Concerns over possible silent clusters of COVID-19 infection were raised when it was revealed on Monday (21 December) that 11 staffers at the Mandarin Orchard hotel showed signs of past infection based on serological tests.

Tests were done on 571 staffers with 11 having come back positive, meaning they have been infected by the virus in the past, said the Ministry of Health (MOH) in a statement.

This was just two days after the ministry said on it has detected 13 COVID-19 cases among quarantined travellers at the hotel which had “high genetic similarity” despite them coming from 10 different countries. The infected travellers stayed at the hotel between 22 October and 11 November, serving out their Stay at Home Notices (SHN).

This raised the suspicion that they may have been infected in the hotel. The MOH said that preliminary investigations indicated that it “cannot exclude” that transmissions could have occurred at Mandarin Orchard.

The hotel, which houses quarantined travellers in designated floors as well as regular guests, has scaled back operations as authorities look into the situation.

Mandarin Orchard is one of the hotels which had been approved by relevant authorities to offer packages to Singapore residents for staycations as they cannot leave the country due to travel restrictions.

In response to the question of the possibility that the hotel is actually a cluster of infection, infectious diseases specialist and President of the Asia Pacific Society of Clinical Microbiology and Infection Dr Paul Tambyah said that the rate of 11 out of 571 staffers being seropositive indicates a prevalence of 1.92 percent, which is ten time higher than the seropositivity rate of the general population which the MOH said is at 0.25 percent.

The 1.92 percent is a little higher than the seropositivity rate reported by National Centre for Infectious Diseases (NCID) researchers who looked into work and social contacts in investigations of other clusters in Singapore, published in the medical journal The Lancet.

Dr Paul, who is also Chairman of the Singapore Democratic Party (SDP), said: “It is possible that this is a coincidence but it does suggest that the hotel may have been a silent cluster for some time.”

Dr Paul also noted that a detailed investigation using serology and contact tracing would be more useful that using polymerase chain reaction (PCR) tests which are used to detect if a person is currently infected with the virus.

He added, “Singapore pioneered this technique and this helped solve the mystery of the links between two church clusters early in the outbreak in Singapore. I have no doubt that this will be done effectively.”

On the question of other possible silent cluster clusters in the community, Dr Paul said, “Yes, these could be silent clusters as has been reported from Quarantine hotels in Melbourne – the inquiry report was released today coincidentally.”

“The alternative is that there was an environmental source as is believed in the original SARS outbreak at the Metropole hotel in Hong Kong where individuals staying on the ninth floor who did not have contact with each other were infected. The detailed contact tracing investigation will tell us which is the case.”

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