Three Thais returning from Tachilek were among 11 more coronavirus infections reported on Friday, taking the number of confirmed cases to 4,180.
The Centre for Covid-19 Situation Administration (CCSA) said the two women, who worked at an entertainment centre, and one man were in quarantine at Chiang Rai Prachanukroh Hospital in Muang district.
The women returned to Mae Sai district on Dec 4 and their tests on Wednesday were positive. The man, whose work was not given, crossed back through the border on Sunday, and a test on Tuesday came back positive.
Chiang Rai Prachanuikroh Hospital is the provincial quaantine facility for Thais returning from Tachilek.
The government has been urging Thai returning home to do so through the official checkpoint, instead of illegally crossing the border as many of them do, so the Covid-19 outbreak in Myanmar can be kept out of Thailand.
All returning through the official crossing are promptly sent into quarantine for 14 days.
There were 49 cases related to returnees from Tachilek, 37 of them in Chiang Rai and five in Chiang Mai, spokesman Dr Thaweesilp Visanuyothin said.
Elsewhere, there was one case in Phayao, three in Bangkok, one in Phichit, one in Ratchaburi and one in Sing Buri.
He said 17 were people who crossed the border via natural passes, 30 were in local quarantine facilities and two were local infections.
The cases coming from Tachilek caused alarm ahead of the holiday season highlighted by a long New Year break.
The two provinces are popular choices for holidaymakers. Health officials have hastened to assure intending visitors the spread of the virus has been well contained and they are in no danger of contagion.
Disease Control Department chief Opas Kankawinphong on Friday said rumours that tourists to the two provinces would have to stay at home afterwards for 14 days were nonsense.
"There are no plans to quarantine visitors to Chiang Rai and Chiang Mai after they return home. They are safe, just like other provinces," Dr Opas said.
Dr Thaweesilp said a man who had sneaked back to Mae Sot district of Tak from Myawaddy and travelled to Nakhon Phanom had tested negative.
The man crossed the border on Monday and took a bus to Mukdahan, then a van to Nakhon Phanom. People in his neighbourhood reported his arrival. He had been quarantined at Thatphanom Crown Prince Hospital.
Other cases incuded two Thai women returning from Saudi Arabia, two from Bahrain and another from Portugal.
A Swedish man, an American woman and a South Korean man had also tested positive after they arrived in the country.
Of the total 4,180 people diagnosed with Covid-19, 3,903 (93%) had recovered, including 15 discharged from hospitals over the past 24 hours, and 217 people remained in hospitals. The death toll was unchanged at 60, the CCSA said.
Global Covid-19 cases rose by 676,806 over the previous 24 hours to 70.71 million. The worldwide death toll soared by 12,625 to 1.58 million.
The US had the most cases at 16.03 million, up 217,779 followed by India with 9.79 million, up 34,666. Thailand ranked 151st.
