Coronavirus: WHO cautions data still limited on mixing COVID-19 vaccines, but Canadian officials say it’s OK


Canadian officials and doctors are defending sanctioning the mixing and matching of COVID-19 vaccines, after the World Health Organization’s (WHO) chief scientist issued a new caution about citizens opting to receive vaccines from different manufacturers, saying that the data remains limited on the practice.

On Monday, WHO Dr. Soumya Swaminathan said that the global health agency has received “a lot of queries” about people who have received a first dose of a COVID-19 vaccine from one pharmaceutical company and were planning to take another shot from a different authorized drug maker.

She called it: “A little bit of a dangerous trend.”

The comment was made in the context of a conversation about booster doses, and has reached out for more clarity about the WHO’s position.

“We are in a data-free, evidence-free zone as far as mix and match. There is limited data on mix and match,” she said, cautioning that it could become a “chaotic situation” in countries where citizens decide to take “a second, or a third, or a fourth dose.”

While this may cause concern among Canadians, who have been told that mixing vaccines is acceptable based on National Advisory Committee on Immunization (NACI) guidance, federal and provincial officials were quick to defend Canada’s approach and the health experts who suggested it, saying that all decisions are being made with peoples’ health and safety at the forefront.

“In terms of the dose regimens and the recommendations, those are decisions that are at the end of the day made by the provinces and territories who are responsible constitutionally for the administration of health care in their individual jurisdictions,” said Procurement Minister Anita Anand. “For our part, the federal government will continue to follow the science… as well as the guidance that’s coming out from other bodies.”

Anand also noted that she is among the numerous Canadians who have taken a mixed vaccine regime, having a first shot of Pfizer followed by a second dose of Moderna’s vaccine.


Swaminathan noted that while there has been data on the efficacy of a first dose of AstraZeneca followed by a second dose of the Pfizer-BioNTech vaccine, there is limited data on the interchangeability of mRNA vaccines.

This was also noted by NACI when it updated its guidance, but the volunteer body said it made the recommendation based on the similarity of mRNA technology between Pfizer and Moderna.

“At this time, there is no reason to believe that mRNA vaccine series completion with a different authorized mRNA vaccine product would result in any additional safety issues or deficiency in protection,” NACI said when it issued its guidance on mixing in early June.

Specifically, NACI—which is separate and independent from Health Canada, who authorizes vaccines and therapies for use in this country— have said that either Pfizer or Moderna was fine as a second shot for those who received AstraZeneca for their first.

The agency also said that while Canadians should be offered the same mRNA vaccine for their second as they received for their first if it’s available, a second shot of either Pfizer or Moderna was acceptable.

Asked about the WHO’s latest, Chief Medical Officer of Health for Nova Scotia Dr. Robert Strang said Monday that people should still take the first available dose offered.

“We have lots of available Moderna, lots of people had their first dose of Pfizer. The best thing they can do and it’s very safe and effective, and NACI agrees with that, is to take the first appointment they can get,” he said.

Manitoba’s Chief Provincial Public Health Officer Dr. Brent Roussin also backed up Canada’s approach as one that has worked in the past with other vaccines.

“We will always be looking at evidence as it comes out… The evidence that we’ve seen to date is that the efficacy of mixing doses is the same… the basic premise of interchangeability of vaccines is there,” he said.

Swaminathan said that while, in time, mixing and matching vaccines may be found to be a “very good approach,” the evidence is not there yet.

In an interview on CTV News Channel, ICU physician and associate professor of medicine at the University of Saskatchewan Dr. Hassan Masri questioned the WHO official’s messaging, saying that it wasn’t reflective of the more recent studies into mixing, didn’t take into account the historical scientific premise, and could lead to more vaccine hesitancy.

“There should be no scientific reason to have a problem with mixing and matching for example, Moderna and Pfizer, they’re almost identical mRNA vaccines,” he said. “There should be no reason really to say that it’s not appropriate, or that it’s not safe.”

He also defended NACI’s decision to sign off on mixing in the name of seeing as many shots get in arms as quickly as possible.

“What we see here in Canada, is that our numbers are coming down quite quickly,” he said, referring to daily COVID-19 case counts. has asked NACI for comment on the latest messaging from the WHO.


Further, Swaminathan said that in some countries where vaccine supply is flush, people are “voluntarily” starting to think about taking an additional dose. This is something she warned against, both from a global vaccine supply perspective—using up doses for third shots for some before others in developing nations have even had a first—as well as from a data perspective.

“There is no scientific evidence to suggest that boosters are definitely needed,” she said. “We have seen in some countries an increase in infections, but no significant increase in hospitalization or deaths. And it may well be that you need boosters after a year or two years, but at this point after six months of the primary dose, there doesn’t seem to be any indication.”

This comes after Pfizer has signalled that a third dose of its COVID-19 vaccine may be beneficial after seeing indications of lessened immunity over time. While Israel is making plans to offer booster shots to high risk citizens, so far both U.S. and Canadian health officials are cautioning that it may be too soon to make that call.

Anand said Monday that while Pfizer has yet to ask Health Canada to consider authorizing a booster dose, there are contracts in place for booster doses this year and in years following if they are required.

Swaminathan said that the WHO will make recommendations on boosters “when we think that they are needed” and based on the data rather than on “individual companies declaring that their vaccines should now be administered as a booster dose.”

Masri said that from his front-line perspective he agrees that pursuing booster doses should be better organized and not left up to individual citizens, and if that was the message the WHO was trying to get across, it could have been done more clearly.

“To come out and make such a broad statement that mixing and matching is dangerous, I think that announcement itself is dangerous.”

With files from’s Brooklyn Neustaeter

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