Canada was an outlier on mixing COVID-19 vaccines, but more countries now following suit

Canada became an outlier in the global fight against COVID-19 by allowing residents to mix various coronavirus vaccines.

But while the approach, in place for more than a month, has been controversial — prompting concern among some Canadians, and preventing people from travelling abroad to certain destinations — it's now increasingly being explored by other countries.

"It's not unusual to mix and match," said Angela Rasmussen, a virologist with the Vaccine and Infectious Disease Organization in Saskatoon.

"I think that it does create a more flexible situation which is really good overall, because we don't need to just think about vaccinating everybody in Canada — we need to think about vaccinating everybody in the world."

A growing number of countries are considering swapping in different COVID-19 vaccines into their programs as second doses or boosters, in light of supply delays and safety concerns tied to certain other shots. 

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Bahrain, Bhutan, Indonesia, Italy, Thailand, the United Arab Emirates, Uruguay and Vietnam are among those now exploring or actively pursuing mixed-dosing strategies.

It's a move increasingly backed by new research, though it's rooted in decades of vaccine science, according to Rasmussen and other vaccine experts.

"Combining vaccines is nothing new," she said. "There's no reason to expect that it wouldn't be safe."

"It is already accepted around the world for other vaccines," she said. "Do you ask what flu vaccine you get every year? They're made by different manufacturers."

For some countries, including Italy and Vietnam, the goal is to provide people who were first immunized with the AstraZeneca-Oxford vaccine with a second dose of an mRNA product, such as Pfizer-BioNTech.

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That's similar to Canada's guiding recommendations from the National Advisory Committee on Immunization (NACI), which allow swapping in different vaccines for different doses in certain situations. 

The advisory body recommends that either AstraZeneca or mRNA-based vaccines — Moderna's is the other approved in Canada — can be offered as a second dose for people who had a first shot of AstraZeneca.

But the group notes that mRNA options are preferred as a second dose, thanks to emerging safety evidence and the possibility of a better immune response.

Mixing doses of the Pfizer and AstraZeneca vaccines actually generate a stronger immune response than having two rounds of AstraZeneca, according to not-yet-peer-reviewed U.K. research published in June.

More recently, as Reuters first reported, a similar study of nearly 500 medical workers in South Korea also showed that approach — AstraZeneca first, followed by a Pfizer booster — increased neutralizing antibody levels by six times, compared to two doses of AstraZeneca.

Canada's NACI also calls for the same mRNA vaccines to be used for a second dose if possible, but says another mRNA shot "can be considered interchangeable" if the first type is unavailable.

'Accepted' approach

When it comes to rising global uptake of a mixed vaccine approach, there's a "certain degree of pragmatism," said Dr. Alexander Wong, an infectious diseases specialist with the University of Saskatchewan. 

Not every country has "superb access" to mRNA shots, which are "really kind of the bottom line," he said. 

Some countries, like Thailand and the United Arab Emirates, are allowing the Pfizer shot to be used as a booster for those first vaccinated with one of the leading Chinese vaccines, made by Sinovac and Sinopharm — though there's no research on whether that would hike people's level of protection.

"We just need some more hard and fast clinical data," he said.

Wong also says there's still controversy around the world about mixing different brands and technologies.

While some countries are adopting a mixed dosing schedule, others are outright blocking people from entry if they mixed certain vaccines — and it's a similar situation with some cruise lines and overseas tourist attractions.