Health authorities labelled a couple in the northeastern province of Kalasin Covid-19 super-spreaders after hundreds of people in multiple provinces contracted the Omicron strain of the virus.
Opas Karnkawinpong, director-general of the Department of Disease Control, said since the Thai couple returned from Belgium on Dec 10, Omicron infections have been detected in Kalasin, Udon Thani, Maha Sarakham, Khon Kaen, Roi Et, Nong Bua Lam Phu, Sakon Nakhon, Lamphun, Ubon Ratchathani, Phitsanulok, Lampang and Phetchabun.
Kalasin alone had 248 cases from the cluster, with a further 13 in Khon Kaen, 103 in Roi Et and four in Lamphun.
"This is a superspreader case because a couple spread it to hundreds of others," Dr Opas said.
He said the infected couple visited several entertainment places in Kalasin.
One in particular is believed to have spread the disease throughout the Northeast and to some provinces in the North because it failed to comply strictly with disease control measures, he said
Customers filled 90% of the bar's capacity, failed to observe social distancing and stayed for a long period of time to watch music performances.
Staff quarters had poor ventilation, he added, and although staff were fully vaccinated, they did not undergo antigen tests. They shared meals, and those who felt unwell did not take sick leave.
The Kalasin cluster was the biggest Omicron cluster in the country, said Supakit Sirilak, director-general of the department.
He said that the kingdom now had 740 Omicron cases -- 489 arrivals from abroad and 251 local transmissions. Infections with the highly-transmissible new variant have now been detected in 33 provinces, with local transmissions in 19 of them.
Omicron cases rose sharply over the past two days and formed 5-6% of all new Covid-19 cases, including 75% of the infections found in new arrivals, he said. All the cases had been linked to infected arrivals.
"It has been quite clear that Omicron spreads fast but its symptoms are not particularly severe," Dr Supakit said. The Delta variant was still dominant in Thailand.
Omicron outbreaks have occurred in 108 countries, including in all 50 US states, he said.