Coronavirus: why Singapore fears a ‘hidden reservoir’ of Covid-19 cases

Coronavirus: why Singapore fears a ‘hidden reservoir’ of Covid-19 cases

Coronavirus cases in the city state have ballooned past 10,000 from just 1,000 at the beginning of this month
While four in five have been traced to migrant worker dormitories, there are also concerns about cases where the infection’s origin remains unknown

The number of coronavirus infections in Singapore rose to 10,141 on Wednesday, a remarkable increase given the city state had only 1,000 cases on the first day of the month.


Almost 80 per cent of these infections are linked to migrant workers living in 43 mega-dormitories across the country. Equally worrying for authorities is that incidents of local transmission within the rest of the community are still occurring, despite a two-week partial lockdown in which schools have been closed and people told to stay at home as much as possible.


One statistic in particular is troubling: on average, the source of infection for 17 out of 25 new patients daily (who are Singaporeans or residents but not migrant workers living in dorms), cannot be found. This means about 68 per cent of community cases are considered “unlinked”, fuelling suspicions there is a “larger hidden reservoir” of cases within the rest of society.

UNLINKED CASES, WHAT ARE THEY?

Every day, Singapore announces how many new cases have been discovered in the 24 hour period up to 12pm that day. “Unlinked” cases are those in which the source of infection cannot be found, despite aggressive contact tracing and other efforts aimed at establishing the places an infected person has visited and the people he or she has been in contact with.

In most cases, all the close contacts of an infected person are quarantined and monitored for the disease and during this period authorities are usually able to identify the source of the infection.

If a source cannot be identified, one possibility is that this is because there are undetected cases in the community who have the potential to keep spreading the virus

WHAT’S BEHIND THESE UNLINKED CASES?

There may be a few explanations. One, mooted by Health Minister Gan Kim Yong, is that unlinked cases are a sign of “continued seeding” of the disease within the community. There might be virus carriers who display mild or no symptoms but are still infectious.

Recent studies suggest that some Covid-19 patients do not exhibit any symptoms even as they carry and spread the disease. A study in the British Medical Journal on April 2 suggested that 78 per cent of 166 new infections had no symptoms. Singapore researchers have also discovered that patients can spread the disease before they start exhibiting symptoms.

Infectious diseases expert Leong Hoe Nam said it was likely these asymptomatic and pre-symptomatic patients were causing the unlinked cases.

A second explanation may be that in some cases contact tracers have not yet fully investigated the patient’s background and history, due either to time constraints or because the patient hasn’t provided sufficiently detailed information. However, contact tracing becomes harder the more cases there are to trace.