A UK vaccine study found that four weeks after receiving Oxford-AstraZeneca, a dose from Pfizer-BioNTech vaccine produced a stronger immune response than two doses.

These results are similar to the ones from earlier in the year in Germany and Spain. They will support the decision to mix and combine vaccines in large parts of Canada.

According to the National Advisory Committee on Immunization Canada, June 1 was the first time that enough evidence had been collected about the safety of mixing vaccines. Provinces may now offer Pfizer or Moderna’s mRNA vaccine as a second dose for people who have received AstraZeneca.

NACI stated that growing evidence suggests that vaccines mixed together can improve immune responses. AstraZeneca recipients should now be able to choose between Moderna or Pfizer second.

NACI voted against the AstraZeneca vaccination in April after it was linked to serious, rare and severe vaccine-induced blood clots. This has not been seen in those who have received Moderna or Pfizer.

Health Canada claims that the vaccine is safe and effective. However, the risks of COVID-19 are higher than those associated with vaccine-induced bloodclots. NACI stated that mRNA is preferred if it is available.

Most provinces have stopped using AstraZeneca as a first dose by May middle. Out of the more than 5.4 millions shots across Canada, less than 18,000 AstraZeneca shots were administered in the last two weeks.

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Many provinces still limit first doses for AstraZeneca to people who are unable to get Moderna or Pfizer because of an allergy. However, supplies are also a problem. Moderna and Pfizer have shipped over 30 million doses between May 1st and Canada has received 1.45 million AstraZeneca tablets.

All provinces began offering Moderna or Pfizer as a second dose to AstraZeneca-first-time recipients in June. Health Canada claims that 459,000 people received a mixed vaccine series in the first three weeks. However, this number is not broken down according to vaccine. This will also include those who received both mRNA vaccines.