U.K. study reports mixing AstraZeneca, Pfizer vaccines produces better immunity

OTTAWA – A vaccine study in the United Kingdom reports that getting a dose of the Pfizer-BioNTech vaccine four weeks after a dose of Oxford-AstraZeneca produced a much stronger immune response than two doses of AstraZeneca.

The results are similar to those reported earlier this year from small studies in Germany and Spain and will reinforce the decision to mix and match vaccines in much of Canada.

The National Advisory Committee on Immunization in Canada said June 17 that it is now "preferred" that every Canadian whose first dose was AstraZeneca get an mRNA vaccine from Pfizer or Moderna for their second dose.

NACI cited growing evidence that getting mRNA after AstraZeneca was proving to have better results, and eliminated further risks of vaccine-induced blood clots that have been potentially linked to getting the AstraZeneca vaccine.

The U.K. study at the University of Oxford, where the AstraZeneca vaccine was developed, found mixing Pfizer and AstraZeneca in any order produced better results than two doses of AstraZeneca but that getting AstraZeneca first generated better results than getting it second.

Lead investigator Matthew Snape, an associate professor in pediatrics and vaccinology at Oxford, says the results show the vaccines can be used interchangeably, adding flexibility to the rollout of vaccines around the world.