vaccine should only be used interchangeably if the same first dose is unavailable or unknown, so the recommendations will have a significant impact on Canada's vaccine rollout. The updated guidelines come after some provinces decided to combine different shots due to AstraZeneca supply issues and a rare but serious type of blood clot that can occur after the shot called vaccine-induced immune thrombotic thrombocytopenia (VITT), which NACI also cited in its guidance as a reason for its decision.

As of May 22, more than 13 million people in Canada have received at least one dose of the Pfizer vaccine, compared to over 3.5 million for the Moderna vaccine and more than 2.1 million for the AstraZeneca vaccine, according to the most available federal government numbers. am stated that NACI considered the "growing" data on AstraZeneca's mRNA vaccine to be sufficient to revise its recommendation and that Canadians should consider "all the facts in front of them" concerning the vaccine's hazards.

According to Prof. Alyson Kelvin, a virologist at the Canadian Center for Vaccinology and the Vaccine and Infectious Disease Organization in Saskatoon, the rules were "appropriate" given existing data. Because the clinical trial data and real-world studies on the two mRNA based vaccinations are similar, Kelvin believes the Pfizer and Moderna vaccines might be interchanged effectively.

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