Mixing COVID-19 vaccines in Canada: AstraZeneca, Pfizer and Moderna vaccines can be used interchangeably between doses, experts say

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The National Advisory Committee on Immunization (NACI) has now said that Canadians who got one dose of a AstraZeneca COVID-19 vaccine can receive the same type of vaccine for their second dose or an mRNA COVID-19 vaccine product — either the Pfizer or Moderna vaccines.

For anyone who received an mRNA vaccine, they should be offered a second dose of vaccine from the same manufacturer, but if the vaccine is "not readily available" then another mRNA vaccine can be offered.

"The interchangeability of vaccines means that you can receive one vaccine product for your fist dose and safely receive a different vaccine for your second dose to complete your two-dose vaccine series for optimal protection from COVID-19," Dr. Theresa Tam, Canada’s chief public health officer, said on Tuesday.

The report from NACI states that "no data currently exist on the interchangeability of COVID-19 mRNA vaccines" but currently there is "no reason to believe that mRNA vaccine series completion with a different authorized mRNA vaccine product would result in any additional safety issues or deficiency in protection."

Dr. Tam explained that studies on the interchangeability of mRNA vaccines are ongoing but stressed that the vaccines are very similar and they are both targeting the spike protein in a similar formation.

"The key aspects of their makeup as well as that specific spike protein target produced by Pfizer-BioNTech and Moderna are essentially very similar," she said.

Dr. Howard Njoo, deputy chief public health officer, added that in the past for different diseases, vaccines from different companies can be used interchangeably with "great success."

NACI looked at studies of mixed scheduling from the U.K., Germany and Spain, which also showed that there is a possibility of increased short-term side effects when mixing vaccines, including headaches and fatigue, but Dr. Tam stressed these are "temporary and without complications."

"This is a pandemic, we will get real-life data and it is great that we do, which means that the committee has to update its recommendations on an ongoing basis," Dr. Tam said. "But on top of the real-life data...are those general immunological and vaccine knowledge that have been built over decades of vaccine programatic use and the immunology that goes with it."

"Canadians should be reassured that we are here to be transparent and give them the information as it becomes available."