Canadians should receive the same COVID-19 vaccine for both shots — except in very specific and unlikely situations, according to the Public Health Agency of Canada.

"Currently, no data exist on the interchangeability of COVID-19 vaccines," according to PHAC's recommendations on the use of COVID-19 vaccines.

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.

"The spike proteins encoded by either of the authorized mRNA vaccines have the same sequence and are stabilized in the same manner to remain in the pre-fusion confirmation, though other vaccine components like the lipid nanoparticle may be different," the recommendations read.

"Active surveillance of effectiveness and safety of this mixed schedule will be important in these individuals. Accurate recording of vaccines received will be critical."

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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 told CBC News on Sunday that 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, particularly if one dose is a mRNA vaccine and the other is an adenovirus-based vaccine like those produced by AstraZeneca, Johnson and Johnson or CanSino.

While studies on vaccine mixing could yield interesting developments, he said the theory isn't meant to be part of public policy yet if there is enough access to vaccine products to ensure people receive the same vaccine for both doses.