WHO Scientist Cautions Against Mixing and Matching COVID-19 Vaccines, Calls It 'Dangerous Trend'

The CDC also currently advises that COVID-19 vaccines "are not interchangeable"

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COVID-19 vaccine. Photo: Chip Somodevilla/Getty

A chief scientist from the World Health Organization is speaking out against mixing and matching different COVID-19 vaccine doses, saying more research has to be done about the method.

During a press conference Monday, WHO's Soumya Swaminathan said that supplementing different COVID-19 vaccines in the series of doses is "a little bit of a dangerous trend" since there isn't sufficient data yet.

"I really want to caution folks, because there is a tendency now for people in countries with enough availability of vaccines to voluntarily start thinking about an additional dose. There are people who are thinking about mixing and matching," said Swaminathan. "We receive a lot of queries from people who say they've taken one and planning to take another one."

"It's a little bit of a dangerous trend," Swaminathan continued. "We're in a data-free, evidence-free zone as far as mix-and-match. There is limited data on mix-and-match. There are studies going on; we need to wait for that. Maybe it will be a very good approach. But at the moment we only have data on the Oxford-AstraZeneca vaccine followed by Pfizer. It will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, or a third and or a fourth dose."

In a tweet later on Monday, Swaminathan clarified that "individuals should not decide for themselves" to mix and match, "public health agencies can, based on available data. Data from mix and match studies of different vaccines are awaited - immunogenicity and safety both need to be evaluated."

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Canada's National Advisory Committee on Immunizations has said, according to Canadian news outlet CP24, that Pfizer and Moderna COVID-19 vaccines can be swapped and used with each other when a matching second dose isn't available.

A spokesperson for Health Minister Christine Elliott said in a statement, per CP24, that "Ontario continues to follow the advice of the National Advisory Committee on Immunization (NACI) which recommends that it is safe to mix vaccines based on studies from the U.K., Spain and Germany that have found that mixing vaccines is safe and produces a strong immune response."

In the United States, however, the Centers for Disease Control and Prevention still state that "COVID-19 vaccines are not interchangeable." According to the CDC, "The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product."

The CDC added that in "exceptional situations" where the same vaccine used for a first dose is no longer available, any approved "available mRNA COVID-19 vaccine may be administered." But, they said in scenarios where the same vaccine is only temporarily unavailable, "it is preferable to delay the second dose (up to 6 weeks) to receive the same product than to receive a mixed series using a different product."

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