Canadians should get same COVID-19 vaccine for both doses — except in 'extremely unlikely’ cases.

However, the recommendations state that „attempts should be made to complete the vaccine series with a similar type of COVID-19 vaccine” if the product used for the first dose is unavailable or unknown. The two vaccines currently approved for use in Canada — manufactured by Pfizer-BioNTech and Moderna — are both messenger RNA (mRNA) vaccines.

Dr. Zain Chagla, an infectious disease physician and associate professor of medicine at McMaster University in Hamilton, said it is „extremely unlikely” that someone wouldn’t know which vaccine they were given. Chagla said it’s been one of the government’s mandates that people have documentation on which vaccine they received, along with a lot number in case any adverse reactions are linked to a particular vial.

Chagla said the prospect of mixing vaccines requires further study in clinical trials, but he does not recommend mixing vaccines until there is evidence to support it. „Theoretically, yes, they could be synergistic, but theoretically they could blunt each other out, you might make the wrong response to one and then have the other on board,” Chagla told CBC’s Benjamin Blum. „And so as much as we think one plus one equals two, it may not. It may be one plus one equals zero in this sense.”