The World Health Organization’s (WHO) chief scientist issued a new caution about mixing COVID-19 vaccines from different manufacturers.

On Monday, Dr. Soumya Swaminathan called the mixing of vaccines “a little bit of a dangerous trend” that could lead to a “chaotic situation” in countries where citizens decide to take “a second, or a third, or a fourth dose.”

“We are in a data-free, evidence-free zone as far as mix and match. There are limited data on mix and match,” she said.

Swaminathan also said 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 are limited data on the interchangeability of other vaccines.

In Canada, the National Advisory Committee on Immunization (NACI) has advised Canadians that mixing vaccines is both safe and effective. 

In June, federal officials advised Canadians that a first dose of the AstraZeneca-Oxford vaccine can be combined with a dose of the Pfizer-BioNTech or Moderna shots “unless contraindicated.” NACI also advises that the Pfizer and Moderna vaccines can be mixed for first and second doses.

“NACI has worked to quickly adapt this guidance on the use of COVID-19 vaccines in Canada to ensure optimal protection of Canadians at pace with the ever changing circumstances during this pandemic,” Canada’s Chief Public Health Officer Dr. Theresa Tam said at the time.

“This advice provides provinces and territories with safe and effective options to manage the vaccine programs,” she added.

The WHO’s comments left Canadian officials and doctors scrambling and defending the approach of mixing vaccines. 

“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.”

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