A vaccination study in the UK 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 by small studies in Germany and Spain and reinforce the decision to mix and match vaccines in much of Canada.
The National Advisory Committee on Immunization in Canada said that on June 1 there was enough evidence on the safety of mixing two vaccines to say that provinces could start offering Pfizer or other mRNA vaccine from Modern as second dose to people who have had AstraZeneca before.
On June 17 NACI went further and said that growing evidence that mixing vaccines creates a better immune response meant it was now “preferred” for AstraZeneca recipients to use Pfizer or Moderna accordingly.
NACI dropped on the AstraZeneca vaccine in April when it was potentially linked to rare but severe vaccine-induced blood clots that were not found in people who received Pfizer or Moderna.
Health Canada says the vaccine is still safe and effective, with the risks posed by COVID-19 being higher than the risk of vaccine-induced blood clots. NACI however said that if there is mRNA available it is the preferred type of vaccine.
As a result, most provinces stopped using AstraZeneca for the first doses in mid-May. In the last two weeks of May, less than 18,000 new AstraZeneca shots have been administered, out of more than 5.4 million shots given across Canada.
Most provinces still limit the first doses of AstraZeneca to those who cannot use Pfizer or Moderna for some reason, such as an allergy. But supplies are also a problem. Pfizer and Moderna have collectively shipped more than 30 million doses since the beginning of May, while Canada has received about 1.45 million doses of AstraZeneca.
In early June, all provinces moved to start offering Pfizer or Moderna as a second dose to those who received AstraZeneca earlier. Health Canada says that in the first three weeks of June 459,000 people received a series of mixed vaccines, although this number is not broken down by vaccine. It will include people who have also mixed the two mRNA vaccines.
The UK study at Oxford University, where the AstraZeneca vaccine was developed, found that mixing Pfizer and AstraZeneca in each order produced better results than two doses of AstraZeneca but that getting AstraZeneca earlier generated better results than it did. the second.
Two doses of Pfizer have produced the best antibody results – proteins that help neutralize viruses when they enter your body. A dose of AstraZeneca followed by Pfizer produced the best T cell results. T cells are a type of white blood cell that attacks foreign particles in the body, including viruses.
However, the study authors warn that the results so far are based on a four-week interval and it is already known from other studies that AstraZeneca worked best if the second dose was given eight to 12 weeks later.
Further results with a 12-week interval will be available in July.
Lead researcher Matthew Snape, an associate professor of pediatrics and vaccinology at Oxford, says the results show that vaccines can be used interchangeably, adding flexibility to the development of vaccines around the world.
The study is also currently looking at mixed vaccine programs that include Moderna and the vaccine also not authorized by Novavax.