Is it OK to mix and match different vaccines? Your COVID-19 questions answered

Health officials don't recommend having different types of COVID-19 vaccines for your two shots, but doctors say a unique clinical trial in the United Kingdom could inform a more flexible rollout in Canada.

U.K. trial explores scientific potential of more flexible vaccine rollouts

A woman gives the thumbs up after receiving her first dose of Pfizer-BioNTech's COVID-19 vaccine at a mass vaccination clinic in Toronto's east end run by the Scarborough Health Network last week. (Evan Mitsui/CBC)

We're answering your questions about the pandemic. Send yours to, and we'll answer as many as we can. We publish a selection of answers online and also put some questions to the experts during The National and on CBC News Network. So far, we've received more than 72,000 emails from all corners of the country.

Does your second dose of COVID-19 vaccine have to be the same brand as your first?

Health officials don't recommend having different types of COVID-19 vaccines for your two shots, but doctors say a unique clinical trial in the United Kingdom could inform a more flexible rollout in Canada.

People like Joseph Frisina of Montreal are asking CBC News about the pluses and minuses of receiving a first dose of one vaccine and a different brand for the second shot.

Frisina said he received his first dose of the AstraZeneca-Oxford vaccine in Quebec and plans to be in the U.S. when time comes for his second, where he'll likely be offered Pfizer-BioNTech. He wants to know if it could be risky.

Dr. Lisa Barrett, an infectious diseases physician and researcher at Dalhousie University in Halifax, said she doesn't see any reason to be worried about safety.

WATCH | Does it matter if your first vaccine dose is the same brand as your second?:

New trial looks into mixing COVID-19 vaccines

3 years ago
Duration 2:04
A new trial is looking into the possibility of getting different types of COVID-19 vaccines for each dose and what it could mean for the fight against COVID-19.

Barrett likens the first shot of the two-dose coronavirus vaccines to giving your immune system an elementary school education. The second dose broadens the response to a university level.

"What your first shot does is it sends up a battle cry, if you will, to your immune system to rally those specific troops together to get an immune response that's good to this particular part of COVID vaccine and then it adds some memory," Barrett said.

"The quality of the immune response after one shot is generally OK, but still not highly sophisticated."

That's why federal, provincial and territorial health officials all recommend that people be fully immunized to protect against COVID-19.

What's guiding mix-and-match decisions?

Barrett said she is looking forward to when results from a trial at the University of Oxford shed light on mix-and-match vaccinations.

Unlike typical clinical trials comparing a vaccine to a dummy shot, the 800 volunteers aged 50 and over in England will either be given:

  • A Pfizer "prime" dose followed by a "booster" from AstraZeneca.
  • AstraZeneca followed by Pfizer.
  • The standard approach of receiving the same vaccine twice as a comparison.

"There are definite advantages to having data that could support a more flexible immunization program, if needed and if approved by the medicines regulator," said Jonathan Van-Tam, the U.K.'s deputy chief medical officer, in announcing the 13-month study in February. 

If clinical trials support mixing and matching doses of different COVID-19 vaccines, doctors say it could help in containing the pandemic faster. (Clodagh Kilcoyne/Reuters)

Participants will also be administered their first and second doses either four or 12 weeks apart to see what works best.

"With that knowledge, we can have complete confidence that a number of different vaccines can be rolled out and implemented in a population very quickly, very efficiently, without any concern about inadvertent mixing of the combination," Prof. Paul Heath, director of the Vaccine Institute at St. George's, University of London, told CBS News.

What's more, Heath said, if mixing vaccines succeeds, it could trigger better protection against more contagious variants of concern. 

The makers of mRNA vaccines are already tweaking their vaccines in face of the B1351 variant first identified in South Africa. 

Some vaccinologists suspect that getting a different type of COVID-19 vaccine for the second dose could stimulate immune cells to focus on the right target to tackle variants of concern.

With files from CBC's Christine Birak