Canada's National Advisory Committee on Immunization said Thursday people who got the AstraZeneca vaccine as their first dose should get Pfizer-BioNTech or Moderna for their second shot.

On June 1, committee had said AstraZeneca recipients "could" get Pfizer or Moderna for their second shot if they wanted, but Thursday went further to say an mRNA vaccine was the "preferred" choice.

Since the advisory committee "first looked at mixed vaccine schedules, new evidence is starting to emerge suggesting immune responses are better when a first dose of the AstraZeneca vaccine is followed by an mRNA vaccine as a second dose,″ said its vice-chair Dr. Shelley Deeks, in the new guidance documents.

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The committee also updated its previous recommendation that people at high risk of exposure to, or serious illness from, COVID-19, could opt to get AstraZeneca rather than waiting for Pfizer or Moderna. Now it says everyone should always get the mRNA vaccines first, unless they are allergic to them.

Deeks said the advice is based on the growing supply of Pfizer and Moderna, and the risk of vaccine-induced blood clots associated with AstraZeneca. But she is still trying to reassure people who got one or two doses of AstraZeneca’s vaccine that they are nevertheless well protected.

"Anyone who has already received two doses of AstraZeneca/Covishield can rest assured that they are protected, particularly against severe illness," she said. "There is no need for a third dose at this time.″

Dr. Theresa Tam, the chief public health officer of Canada, said the new evidence in favor of mixing different types of vaccines included four studies in Germany.

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"It’s really the immunogenicity, that immune response, after that mixed-dose schedule that’s resulted in NACI updating this recommendation, because all the accumulated studies essentially suggest that immune response is a better response,″ said Tam at a news conference in Ottawa.

One of those studies from Germany’s Saarland University, published early data Wednesday saying giving Pfizer as the second dose after AstraZeneca, or two doses of Pfizer only, generated far more antibodies and T cells as two doses of AstraZeneca.

Almost 25 million Canadians have now received at least one dose of vaccine, and almost 6 million are now fully vaccinated. As of June 5, 2.1 million people had received one dose of AstraZeneca, and 15,186 had received two doses.

There are no further shipments of AstraZeneca currently scheduled, but there are about 600,000 doses still left from previous deliveries.

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There are 14 million doses of Pfizer-BioNTech and Moderna being delivered this week and next, including a donation of 1 million doses of Moderna’s COVID-19 vaccine from the United States that was to arrive in Canada Thursday night.