Dr Opass Putcharoen, chief of Thai Red Cross Emerging Infectious Diseases Clinical Centre, said all three cases were found to be carrying the B1.351 South African strain in Thai quarantine.
The first was a 41-year-old Thai gemstone merchant who tested positive on February 3 after returning from Tanzania. He suffered severe pneumonia but is recovering after being treated with Remdesivir.
However, two more people have since tested positive for the South African variant in quarantine, said Dr Opass.
Thailand has ordered 61 million doses of the AstraZeneca Covid-19 vaccine, which is thought to be only 10 per cent effective against mild and moderate cases caused by the South African strain.
Dr Opass urged the government to speed up vaccination and efforts to contain virus transmission, adding that health officials must monitor for Covid-19 variants that might affect the vaccine’s efficiency.
The South African variant is one of three Covid-19 strains being monitored around the world.
The other two are the Brazilian strain and the highly infectious UK strain – of which nine cases have been detected so far in Thai quarantine. Dr Opass warned that another strain similar to the South African variant had recently emerged abroad.