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Editorial Cartoon by Graeme MacKay, The Hamilton Spectator – Wednesday May 12, 2021

What happens when people get two different COVID-19 vaccines?

As some experts continue to warn of very rare side effects associated with the AstraZeneca vaccine, Canadian health officials are now reviewing the research on mixing various COVID-19 shots.

A study of a “mismatched” vaccine regimen is underway in the U.K. — but some scientists say there’s reason to believe that administering two doses of different products could boost a person’s immune response beyond what can be achieved by giving the same shot twice.

The National Advisory Committee on Immunization (NACI) caused some confusion earlier this month when it said the viral vector shot from AstraZeneca is not the “preferred” product given its associated risk of vaccine-induced immune thrombotic thrombocytopenia (VITT) — a condition that causes blood clots. That warning came out after hundreds of thousands of Canadians had received the AstraZeneca vaccine already.

According to the Ontario Science Table, estimates of the frequency of VITT in individuals who have received the AstraZeneca vaccine now range from 1 case in 26,000 to 1 case in 127,000 doses administered.

The risk of developing this side effect, combined with an uncertain delivery schedule for future supply, has prompted some provinces to consider pausing AstraZeneca vaccinations altogether.

Researchers at Oxford University in the U.K. launched a study in early February to explore the possible benefits of alternating different COVID-19 vaccines. According to the lead scientists, the study is “looking for clues as to how to increase the breadth of protection against new virus strains.”

The study — otherwise known as the COVID-19 Heterologous Prime Boost study, or “Com-COV” — is collecting data to determine whether receiving two different types of vaccine generates an immune response at least equal to the response that follows receiving the same product twice. (A “heterologous” vaccination regimen is one that uses more than one product.)

Dr. Helen Fletcher is a professor of immunology at the London School of Hygiene and Tropical Medicine in the U.K. She said a “mismatched” vaccine program would deliver some practical benefits — vaccine delivery logistics would be greatly simplified — but there could be another good reason to pursue a mixed-dose regimen.

“I’m excited about the study because I think it’s likely that the immune response will be even better if you mix and match vaccines,” Fletcher said in an interview with CBC News.

Dr. Theresa Tam, Canada’s chief public health officer, said last week the current guidance is for AstraZeneca recipients to get a second dose of the same product, but NACI is now reviewing the Oxford research on mixing AstraZeneca with an mRNA shot.

“There will be further advice forthcoming on that second dose based on the evolving science. We should watch this space,” Tam said. 

Will Canada shorten the time between shots? Possibly. NACI said in early March that, given the limited vaccine supply, provinces and territories may want to wait up to 16 weeks between first and second doses to give more people at least some level of protection.

The provinces have since followed this guidance, with a few exceptions. For example, many long-term care home residents have been fully vaccinated on the timeline recommended by the vaccine makers. Pfizer calls for a second dose 21 days after the first, while Moderna stipulates the second shot should come 28 days later. (CBC)