Thailand will purchase an additional 12 million doses of Sinovac's coronavirus vaccine to try to expedite its rollout, a senior official has said, despite concerns over its efficacy against the Delta variant of Covid-19.

Thailand's strategy had planned to use mainly locally produced AstraZeneca vaccines but with supply short of the government's target, Sinovac shots are being used in a mix-and-match approach to inoculate the population faster.

Sinovac's inactivated virus vaccine will be used as a first dose and AstraZeneca's viral vector vaccine the second, a mix that officials say protects against the Delta variant, according to a Reuters report.

Thailand was the first country to announce it would use this combination.