SARABURI: Thai scientists administered a second dose of an experimental COVID-19 vaccine to monkeys on Monday, looking for another positive response to enable clinical trials in humans as early as October.

The Thai vaccine is one of at least 100 being worked on globally as the world reels from a devastating virus that has infected more than 8.7 million and killed 461,000, with Sunday's 183,000 cases the highest reported in a single day.

Thirteen monkeys were immunized on Monday and the next two weeks will be critical in determining whether researchers can proceed with further tests.

"We're going to analyse the immune response once again. If the immune response is very, very high, then this is a good one," said Kiat Ruxrungtham, lead researcher of the COVID-19 vaccine development programme at Bangkok's Chulalongkorn University.

Thailand's government is backing the trials and hopes it can have a cost-effective vaccine manufactured domestically and ready for next year.

The monkeys are divided into three groups, with one getting a high dose, another a low dose and the last none. They are receiving three injections in total, each a month apart.

The first dose on May 23 prompted positive responses from all but one animal in the high-dose group and from three in the low-dose group, an outcome Kiat called "very impressive".

If there is a similar response after the second dose, Kiat said, the programme would order 10,000 doses made for a human trial, adding that his group had been flooded with offers from volunteers.

"The earliest we can get may be late September," he said of the doses. "But we don't expect it that soon, and the latest may be by November."