BANGKOK, June 11 (Reuters) - Thailand has signed a purchase order for 20 million doses of the COVID-19 vaccine develop by Pfizer and BioNTech to be delivered before the end of this year, its health minister said.

Details of the deal, delivery time frame and the vaccine's approval by the Food and Drug Administration would be worked out in about a month, Thai Health Minister Anutin Charnvirakul said on Thursday (Jun 10).

Thailand also planned to buy 5 million doses of Johnson & Johnson's single-shot vaccine within this year, Anutin said.

Thailand is racing to secure vaccines for its mass immunisation campaign that started on Monday, two months after it was hit by its biggest coronavirus outbreak so far, driven by highly contagious variants.

The programme had relied heavily on its reserved 61 million doses of locally-made AstraZeneca vaccines, but the government has tried to diversify its sources to meet its target of acquiring 100 million doses by year-end.

About 4.1 million of Thailand's more than 66 million people have so far received a first dose of a COVID-19 vaccine.