Mixing different COVID-19 vaccines is a “dangerous trend” and could lead to confusion, WHO Chief Scientist Soumya Swaminathan warned on Monday. “There are people who are thinking about mixing and matching.

We receive a lot of queries who say they’ve taken one and they’re planning to take another one. It’s a little bit of a dangerous trend here where we’re in a data-free, evidence-free zone as far as mixing and matching,” Swaminathan told a press briefing. Swaminathan noted the limited clinical data on mixing and matching COVID-19 vaccines, citing the UK-based trial of a mixed dosing regimen using AstraZeneca and Pfizer’s COVID-19 vaccines, but said that the issue could cause chaos in other countries.

“It will be a chaotic situation in countries if citizens start deciding when they should be taking a second, or a third, or a fourth dose,” the WHO’s chief scientist said.