Across Canada, people have begun receiving their second doses of one of three COVID-19 vaccines. However, due to the AstraZeneca vaccine no longer being administered and a lack of supply with others, the first vaccine you received may not be available for your second dose.
Many Canadians have been left wondering; Is it ok to receive another vaccine for my second dose, or should I wait a few weeks for my “preferred” vaccine to arrive?
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6ixBuzz: Is it “safe” to mix COVID-19 vaccines?
Dr. Krishana Sankar: Yes! It’s a strategy that has worked before for lots of other diseases. A mixed vaccine schedule is not a new concept and has been used to help complete vaccine series’ for influenza, hepatitis A and other diseases.
Many experts agree that mixing two mRNA vaccines (i.e. Pfizer or Moderna) would still be safe while providing protection, especially against variants.
Having a mixed vaccine schedule may even be advantageous for our immune system to create further protection.
6B: How are the Pfizer and Moderna vaccines similar?
Dr. Sankar: The Pfizer and Moderna vaccines are very similar. Both vaccines are mRNA vaccines, which means they use the same technology to generate an immune response to protect our bodies against COVID-19.
The mRNA used in both are the same cellular instructions, just wrapped in different cases.
While their chemical composition of lipids, salts and sugars are slightly different, both Pfizer and Moderna do the same job of inducing our bodies to create antibodies to fight COVID-19 in the same way.
They are both also extremely effective: Pfizer 95% and Moderna 94% at reducing symptoms during clinical trials.
6B: Which vaccine should I get for my second dose if the first one I received isn’t available?
Dr. Sankar: We look to the National Advisory Committee on Immunization (NACI) for guidance, the independent body in Canada responsible for looking at all the available evidence to recommend the best vaccination strategies.
NACI recommends that, if available, the same mRNA vaccine should be used for both doses.
So, if you received Pfizer for your first dose, you would receive Pfizer for your second dose. If you received Moderna for your first dose, you would receive Moderna for your second dose.
But, if one isn’t readily available, they state, “another mRNA vaccine is considered interchangeable and should be used to complete the series.”
That means, if you received Pfizer for your first dose, you can receive Moderna for your second dose. If you received Moderna for your first dose, you can receive Pfizer for your second dose.
For those who had Astrazeneca first, NACI recommends an mRNA vaccine (i.e. Pfizer or Moderna) for the second dose.
6B: Are mixing vaccines more effective or less effective?
Dr. Sankar: As of right now, we don’t actually know. Studies are ongoing. Current results suggest that mixing vaccines could be more effective.
6B: If Canada is saying mixing vaccines is ok, why is the Centre for Disease Control (CDC) in the United States saying it’s not?
Dr. Sankar: Every country has its own independent regulatory processes, so they won’t always match each other.
Germany, Sweden, France, Spain, Italy and Canada all allow some degree of vaccine mixing.
NACI looks at all available evidence and considers important factors such as equity, supply and logistics before making recommendations.
6B: Why should I get a different vaccine than my first dose instead of just waiting for a shipment of the same dose to arrive?
Dr. Sankar: One of the biggest reasons to mix vaccines is to get more second doses into arms quicker. With more dangerous variants like Delta circulating, getting a second dose as soon as possible is even more important than ever.
Being fully vaccinated offers greater protection than a single dose, especially against variants.
If you’re concerned, you should always talk to your healthcare provider about what decision is best for you.