Canada, like several other countries, has been mixing and matching COVID-19 vaccines for weeks amid safety concerns over the AstraZeneca shot.

On Monday, the chief scientist at the World Health Organization (WHO) cautioned against that approach, calling it “a dangerous trend” for a subsequent dose as well as booster shots saying there was insufficient evidence available about the health impact.

“There is limited data on mix and match,” said Soumya Swaminathan during a virtual news conference.

“Maybe it will be a very good approach but at the moment we only have data on the Oxford-AstraZeneca vaccine followed by Pfizer,” she added.

However, her concerns stemmed from individuals deciding to mix vaccines or take additional doses on their own without public health guidance.

In Canada, the National Advisory Committee on Immunization (NACI) has recommended since June that people who received a first dose of the AstraZeneca vaccine should get an mRNA vaccine — Pfizer-BioNTech or Moderna — for their second dose, unless contraindicated.

People who have received a first dose of an mRNA vaccine should be offered the same vaccine for their second dose, NACI said. But mRNA vaccines can be interchangeable if the same product is not readily available for the second dose, it added.

The non-binding recommendations were based on a range of factors from safety concerns to vaccine supply, said Theresa Tam, Canada’s chief public health officer, during a news conference on June 1.