The mystery behind Singapore's largest coronavirus infection cluster - that of Grace Assembly of God church - has been solved using a new test that can detect past infections even in recovered patients.

Using a serological test developed by Singapore's Duke-NUS Medical School, the authorities tracked the infection to two travellers from Wuhan, cases 8 and 9.

The Wuhan couple started a chain of infections linked to The Life Church and Missions cluster, a Chinese New Year family gathering at Mei Hwan Drive and, finally, to Grace Assembly of God church, which is linked to 23 cases.

The Health Ministry said the Grace Assembly of God church cluster began with case 66, a 28-year-old Singaporean who works at the church and also attended the family gathering at Mei Hwan Drive.

Investigations led contact tracers to a married couple, cases 83 and 91, who attended the same family gathering and who were also at The Life Church on the day the Wuhan couple were there.

Case 91 was the only new confirmed case as of yesterday.

Prime Minister Lee Hsien Loong wrote in a Facebook post yesterday that cases 83 and 91 had only mild symptoms earlier.

But the new serological test subsequently confirmed they had been infected earlier and linked the two clusters together, he noted. Instead of detecting the virus itself, the new test detects antibodies in the blood.

Meanwhile, Singapore has decided to bar visitors from Cheongdo county and Daegu city in South Korea, as the number of coronavirus cases in those areas continues to climb.