GENEVA, July 12 (Reuters) - 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."