The Ministry of Public Health on Monday announced that passengers on four flights from Chiang Rai's Mae Fah Luang International Airport last week are now classified as at "low-risk" of contracting Covid-19.
It told the passengers to self-monitor their health for 14 days.
The ministry in a statement on Monday identified the four flights as DD8717 (Nok Air) on Nov 28 with a 1.40pm boarding time; SL533 (Thai Lion Air) on Nov 29 with a 10.40am boarding time; WE137 (Thai Smile) on Nov 29 with an 8.30pm boarding time; and SL545 (Thai Lion Air) on Nov 30 with a 7.15pm boarding time.
All four flights were from Chiang Rai to Bangkok. The health ministry did not disclose the full number of coronavirus carriers on the four flights.
The passengers on the four flights were asked to observe their health condition for 14 days. The ministry said those displaying symptoms of Covid-19 -- fever, coughing, sore throat and sensory losses -- must immediately contact the 1422 hotline or go to the nearest hospital.
The announcement followed the discovery of a Sing Buri woman who contracted the disease after arriving in Chiang Rai via Mae Fah Luang International Airport.
It was the same airport used by Thais who returned to the kingdom after staying at the 1G1-7 Hotel in Myanmar's Tachileik district. The Thais tested positive for Covid-19 after returning home, prompting fears that the disease is making a comeback in the kingdom.
Dr Sophon Iamsirithaworn, director of the Communicable Disease Division at the Department of Disease Control (DDC), asked the public not to panic, insisting that the passengers on the four flights should be okay.
However, he did say the passengers should wear a face mask and practise social distancing at home.
"They are classified as 'low-risk' people as long as they wore their mask at all times," Dr Sophon said.
He said the Sing Buri woman, 51, did not wear her mask properly, noting it was possible that she might have become infected at Chiang Rai airport.
The woman is considered a local case as she did not travel overseas.
She tested positive for the disease on Dec 3. Thirty-seven people who came in close contact with her around the time are now being tested for Covid-19. Thirty-two of them have shown negative results, and five others are waiting for their test results.
According to the ministry, a 30-year-old man tested positive for Covid-19 on Dec 5 after returning from Tachileik in Myanmar. He boarded Lion Air flight SL545 on Nov 30, and the DDC has classified 24 people who came in close contact with him as at "high-risk".
For the flight WE137 (Thai Smile) on Nov 29, a 26-year-old woman -- a Thai returnee from Myanmar -- boarded the flight and tested positive for the disease on Dec 4. Ten persons were designated as "high-risk" while 68 others were labelled "low-risk".
Regarding the Tachileik cluster case, the department has identified 38 cases of Covid-19 from Nov 24 to Monday, 20 of whom were already in quarantine. They are mostly women in their 20s.
Dr Sophon said the 1G1-7 Hotel -- with its pub, casino and karaoke attractions -- was the source of the disease.
It has been estimated that over 100 Thai nationals worked there before it was closed on Nov 24 due to a local outbreak.
A Defence Ministry representative said more soldiers have been assigned to patrol the Myanmar-Thai border.