The World Health Organization (WHO) issued a warning on Monday advising against the mixing of different types of two-shot COVID-19 vaccines.
Experts at the WHO say there is little data available about the health impact from mixing different jabs.
The warning comes after Thailand announced it would allow patients to get one shot each of the AstraZeneca and Sinovac vaccines to combat a current surge of COVID-19 cases.
Canada also announced last month that residents who receive the AstraZeneca vaccine for their first shot should receive either the Pfizer-BioNTech or the Moderna vaccine for their second dise.
'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,' Dr Soumya Swaminathan, the WHO's chief scientist said during a briefing on Monday.
'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.'
Thailand will mix the Sinovac and AstraZeneca vaccines for health workers, a move the WHO advises against. Pictured: A Thai citizen receives a vaccine in Bangkok
Thailand is currently suffering through what is the largest case surge it has experienced since the pandemic began.
The southeast Asian nation is recording an average of 6,695 new cases a day with the total likely to grow even more in the coming days.
Cases are 162 percent up from where they were a month ago on June 11 and 1,239 percent up from where cases were three months ago on April 11.
The infections included 618 breakthrough cases among 677,000 medical staff who were fully vaccinated with the Sinovac vaccine - or 0.1 percent of people.
The Sinovac vaccine was developed in China and distributed to neighboring Asian countries.
Thailand now plans to give medical staff with only one shot of the Sinovac vaccine the
