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Is it a good idea to mix and match COVID-19 vaccines? Experts aren’t so sure

TORONTO — Canadian experts are warning that mixing various COVID-19 vaccines, if a second dose of the vaccine a patient originally received isn’t available, may compromise the overall efficacy of the inoculation.

Barry Pakes, a public health and preventative medicine physician with the University of Toronto’s Dalla Lana School of Public Health, told CTVNews.ca that getting two different COVID-19 vaccine doses would not likely be dangerous, however, he says experts do not know the degree of protection it would provide against the virus.

“I don’t have any doubt really and I don’t think anyone who works in the area of vaccines would worry about the safety of using two different vaccines of this kind, but we would have legitimate concerns about efficacy,” Pakes said in a telephone interview on Monday.

Britain changed its vaccine guidance last week to allow for health officials to mix and match various coronavirus vaccines in “extremely rare occasions,” despite a lack of evidence on the efficacy of the process.

“[If] the same vaccine is not available, or if the first product received is unknown, it is reasonable to offer one dose of the locally available product to complete the schedule,” Britain’s government said in its updated vaccine guidance, noting that there is not yet any “evidence on the interchangeability of the COVID-19 vaccines, although studies are underway.”

Mary Ramsay, head of immunizations at Public Health England, told Reuters that it is “better to give a second dose of another vaccine than not at all,” should supply be limited.

But Pakes explained that there isn’t any data to show that mixed doses provide the same amount of protection as uniform doses. He added that, without clinical trials to assess this, mixing COVID-19 vaccines could create a false sense of protection against the disease.

“It’s only dangerous in the sense that they may think they’re protected, but they’re not,” Pakes said.

Pakes explained that the Moderna and Pfizer coronavirus vaccines are both mRNA vaccines and target the same spike protein of SARS-CoV-2 making it “reasonable to think that they’re somewhat interchangeable,” he says.

However, Pakes said there is currently no evidence to support this. He added that seeing if a mix and match approach works should not be a priority for Canada, but says health officials could learn from other countries who try it.

“All of the vaccines that are coming out now do target the same spike protein in different ways and it could be the case that targeting in different ways, one with an adenovirus vector vaccine and another one with an mRNA vaccine could actually be more efficacious. These are things we just don’t know yet,” Pakes said.

The U.S. Centers for Disease Control and Prevention has advised against the mixing of COVID-19 vaccines, noting that they “are not interchangeable.”

Canada’s current COVID-19 plans are to give the required number of doses of the same product, and infectious disease expert Dr. Isaac Bogoch says that is unlikely to change.

“Until we have compelling data that mixing vaccine products provides durable protection and is safe, and in the context of having excellent data for two of those series of the same product, I don’t see an immediate need to change the current plans here in Canada,” Bogoch said in a telephone interview with CTVNews.ca on Monday.

Bogoch noted that both the Moderna and Pfizer vaccines – the two COVID-19 vaccines currently approved in Canada – are both meant to be administered as two shots, given several weeks apart, but says the products were not designed to be mixed together.

Bogoch said there is emerging data on the efficacy of COVID-19 vaccines in comparison to one another, but said more research is needed before implementing a mix-and-match approach in Canada.

“I’m totally open minded to that being a good idea, but we should be hesitant implementing that in the absence of compelling data when we have excellent data to support two vaccines,” Bogoch said.

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