The Public Health Ministry is speeding up a study looking into the need for a third Covid-19 vaccine shot.

Dr Opas Karnkawinpong, director-general of the Department of Disease Control (DDC), said on Monday it was still too soon, however, to say whether a booster shot is actually needed and if the shot can come from a different manufacturer.

"Knowledge [about the virus and the vaccines is updated] constantly so we need to wait until we have sufficient information to support any new decisions to make," he said. "We have a team that is following up on this and will decide what will be the most suitable [for Thailand's situation]."

Dr Opas was responding to questions over the need for a booster shot for vaccine recipients in Thailand and whether it could come from a different brand.

The number of people in Thailand infected with the Delta coronavirus variant, which first emerged in India and is more infectious than earlier variants, is increasing and likely to replace the Alpha variant, first identified in Britain, as the dominant strain causing infections, Dr Yong said.

Citing findings from a new study published on June 14 by the British medical journal The Lancet, he said a considerably higher level of immune response to a Covid-19 vaccine is needed as an efficient defence against the Delta variant.

Both the Pfizer and AstraZeneca vaccines were cited in this study, he said, adding a study carried out in Scotland showed they were less effective in preventing infections caused by the Delta variant.