The National Advisory Committee on Immunization says it is now recommending people who got the Oxford-AstraZeneca vaccine first should get Pfizer-BioNTech or Moderna for their second shot.
Previously, NACI had said AstraZeneca recipients could choose whether to get a second dose of the same vaccine, or an mRNA vaccine. But in new guidance released Thursday, NACI says Pfizer or Moderna are now "preferred" as the second dose.
The guidance is based on growing evidence that a second dose of an mRNA vaccine produces a stronger immune response, and because of the low but serious risk of vaccine-induced blood clots associated with getting AstraZeneca.
The advice doesn't mean people can't still choose AstraZeneca if they want, or if they are allergic to the mRNA vaccines.
"People who received two doses of AstraZeneca/COVISHIELD vaccine can rest assured that the vaccine provides good protection against infection and very good protection against severe disease and hospitalization," NACI's advice says.
As of June 5, 2.1 million Canadians had received one dose of AstraZeneca, and 15,186 had received two doses.
There are no further shipments of AstraZeneca scheduled, but there are still about half a million doses of it in provinces.
There are 14 million doses of Pfizer-BioNTech and Moderna being delivered this week and next, including a donation of one million doses of Moderna's COVID-19 vaccine from the United States that arrived in Canada on Thursday.
The donated doses are part of an American promise to donate 80 million doses of vaccines by the end of June.
Jeff Zients, the White House COVID-19 response co-ordinator, said the shipment to Canada followed one to Mexico on Tuesday.
The donation won't cost Canada anything, and will be on top of the 44 million doses Canada has purchased directly from Moderna.
It comes less than a week after Canada promised to donate 13 million doses to the COVAX vaccine-sharing alliance, which will entirely come from the remaining share of doses Canada had purchased from COVAX itself.
Canada has been heavily criticized for buying three times as many doses of COVID-19 vaccine than it needs and not sharing any doses from its own direct purchase agreements with vaccine makers.