The Ministry of Public Health has requested further information about the vaccine it has ordered from China following a report it might not be as effective as first thought.

The move came after Brazil officials revealed on Tuesday that the coronavirus vaccine developed in partnership with Sinovac Biotech Ltd was 50.4% effective in preventing coronavirus cases, a number that meets the threshold required by global regulations for approval but is well below the 78% figure announced by Sinovac Biotech last week.

He said the request for more details about the efficacy and safety of the vaccine had already been sent to Sinovac Biotech and was in line with the process for submitting vaccines for approval by the Thai Food and Drug Administration (FDA).

"We are waiting for answers," he said, "but please understand that the vaccine's efficacy is only at an initial stage after mass human trials. The confirmed result might take about a year or one and a half years." He was confident in the company's methodology, adding: "Sinovac [Biotech] has applied traditional technology, using killed viral particles to create the body's immune system."

Dr Supakit added the vaccine had already been given to about 100,000 medical and military staff without any harmful side effects reported.

He pointed out that factors taken into account before placing vaccine orders covered quality, efficacy and price.

Sinovac Biotech had made a promise to ship the first two million shots by next month, he said, but other, possibly more effective vaccines might not be shipped until later in the year. "That is the factor that we have seriously thought about, whether we can wait until that time," said Dr Supakit. "We have relied on the World Health Organization (WHO) suggestion that 50% efficiency is enough for emergency use.

"We are still committed to the original plan to get the vaccine by next month."

Explaining the clinical test result, the researcher team at the Butantan Institute in Brazil said the previous rate of 78% efficacy did not include data from a group of "very mild infections" among those who received the vaccine and did not require clinical assistance.

However, they said the vaccine had been 78% effective in preventing mild cases that needed some treatment and 100% effective in staving off moderate to serious cases. The government's agreement with Sinovac Biotech is for the company to ship the first 200,000 shots of Covid-19 vaccine by the end of next month, another 800,000 shots in March and one million shots in April.

The two million shots will be enough for one million prioritised people, including frontline officials, those living in highest at-risk areas and elderly people. The ministry's immediate goal is to give a Covid-19 vaccine to 70% of the population to curb the outbreak and create herd immunity. Another 26 million doses from the UK-based AstraZeneca company is expected to arrive by the middle of this year.

Meanwhile, Surachok Tangwiwat, the FDA's deputy secretary-general, said that only AstraZeneca and Sinovac Biotech had applied for their vaccines to be registered in Thailand and the Thai FDA was doing its best to speed up the approval process.

However, it could only be done based on sufficient information, which needed to arrive by the end of this month to meet the roll-out schedule.

The Chinese Food and Drug Administration has not yet approved the Sinovac vaccine because it is also still awaiting the company's efficiency and safety data in China. Prime Minister Prayut Chan-o-cha said on Wednesday that local administrative organisations (LAOs) had the right to buy Covid-19 vaccines for local inoculation, provided they had prior approval from the Thai FDA.

Gen Prayut also applauded the efficacy rate of the Covid-19 vaccine produced by Siam Bioscience, a Thai pharmaceutical company.

A local administrative organisation in Yala, meanwhile, is calling for local bodies to procure vaccines.

Pongsak Yingchoncharoen, mayor of Nakhon Yala, said his local authority was ready to spend 100 million baht to buy vaccines for 70,000 residents if they received permission to do so.

His Majesty the King is to donate medical equipment to the Public Health Ministry for use to boost the country's fight against the pandemic.