Canada to recommend 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 will advise Canadians to combine either the AstraZeneca-Oxford, Pfizer-BioNTech or Moderna shots interchangeably in certain situations.

A registered nurse delivers a Pfizer COVID-19 vaccination to a front-line worker at Vancouver General Hospital in Vancouver, B.C. on March 4. NACI will recommend that a first shot of the AstraZeneca vaccine can be followed by Pfizer, and that the Moderna and Pfizer shots can be used interchangeably. (Ben Nelms/CBC)

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 United Kingdom 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).

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.

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.

"I don't have any concerns with the mixing and matching, knowing the components of the vaccine," she said, adding there were only slight differences in non-serious side effects with Pfizer having slightly higher reports of mild symptoms. "They're fairly on par."

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-BioNTech or Moderna vaccine if they meet provincial eligibility requirements.

B.C. Provincial Health Officer Dr. Bonnie Henry said during a press conference Monday that British Columbians who had an initial dose of Moderna or Pfizer will have the option of receiving either mRNA vaccine as a second shot, with more guidance expected this week on AstraZeneca.

Quebec officials announced in April the province would begin administering second doses to its most vulnerable residents — even if the second dose didn't match the first.

The Public Health Agency of Canada (PHAC) now estimates the rate of VITT in Canadians who have received the AstraZeneca vaccine is between 1 in 83,000 and 1 in 55,000, with a fatality rate of between 20 and 50 per cent, but that is subject to change as more data emerges.

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."

Registered nurse Sarah Moslehi prepares a dose of the Pfizer-BioNTech COVID-19 vaccine at a temporary clinic at the Woodbine racetrack and casino, in northeast Toronto, on May 5, 2021.(Evan Mitsui/CBC)

Kelvin said the future use of the AstraZeneca vaccine needs to "go hand in hand" with the development of a screening mechanism for the condition to identify who's most at risk for the rare blood clots.

"Until we can do that, then we need to at least provide information on what your risk is in general, and discuss using other vaccines as your second dose," she said.

"Having a profile to identify who might be at risk for severe blood clots after the AstraZeneca vaccine would be important moving forward."

Early research shows mixing vaccines safe and effective

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.

It also found that just 1.7 per cent of the participants reported severe side effects, which were limited to headaches, muscle pain and general malaise.

Early results from another recent study out of the U.K., which also informed NACI's guidance, found that mixing different COVID-19 vaccines for second doses could increase the chance of someone having mild or moderate reactions like fatigue, headache or a fever — symptoms which could signal a strong immune response.

The findings were published May 12 as correspondence in The Lancet medical journal and came from the Oxford Vaccine Group's Com-Cov vaccine trial, which is studying the use of different combinations of approved COVID-19 vaccines for first and second doses.


Adam Miller is a senior health writer with CBC News. He's covered health, politics and breaking news extensively in Canada, in addition to several years reporting on news and current affairs throughout Asia.

    With files from Lauren Pelley and Reuters

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