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

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."

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.

WATCH | Why Canada has been slow to get COVID-19 vaccines in arms:

Why Canada has been slow to get COVID-19 vaccines in arms

4 years ago

Duration 3:22

The provinces are behind targets of getting COVID-19 vaccine into Canadians' arms, and experts say logistical challenges are largely to blame but are hopeful the arrival of the Moderna vaccine will help speed things up.

While Canada's approach could change based on any gleanings from these studies, 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," he said. "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."

British guidelines OK mixing in certain instances

New guidelines from the British government also said there is no evidence to support vaccine interchangeability, "although studies are underway."

The advice said that while every effort should be made to complete the dosing regimen with the same vaccine, patients can be given different vaccines if they are at "immediate high risk" or are considered "unlikely to attend again."

"[If] the same vaccine is not available, or if the first product received is unknown, it is reasonable to offer one dose of the locally available product to complete the schedule," according to the U.K. guidelines, which were published on New Year's Eve.

Dr. Mary Ramsay, head of immunizations at Public Health England, said this would only happen on extremely rare occasions, and that the government was not recommending the mixing of vaccines, which require at least two doses given several weeks apart.

"Every effort should be made to give them the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all," she said.