Thailand defended its move to mix-and-match vaccines to boost protection against Covid-19 after a World Health Organisation (WHO) scientist warned of uncertainty over the safety and efficacy of using different vaccine combinations.

Senior virologist from Chulalongkorn University Dr Yong Poovorawan said a shot of China's Sinovac, an inactivated virus vaccine, with a second dose of AstraZeneca, a viral vector vaccine, have been working well in boosting immunity.

“It can achieve a booster effect within...