Three employees of the Mandarin Orchard Singapore hotel were likely to have been previously infected with the coronavirus, said the Ministry of Health (MOH) yesterday.

Serological tests conducted on them have come back positive, indicating probable past infections.

The tests were conducted on the hotel staff as a precautionary measure as 13 travellers who stayed at the hotel were suspected to have been infected after arriving here. Epidemiological investigations of these cases are in progress.

Another three people serving their stay-home notice (SHN) at the Mandarin Orchard Singapore have now tested positive for Covid-19. But it is likely these are imported cases and not linked to the 13 cases being investigated.

The probe into the 13 cases was launched as it was found they had been infected by coronavirus strains that have "high genetic similarity", implying the infections may have come from the same source, MOH said on Saturday.

The 13 had flown in from 10 countries. But they all stayed in the same tower of Mandarin Orchard between Oct 22 and Nov 11.

In a press statement yesterday, MOH said 394 people serving SHN at the hotel have been swabbed, and 384 test results are ready, while 10 are pending. Of these results, 381 have come back negative for Covid-19.

The guests were tested following the news of the possible transmission chain at the hotel. The test is usually done towards the end of their SHN.

While the three new cases announced yesterday are likely to be imported, the National Public Health Laboratory will conduct whole genome sequencing and phylogenetic analysis to determine if they could be linked to the 13 cases under investigation, MOH said. It takes about four weeks to culture the virus and complete the genome sequencing.

So far, polymerase chain reaction (PCR) tests for 158 staff of Mandarin Orchard Singapore have come back negative for Covid-19. As at yesterday, 571 hotel staff have been tested for Covid-19.

Mandarin Orchard Singapore has stopped accepting new guests since Saturday and is checking out existing guests progressively, MOH said.

Guests serving SHN at the hotel have been taken in dedicated vehicles to alternative SHN facilities.

When The Straits Times arrived at the hotel around 11am yesterday, some of these guests were still being moved out. People in personal protective equipment (PPE), believed to be hotel staff and government personnel, were seen ushering guests into waiting vehicles. Others were helping to load the guests' luggage, or directing incoming vehicles to waiting spots.

The vehicles and personnel in PPE were gone by around 2.15pm. ST understands that some SHN guests were transferred to Swissotel The Stamford.

A 64-year-old Briton and his Singaporean wife, 59, are among the 13 cases being investigated. They arrived in Singapore on Oct 26 to visit his wife's family.

The Briton, a retiree who used to work in banking, said it was frustrating to think that he could have contracted Covid-19 while under SHN in Singapore, given that he and his wife had taken extra precautions before travelling here, including self-isolating at home in Britain for more than two weeks before their flight.

"We wanted to ensure that we were clean before we came to Singapore," he said.

After his wife tested positive for Covid-19 on Nov 6, he was transferred to another hotel, and he too tested positive, on Nov 8, after developing symptoms.

The couple were discharged from a community care facility late last month after recovering from Covid-19, and are still in Singapore.

"It was very scary to learn that I had Covid-19, given my age and my history of asthma. I always believed that if I caught Covid, I probably wouldn't survive it because of (my risk factors). But it was a huge relief that I managed to come through," he said. He added that he hoped to have a full discussion with the authorities about what happened when investigations have been completed.

"The biggest frustration was that I lost the first 34 days of my holiday... We had been looking forward to getting a little bit of release after what had been a terrible year for everybody."