GENEVA - The World Health Organization's chief scientist on Monday advised against people mixing and matching COVID-19 vaccines from different manufacturers, calling it a "dangerous trend" since more data is needed about the health impact.

"It's a little bit of a dangerous trend here," Soumya 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."

Swaminathan called mixing a "data-free zone" on Monday but the WHO clarified on Tuesday that some data was available and more was expected.

Its Strategic Advisory Group of Experts on vaccines in June said the Pfizer vaccine could be used as a second dose after an initial dose of AstraZeneca, if the latter is not available.

The results of a further clinical trial led by the University of Oxford that will look at mixing AstraZeneca and Pfizer as well as Moderna and Novovax vaccines is underway.

"Data from mix and match studies of different vaccines are awaited - immunogenicity and safety both need to be evaluated," the WHO said in emailed comments.

It should be public health agencies who make decisions, based on available data, and not individuals, the WHO added.