"If we are not screening people who can quietly spread the virus, there will be subsequent outbreaks and mutations, rendering immunization ineffective," Thiravat Hemachudha, head of the Thai Red Cross Emerging Infectious Diseases Centre wrote on Facebook. His comments come amid public criticism of the government for its slow response to the latest outbreak and delayed vaccine roll-out, with some calling for the health minister's resignation.

Thailand has reported more than half of its 59,687 COVID-19 cases this month alone, and 69 of its total 163 fatalities. It has closed parks, gyms, cinemas, and schools in the outbreak epicenter Bangkok, but malls and restaurants remain open. It has also introduced a fine of up to 20,000 baht ($635) for not wearing masks in public, which even the prime minister fell foul of. Three new cases were reported among Government House staff on Tuesday.

The new outbreak coincides also with the detection locally of the highly transmissible B.1.1.7 coronavirus variant blamed for big jumps in infections in many countries. Thiravat earlier told Reuters that Thailand's third wave comes from the saturation of undetected cases accumulated since last year. The roughly 10-20 cases per day prior to the new outbreak were because testing was insufficient, he said.

Thailand has not conducted mass-testing but uses a targeted testing approach. Reliable testing data has been difficult to obtain. Labs are currently processing around 40,000 to 50,000 samples daily, the health ministry said.

"Real proactive surveillance means everyone needs to be tested, with or without symptoms and in all areas," Thiravat added. Testing in Thailand is free only if an individual has had potential exposure and presents flu-like symptoms. Private hospitals offer drive-through testing for 2,500 baht to 5,000 baht ($80 to $160).