A 2nd dose of AstraZeneca, or mix and match? Advice from B.C. immunologists

The more than a quarter million British Columbians who received the AstraZeneca vaccine have an important decision to make: stick with AstraZeneca for their second dose, or opt for Pfizer’s mRNA vaccine?

With Canada’s National Advisory Committee on Immunization now saying that AstraZeneca recipients should be given the choice on which vaccine to take for their second dose, 275,000 B.C. residents will be seeking information on how to decide.

“For folks that are not immunologists or don’t have sufficient understanding of what’s been going on, they just need to be protected, this is a really tough choice,” said Ismael Samudio, founder of the Vancouver research company Immunity Diagnostics.

There are pros and cons related to choosing either Pfizer or AstraZeneca as a second dose, but much more data available on the outcomes associated with opting for AstraZeneca.

“It’s been proven to be safe and efficacious on that schedule with a huge number of people, that’s why it was licensed in the first place,” said Vancouver immunologist Dr. Kelly McNagny. “The cons would be that there is always concern with (blood) clotting.”

The risk for the rare blood clotting condition associated with the AstraZeneca vaccine is lower for those receiving it a second time. About one in 600,000 people who received a second dose of AstraZeneca experienced a blood clot. But there is no risk of clotting after taking Pfizer.

There is preliminary data suggesting mixing and matching with Pfizer after a first dose of AstraZeneca may have more side effects, but that it also creates a stronger immune response.

“Side effects tend to tell you how strong the vaccine response is, and if people are showing more side effects, that’s telling you they’re actually getting a better boost in their immune response,” McNagny said.

Samudio agrees, saying “If you are gung ho about your immunity and you want a really strong immune response, and you are less concerned about any potential side effects, yeah you should mix the vaccines.”

Both McNagny and Samudio say they would personally choose a second dose of AstraZeneca, but McNagny is advising British Columbians to take whichever is offered first.

“Whatever you can get at the soonest date is what I would go with. And with the mRNA vaccines rolling in, I think it’s likely that’s what they’ll be able to get. And they should feel comfortable with that,” he said.

Provincial health officer Dr. Bonnie Henry is expected to release more information about second shots for AstraZeneca recipients, including the interval between doses, on Thursday. But it’s unclear if she will recommend one vaccine over the other, or tell British Columbians it’s totally up to them.

Samudio says the decision is daunting and people are looking for more than just information, they want advice on what to do. “I would prefer health authorities to give more guidance,” he said. “I think it is very confusing for a lot of people, so I understand and I empathize with that.”