Chiang Mai has reported a rise in its daily Covid-19 caseloads fuelled by the recent detection of new infection clusters.
Meanwhile, a similar surge in infections in Hua Hin and Pran Buri districts of Prachuap Khiri Khan has led to calls for the reopening of the seaside resort to be pushed back from Nov 1.
On Saturday, in Chiang Mai, new infections topped 461 with two deaths. Of this number, five were infected from outside the province, according to the provincial communicable disease office.
The daily tally has been rising steadily in recent days. Saturday's figures saw the total reach 9,660 since the beginning of July.
The office said most sufferers are in the "green" group -- those who exhibit little or no symptoms.
Chiang Mai has vaccinated 934,051 residents, or 74% of its population.
Older citizens over 60 make up 195,508, and 100,401 have underlying illnesses.
Songyot Khamchai, head of the disease control unit of the provincial public health office, said several major clusters have been detected in the province in recent days.
They are responsible for infecting 120 people with the largest outbreak having been traced back to Muang Mai market via 67 of the patients.
Mr Songyot said many people had been reluctant to divulge details of visits to parties at nightlife venues and gambling dens, after returning home and passing the virus on to family members.
In Prachuap Khiri Khan, the public health office announced 445 new cases on Saturday, ranking it the fifth-most-infected province in the country.
The bulk of infections were reported in Pran Buri district where 362 people fell sick from the virus.
Most of the sufferers were Myanmar workers in four canned fruit factories and dormitories at the sites of the dual-track railway construction project.
Udom Srimahachota, adviser to the Hua Hin-Cha Am tourism business association, said local businesses were concerned the long weekend could send infections surging. Many have headed to resorts in Hua Hin district where occupancy rates are in excess of 80%.
Some clusters have occurred in Hua Hin already.
Mr Udom said that if possible, the Nov 1 reopening should be postponed until December to protect the public's health.