Lifestyle Health 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" By Benjamin VanHoose Benjamin VanHoose Twitter Benjamin VanHoose is an Associate Editor on the Movies team at PEOPLE. He's worked at PEOPLE for over three years as a writer and reporter across our Entertainment, Lifestyle and News teams, covering everything from the Johnny Depp v. Amber Heard trial to the Oscars. He regularly covers red carpet events and has interviewed stars like Drew Barrymore, Ryan Reynolds and Kirsten Dunst. He previously worked as a copy editor at Topix Media Lab. People Editorial Guidelines Published on July 13, 2021 11:33 AM Share Tweet Pin Email 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." CDC Warns of 'Small Possible Risk' of Rare Nerve Disorder from Johnson & Johnson Vaccine Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories. 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." As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from the CDC, WHO and local public health departments. PEOPLE has partnered with GoFundMe to raise money for the COVID-19 Relief Fund, a GoFundMe.org fundraiser to support everything from frontline responders to families in need, as well as organizations helping communities. For more information or to donate, click here.