PHNOM PENH, Aug 1 (Reuters) - Cambodia will begin offering a booster shot against Covid-19, switching between the AstraZeneca and Chinese COVID-19 vaccines in an effort to fight the spread of the coronavirus in the Southeast Asian country. Prime Minister Hun Sen, launching the vaccination campaign for 12-17 years old, said on Sunday that the third dose will be offered to between 500,000 to one million frontline workers as a priority. "People who have already been vaccinated with Sinopharm and Sinovac should be given AstraZeneca as the third booster dose," Hun Sen said in a speech which was broadcast on social media. "For Cambodians who have been vaccinated with AstraZeneca, Sinovac should be given as the third dose," he added. Hun Sen also said the country will purchase more AstraZeneca vaccines through Covax for the booster shots and the recent U.S funded Johnson & Johnson vaccines will be used to vaccinate indigenous peoples in northeastern Cambodia. Cambodia has launched a lockdown in eight provinces bordering Thailand this week in a bid to prevent the spread of the Delta variant of the coronavirus in the Southeast Asian country. The World Health Organization (WHO) said this week that the Delta variant has been detected amongst migrants returning from Thailand through land borders and is now in the local community. "We are racing against new variants. We must act today, and we must act fast to have no regrets tomorrow," said Li Ailan, WHO Representative to Cambodia, in a statement on Saturday. Cambodia managed to largely contain the virus for most of last year, but an outbreak first detected in late February has driven up total cases to 77,919, with 1,420 deaths.