According to Canada’s National Advisory Committee on Immunization (NACI), people who received the AstraZeneca vaccine first should get the Pfizer-BioNTech or Moderna vaccine for their second shot.

Previously, the advisory committee had stated that AstraZeneca recipients could get Pfizer or Moderna for their second shot if they so desired. On Thursday, the committee noted that an mRNA vaccine was the preferred option. 

The NACI’s reversal comes after multiple studies from Germany emerged suggesting immune responses are better when a first dose of the AstraZeneca vaccine is followed by an mRNA vaccine as a second dose.

Saarland University in Germany published results Wednesday that administering Pfizer as the second dose following AstraZeneca, or two doses of Pfizer, resulted in more antibodies and T cells than two doses of AstraZeneca.

Another reversal by the NACI is that individuals at high risk of exposure should always get the mRNA vaccines first unless they are allergic. Previously the committee recommended the AstraZeneca vaccine rather than waiting for Pfizer or Moderna.

The committee’s guidance update, stated by vice-chair Dr. Shelley Deeks, is also motivated by the increasing availability of Pfizer and Moderna and the risk of vaccine-induced blood clots associated with AstraZeneca.