Canada Recommends Mixing and Matching AstraZeneca, Pfizer, and Moderna COVID-19 Vaccines

Canada is changing its guidelines on mixing and matching second doses of COVID-19 vaccines and is now advising Canadians to combine either the AstraZeneca-Oxford, Pfizer-BioNTech or Moderna shots interchangeably in certain situations.

The National Advisory Committee on Immunization (NACI) updated its guidance to provinces and territories Tuesday and recommended that a first shot of the AstraZeneca vaccine can be followed by either Moderna or Pfizer.

For Canadians who have had the first dose of Moderna or Pfizer, NACI recommends they can now take either of the two shots as a second dose — because they both use a similar mRNA technology — if the same first dose is unavailable or unknown.

The updated NACI guidance is based on emerging research from Spain and the United Kingdom that found mixing and matching AstraZeneca and Pfizer vaccines was both safe and effective at preventing COVID-19.

Canada’s Chief Public Health Officer Dr. Theresa Tam said the decision to combine mRNA vaccines interchangeably was “nothing new” and that the same principle had been applied to different types of vaccines in the past — including shots for influenza and Hepatitis A.

The recommendations will have a major impact on Canada’s vaccine rollout, with previous NACI guidelines stating that a vaccination series that begins with AstraZeneca should follow up with the same type of shot and those mRNA vaccines should only be used interchangeably if the same first dose is unavailable or unknown.

The updated guidelines follow moves by some provinces to combine different shots due to issues with the supply of AstraZeneca and a rare but serious type of blood clot that can result after the shot called vaccine-induced immune thrombotic thrombocytopenia (VITT), which NACI also cited as a reason for its decision in its guidance.

More than 13 million people have received at least one dose of the Pfizer vaccine in Canada, in contrast to over 3.5 million of the Moderna vaccine and more than 2.1 million of the AstraZeneca vaccine as of May 22, according to the latest available federal government data.

Dr. Zain Chagla, an infectious disease physician at St. Joseph’s Healthcare Hamilton and an associate professor at McMaster University, said it is reasonable to mix and match the NACI COVID-19 vaccine, based on available international data. Canadians who want a second dose of AstraZeneca will get it in the coming weeks, he said, but once the supply of Pfizer and Moderna becomes larger, this will be phased out.

Professor Alyson Kelvin, an assistant professor at Dalhousie University and a virologist at the Canadian Immunization Center and the Vaccine and Infectious Diseases Foundation in Saskatoon, said future use of the AstraZeneca vaccine needed must be coupled with a condition screening mechanism to determine who is at risk of developing a blood clot.

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