Canadian officials are defending Canada's mix-and-match approach to vaccinations after the World Health Organization warned Monday about “dangerous trends” in individual vaccination strategies, including mixing and matching.

In an online briefing, the WHO’s chief scientist Soumya Swaminathan suggested to reporters that mixing and matching is dangerous because there is not currently enough data to support it.

"It's a little bit of a dangerous trend here. We are in a data-free, evidence-free zone as far as mix and match," Swaminathan said in response to a question about booster shots.

She added that “it will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, a third and a fourth dose."

Canada’s National Advisory Committee on Immunizations, the body that advises federal health officials on vaccines, has said that the Pfizer and Moderna mRNA vaccines against COVID-19 can be used interchangeably when a second dose of the same vaccine is not readily available.

In a statement, a spokesperson for Health Minister Christine Elliott said Ontario plans to continue following the NACI recommendations.

“Ontario continues to follow the advice of the National Advisory Committee on Immunization (NACI) which recommends that it is safe to mix vaccines based on studies from the UK, Spain and Germany that have found that mixing vaccines is safe and produces a strong immune response,” the statement read. “The health and safety of Ontarians remains our top priority, and we will continue to monitor the data working with NACI and the federal government.”

Swaminathan later clarified on Twitter that she was warning against individuals deciding to mix and match on their own.

“Individuals should not decide for themselves, public health agencies can, based on available data,” she wrote, adding that data from various mix and match studies are still awaited.

In a statement to Monday evening, Health Canada and the Public Health Agency of Canada said the NACI weighed a number of factors in making its recommendations.

“NACI reviewed all available evidence from ongoing studies monitoring the mixing of COVID-19 vaccines,” the statement read. “NACI also considered the risk of Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT) associated with COVID-19 viral vector vaccines, Canada’s current and projected mRNA (Pfizer-BioNTech or Moderna) vaccine supply and principles of ethical decision-making. Updated recommendations were based on the current evidence and NACI’s expert opinion.”

Speaking to reporters Monday, Canada’s Procurement Minister Anita Anand said that she and her family have taken mixed doses based on the recommendation of Canadian scientists and health authorities.

“For our part at the federal government, we will continue to follow the science and we will continue to bring in vaccines, so that there is sufficient supply, regardless of what the recommendations are from the scientists,” she said.

While there have not been any studies specifically examining mixing and matching Pfizer and Moderna, the NACI said it was basing its recommendation on sound principles of vaccinology and the fact that both vaccines have very similar efficacy and side effects and function in a very similar way.

“No data currently exist on the interchangeability of COVID-19 mRNA vaccines,” the NACI said in its recommendation. “However, there is no reason to believe that mRNA vaccine series completion with a different authorized mRNA vaccine product will result in any additional safety issues or deficiency in protection.”

In a tweet, infectious disease specialist Dr. Abdu Sharkawy said the WHO statement should not be taken as a factual assertion that mixing vaccines is not safe.

“Let's be clear. This does NOT mean that mix/match is not effective or safe,” Sharkawy wrote. “Certainly enough emerging data to support both. But decisions should be guided locally, regionally by PH (public health units)/medical expertise not individuals.”

Provincial and local health officials have adopted NACI’s recommendation on mixing and matching mRNA vaccines and have urged people to take whichever vaccine is available to them when they go for either the first or second doses.

The recommendation to mix the two vaccines came as Moderna finally delivered millions of promised doses to Canada at the same time that Pfizer experienced some shipment delays.

Some health officials have dismissed concerns around mixing the two vaccines as nothing more than brand preference and have urged people to get fully vaccinated as soon as possible in order to be protected from the more transmissible Delta variant, first identified in India and quickly spreading in Ontario.

The NACI has also recommended that anyone who got AstraZeneca for their first dose preferentially get an mRNA shot for their second dose, based on preliminary research showing a stronger immune response from mixing the two vaccine types than from taking two shots of AstraZeneca as well as the fact that the mRNA vaccines do not carry the same risk of rare blood clots associated with AstraZeneca.

A number of other countries have also adopted mixing and matching to some extent.

While the U.S. Centers for Disease Control and Prevention has said it does not consider the mRNA vaccines to be interchangeable, it concedes that one can be used in place of another in circumstances where the first dose vaccine is not available for the second.

Health Canada noted in its statement to that “vaccine interchangeability is not a new concept.”

“Similar vaccines from different manufacturers are used when vaccine supply or public health programs change. Different vaccine products have been used to complete a vaccine series for influenza, hepatitis A, and others,” the agency said.

“More results from ongoing studies on mixing COVID-19 vaccines are expected in the coming months (including this Canada-wide research study).NACI continues to closely monitor evolving evidence on mixing COVID-19 vaccines. NACI will update recommendations as needed.”

-          With files from Reuters