Two people working for major companies have contracted Covid-19.
Thepparat Theppitak, president of energy company Electricity Generating (Egco) said on Monday that a staffer working at its head office in Bangkok's Lak Si district was found to be infected on Friday and was hospitalised.
Mr Thepparat notified 31 colleagues who had been in close contact with the infected man and ordered that they be tested too -- 28 of them received the all-clear and three have yet to get their results.
Even those who tested negative have been asked to self-quarantine, he said, and the company also has the whole building sanitised on Sunday.
Meanwhile, True Corporation, one of the country's biggest telecommunications companies, announced that one of its data analysts, working at at its building in Ratchadaphisek Road, Huai Khwang district, tested positive on Dec 31.
The infected person last visited the office on Christmas Day, developed a fever two days later and was immediately instructed to go to hospital.
The company said the staffer did not work in customer services but it nevertheless ordered other workers on the same floor to self-quarantine for 14 days to make sure they remained healthy and could not transmit the virus.
The Public Health Ministry is still trying to establish the cause of the workers' infections. Previous cases have been linked to the Central Shrimp Market in Samut Sakhon province and also entertainment venues in the capital.
The number of Covid-19 cases recorded in Bangkok rose on Monday to 28, trailing only Samut Sakhon province (93) in the total number of transmissions among Thais.
Next on the infection list were Chanthaburi (27), Samut Prakan (20) and Rayong (17).