A second test found all forty of the newly suspected Covid-19 cases in Yala on Sunday turned out to be false positives, as the Public Health Ministry has decided to run another test at its main lab to leave no room for doubt.
Officials are also investigating what went wrong at the Yala lab as controlled test equipment was found to be contaminated.
Those cases which were uncovered during comprehensive testing or "active case finding" have attracted attention from both the public and health authorities who were alarmed by the spike in infections.
Taweesilp Visanuyothin, spokesman of the Centre for Covid-19 Situation Administration (CCSA), said on Monday the second tests conducted by a lab at the medical science centre in Songkhla came back negative in all 40 cases.
But he said the result of those cases would not be confirmed until a third test is run, adding samples will be sent to the Department of Medical Sciences for testing.
"So, the 40 cases in Yala were not confirmed today pending an examination of the process," he told the press briefing, stressing there would not be a cover-up.
A source in the provincial health office, who asked not to be named, said on Monday an investigation is under way to find out what went wrong.
The source said there are several factors potentially contributing to false positives, including the possibilities that the equipment had trace samples from old tests or had had an unstable run.
The source said the Yala lab will not be used for testing for the time being and samples will be sent to the lab in Songkla instead.
Deputy Public Health Minister Sathit Pitutecha said on Monday a team of investigators has been dispatched to Yala to find out what happened during the testing.
"We must find an answer. It could be the solution liquids, the interpretation of test results or even a machine error," he said.
Supat Hasuwannakit, director of Chana district hospital in Songkhla, wrote on Facebook the virus was found in the "No Template Control" tube of the RT-PCR, which is supposed to be free of the virus.
But Dr Supat, a regular government critic, likewise said there is no cover-up in reporting the results.
The Yala provincial public health office on Monday reported no new infections, putting the number of confirmed cases at 126, with 99 of them discharged from hospital. Two deaths were recorded in the province.
Yala is a hotbed for infections among the southernmost provinces, with 126 confirmed cases. Its numbers are behind only Bangkok, Phuket and Nonthaburi.
Health officials in Yala are battling against Covid-19 during Ramadan in the hopes Muslims will be free to celebrate at the end of their holy month of fasting.
The province is conducting active case finding in all districts, targeting risk groups such as those who have had contact with returnees from neighbouring countries.
In Bannang Sata district, health officials have been conducting second active case findings on Monday and today target more than 800 people.
Ramadan, which started on April 24, has posed a challenge for officials, who decided not to do swab tests during the day, when Muslims are fasting. Ramadan is due to end on the evening of May 23.
"We cannot conduct the tests during the daytime as people are fasting, including the Muslim staff at the office," Yala public health chief Songkran Maichum said. "My [Muslim] staff also don't have the energy," he added.
Dr Songkran said cooperation from community leaders was key to success, adding only they can lead villagers to the designated locations where swabs will be taken for testing.
Meanwhile, the CCSA on Monday reported 18 new Covid-19 cases all of whom are illegal migrants detained in Songkhla's Sadao district.
It is reported that the new cases are Rohingya people including a 10-year-old child whose mother did not have the virus. The second floor of the detention centre has since been turned into a field hospital.
Monday was the first day since the outbreak that no cases had been reported among Thai citizens.
As of Monday the confirmed case tally nationwide stood at 2,987 and the death toll was 54.
But Sophon Iamsirithavorn, chief of the Communicable Disease Division, issued a stern warning that a rise in new cases could be seen over the next week now that some lockdown measures have been relaxed.
