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Canada to recommend mixing and matching AstraZeneca, Pfizer and Moderna COVID-19 vaccines

Posted in Canada, Featured

Published on June 01, 2021 with No Comments

NACI guidance based on early research from U.K., Spain that shows mixing shots is safe and effective.

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

The National Advisory Committee on Immunization (NACI) will update its guidance to provinces and territories in the coming days and recommend that a first shot of the AstraZeneca vaccine can be followed by Moderna or Pfizer, according to sources with direct knowledge of the decision who spoke to CBC on condition of anonymity.

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

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

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

The updated guidelines follows 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).

Prof. Alyson Kelvin, an assistant professor at Dalhousie University and virologist at the Canadian Center for Vaccinology and the Vaccine and Infectious Disease Organization in Saskatoon said the guidelines were “appropriate” given available data.

Kelvin said she believes the Pfizer and Moderna vaccines could be effectively interchanged because of the similarity between the clinical trial data and the real world research on the two mRNA-based vaccines. 

Provinces move to change vaccine dose orders

Dr. Zain Chagla, an infectious diseases physician at St. Joseph’s Healthcare Hamilton and an associate professor at McMaster University, says the proposed approach to mixing and matching COVID-19 vaccines by NACI is reasonable based on available international data. 

“[Pfizer and Moderna] are so interchangeable, I doubt there’s going to be any difference between the two,” he  said. “Their immune responses should be pretty similar.”

Manitoba health officials announced Monday that residents who got a first dose of the AstraZeneca vaccine can receive a second dose of either the Pfizer-Bio NTech or Moderna Vaccine  if they meet provincial eligibility requirements.

PHAC said in a statement to CBC News Thursday there have been 27 confirmed cases of VITT to date in Canada out of two million doses administered, with five deaths among those cases.

Chagla said Canadians who want to have a second dose of AstraZeneca will have access in the coming weeks, but says once supply of Pfizer and Moderna shots are more readily available in Canada it will likely be phased out. 

“Second doses make sense,” he said. “First doses are starting to get a little bit tricky with so much vaccine coming to Canada.”

The Spanish study that informed NACI’s guidance analyzed 670 volunteers between the ages of 18 and 59 who had already received a first dose of the AstraZeneca vaccine, with 450 volunteers given a Pfizer dose.

The study found those who had an initial dose of AstraZeneca vaccine and got a second shot of Pfizer had an increase in IgG antibodies  — which are commonly found in the bloodstream and play a key role in creating memory cells that fight the virus — that were 30 to 40 times higher than in a control group who only received one AstraZeneca dose.

Results from the Spanish study were announced in a press conference May 18 and also found the presence of neutralizing antibodies rose sevenfold after a Pfizer dose — significantly more than after a second AstraZeneca shot.


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