The World Health Organization (WHO) warned that people should not mix and match COVID-19 vaccines from different manufacturers and described the phenomenon as a “dangerous trend.” Soumya Swaminathan, WHO’s chief scientist, said Monday that more data is needed first before a proper health impact can be assessed. “It’s a little bit of a dangerous trend here,” Swaminathan told an online briefing. “It will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, a third, and a fourth dose.” However, her comments appear to contradict what WHO’s Strategic Advisory Group of Experts said in June that the Pfizer vaccine can be used as a second dose after an initial dose of AstraZeneca if the second AstraZeneca dose isn’t available. On Monday, Swaminathan noted there are studies on the practice and referred to a paper from Oxford University that examined the result when one was given the AstraZeneca …