Canada’s National Advisory Committee on Immunization (NACI) has updated its guidance, recommending that approved COVID-19 vaccines can be safely mixed and matched in most scenarios.
Under the new recommendations released June 1, people who received a first dose of the AstraZeneca vaccine may receive an mRNA vaccine — Pfizer-BioNTech or Moderna — for their second dose, unless contraindicated.
But it is not recommending AstraZeneca after a first shot of Pfizer or Moderna because of safety concerns and limited data on the use of this combination.
People who have received a first dose of an mRNA vaccine should be offered the same vaccine for their second dose, NACI said. But mRNA vaccines can be interchangeable if the same product is not readily available for the second dose, it added.
In either case, the previous dose should be counted, and the series need not be restarted, the guidance stated.
The non-binding recommendations were based on a range of factors — from safety concerns to vaccine supply, Theresa Tam, Canada’s chief public health officer, said during a news conference Tuesday.
“The interchangeability of vaccines means that you can receive one vaccine product for your first dose and then safely receive a different vaccine for your second dose to complete your two-dose vaccine series for optimal protection from COVID-19,” Tam said.
“This advice provides provinces and territories with effective options to manage their vaccine programs,” she added.
“It is good news that people now have the choice.”
Early data from studies in Europe suggests that mixing doses of COVID-19 vaccines is safe and effective.
Preliminary results from a University of Oxford study published on May 12 found that mixing the Pfizer-BioNtech and AstraZeneca COVID-19 vaccines may increase the frequency of mild to moderate side effects. But these symptoms were short-lived — lasting no longer than a few days — and there were no hospitalizations or other safety concerns.
Meanwhile, a Spanish study released on May 18 showed that the presence of neutralizing antibodies rose sevenfold after people who already received a first shot of AstraZeneca vaccine were given the Pfizer dose, significantly more than the doubling effect observed after a second AstraZeneca shot.
A nationwide study was also launched in Canada last month to look at the safety and effectiveness of mixing and matching different types of shots.
Amid concerns of reports of rare blood clots linked with the AstraZeneca vaccine, NACI said several European countries had begun offering an mRNA vaccine as the second dose to those who received a first shot of AstraZeneca.
The risk of the new blood-clotting syndrome, known as vaccine-induced thrombotic thrombocytopenia, or VITT, was among the considerations for NACI’s updated guidance.
“This is not a new concept,” the Public Health Agency of Canada (PHAC) said in a statement.
“Similar vaccines from different manufacturers are used when vaccine supply or public health programs change.”
Tania Watts, an immunologist and professor at the University of Toronto, said NACI’s new recommendation was “great news,” making it easier to get the second dose into people’s arms.
While NACI makes recommendations for the use of vaccines approved for use by Health Canada, it is ultimately up to the provinces and territories to implement that advice.
Some experts fear that this could lead to wastage of Canada’s AstraZeneca supply.
“It may now be more challenging to get a second dose of the AstraZeneca vaccine into people who remain overly concerned about side effects, despite the fact that VITT, which is quite uncommon, is even less common amongst second-dose recipients,” said Gerald Evans, an infectious disease specialist at Queen’s University in Kingston, Ont.
Tam said it remains to be seen what the actual uptake of AstraZeneca will be following the new guidance.
“We don’t want to be ordering vaccines if we’re not using it, but it can only be ascertained in a more granular way when we see what the vaccine uptake looks like in the coming days,” she said.
— With files from Global News’ Abigail Bimman