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 there was little data available about the health impact.

'It's a little bit of a dangerous trend here.

We are in a data-free, evidence-free zone as far as mix and match,' 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.'