Canada's National Advisory Committee on Immunization (NACI) recommends Canadians who received a first shot of the AstraZeneca vaccine to get a Pfizer or Moderna vaccine for their second shot.

On June 1, NACI said AstraZeneca recipients could choose whether to get a second dose of the same vaccine, or an mRNA vaccine. But in a new guidance released Thursday, NACI said Pfizer or Moderna is now "preferred" as the second dose.

"An mRNA vaccine is now preferred as the second dose for individuals who received a first dose of the AstraZeneca/COVISHIELD vaccine, based on emerging evidence of a potentially better immune response from this mixed vaccine schedule," NACI said in a statement on Thursday afternoon.