The Department of Disease Control (DDC) is on high alert after discovering that a French tourist on Koh Samui, Surat Thani province, is sick with the coronavirus.
The woman had successfully passed through the 14-day state quarantine, making health officials suspect she contracted the disease while in Thailand. Nevertheless, they have urged the public not to panic.
Sophon Iamsirithavorn, head of the Communicable Disease Division, said he suspected the 57-year-old woman became infected while in the country because she twice tested negative for the virus while being kept in the mandatory 14-day alternative state quarantine (ASQ) in Samut Prakan province.
The woman developed the fever on Oct 17, 17 days after arriving in the country, he said.
Moreover, she had flown here from the city of Limoges, where the Covid infection rate is quite low.
However, Dr Sophon stressed that further inquiries needed to be carried out to establish the source of infection.
"The process of disease control is already in motion and all suspected people are under the health authority's supervision," he said. "We have not yet seen anyone else become infected. Health officers are doing their job to control the disease."
Earlier, it was revealed that Buriram United footballer Akbar Ismatullaev, who arrived in Thailand on Aug 11 and also completed the 14-day state quarantine, was found on Sept 10 to be infected with the coronavirus.
Meanwhile, a 37-year-old prison inmate who has not been abroad was found on Sept 2 to also have the disease. He is Thailand's first local transmission for 100 days.
According to the Frenchwoman's timeline, she arrived in Thailand with her husband and son on a Thai Airways International flight on Sept 30 and stayed at alternative state quarantine in Samut Prakan province.
She was twice tested for Covid-19 while in quarantine, on Oct 3 and 11, both tests showing her to be negative.
She and her family left Samut Prakan on Oct 15 and visited the French embassy before flying to Samui on the same day with Bangkok Airways flight PG 167.
On Oct 17, she developed a fever and three days later went to a private hospital. She was admitted for one night but left the hospital on the same day that a laboratory returned her first positive Covid-19 test result.
A further positive test was made on Thursday, after which the entire family was admitted to hospital.
Dr Sophon said the woman's husband and son were found to be virus-free but it remained to be seen whether they had developed antibodies. A friend of the woman is also waiting for the result of a Covid-19 test.
According to the DDC, 15 people are at high risk of infection, including the woman's husband and son, the friend who collected them from the airport in Samui plus 12 passengers and crew from the Bangkok Airways flight.
At low risk are 21 medical staff at the hospital.
Public health officials have also examined the ASQ venue in Samut Prakan province, where 40 members of staff are under supervision.
DDC chief Dr Opas Karnkawinpong said the woman's infection is unexpected given the country's efficient handling of Covid-19 threats, especially in relation to people's cooperation with the authorities.
As many as 97% of people have been wearing masks, he said, and practising social distancing, which is why authorities have been confident the situation is now under control.
Public Health Minister Anutin Charnvirakul flew to Samui yesterday taking with him 1,000 Favipiravir pills to the Ko Samui Hospital, where the woman has been isolated and treated.