Thailand's health ministry has said that its Covid-19 vaccine regimen of China's Sinovac followed by British-developed AstraZeneca was safe and successfully boosted immunity among its first 1.5 million recipients, Reuters reports.

Thailand in July became the first country in the world to mix a Chinese vaccine and a Western-developed vaccine as cases and deaths in the country surged and the government struggled with vaccine supplies.

"The cross formula has been injected to over 1.5m people and it is safe. Please don't say things that would create concern," senior health official Supakit Sirilak told a news conference.

The health ministry said the Sinovac-AstraZeneca combination boosted immunity to the same levels as two AstraZeneca shots and meant vaccinations could be completed faster due to the shorter dose gap.