BANGKOK (REUTERS) - Thailand will purchase an additional 12 million doses of Sinovac's coronavirus vaccine to try to expedite its rollout, a senior official said on Tuesday (Aug 17), despite concerns over its efficacy against the Delta variant of Covid-19.

Thailand's strategy had been 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.

The mass vaccination campaign started in June but the rollout is being hastened by Thailand's worst coronavirus wave so far, which is challenging Bangkok's healthcare system at a time when just 7.3 per cent of the population has been fully vaccinated.

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.

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

"A first shot of Sinovac followed by AstraZeneca significantly boosts immunity," said Dr Chawetsan Namwat, a senior health official at the Department of Disease Control.

Thailand reported a record 239 new Covid-19 deaths on Tuesday, with 7,973 fatalities overall, and is on course to pass the one million mark in cases this week.

It used the Sinovac vaccine initially for priority groups, medical personnel and the resort island of Phuket.

A Thai study carried out on medical personnel from May to July showed two doses of Sinovac offered 72 per cent protection against infection and 98 per cent protection against severe symptoms and death.

"The Sinovac vaccine that we used in Thailand is effective,"said Dr Taweesap Sriprapasiri of the Department of Disease Control. "Even with the new variant, the vaccine is still effective."