Explainer | 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
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.
Prime Minister Lee Hsien Loong warned of this possibility on Tuesday when he announced the extension of “circuit breaker” measures till June 1.
UNLINKED CASES, WHAT ARE THEY?
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.
Storm in a bubble tea cup as Singapore coronavirus circuit-breaker extended to June
However, in some cases, it might simply mean a more thorough investigation is needed. For example, in February when total infections numbered in the hundreds, authorities took four weeks to discover that a cluster of infections at one church was linked to another set of infected church-goers. Tourists from China had visited one church on January 19, a married couple who was at the same church service then attended a Lunar New Year family gathering on January 25 where a staff member from the second church was present. From April 15 to 21, authorities uncovered links for 918 previously unlinked cases.
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.
“When the number of daily new cases is low, the contact tracing teams can perform a much deeper investigation and surveillance,” said Teo Yik Ying, dean of the National University of Singapore’s (NUS) Saw Swee Hock School of Public Health.
“As the number of cases reported daily over the past two weeks has increased considerably, so the contact tracing efforts have become harder – you can also imagine the database of known cases has grown, and every newly reported case now has to be compared against this much larger database.”
In February, for example, Singapore was reporting fewer than 10 new cases a day. On three days this month more than 1,000 new cases were reported.
Coronavirus: in Singapore, foreign workers were locked in dorm room
The contact tracing team has been strengthened to meet this growing need, going from 70 contact tracers working in shifts seven days a week on February 10, to involving 1,300 servicemen from the Singapore Armed Forces at present. Contact tracers interview each patient about their actions and contacts over the previous 14 days, then call each of these contacts to get more details and determine whether they are a “close contact” who should be put in quarantine.
South Korea also does contact tracing, but its process is less labour intensive. Contact tracers there use patients’ health records, credit card transaction data, CCTV and mobile phone locations to trace their movements and find their contacts.
HOW WORRIED SHOULD WE BE?
The circuit breaker measures that have banned social gatherings and reduced public transport usage and traffic volume by 70 per cent have helped to reduce community transmission.
In the week before measures began on April 7, there were an average of 39 cases a day within the rest of the community, defined as Singaporeans and residents other than low-wage workers living in mega dorms, and this has fallen to 25 in the past week. But a higher percentage are unlinked, compared to about a month ago on March 22, when there were 455 infections in total, and of the 23 reported that day, two were unlinked.
“Suppose we have 50 unlinked cases today, it may actually mean that there are at least 50 places out in the community that a person may have been infected,” said Teo, using an analogy he said wasn’t perfect.
Singapore extends coronavirus ‘circuit breaker’ measures to June 1
“Compare this to a situation where all 50 cases are actually linked within a single cluster, this means there is actually a point source where we can rely on active contact tracing to find out other exposed contacts, isolate and quarantine them early, to contain further spread, as early as possible.”
One piece of good news is that unlinked cases are being discovered earlier on in the chain of transmission. According to the health ministry, these cases are now found and isolated from the community 3.5 days after the onset of symptoms. That compares to 11.5 days at the start of February. Leong said this meant “we are catching them earlier and earlier, which is good. There is a lower risk of it spreading”.
WILL THE CIRCUIT BREAKER REDUCE UNLINKED CASES?
Teo said the extended circuit breaker period, by keeping more people at home with additional workplaces closed and restrictions on the number of workers in essential services, would hopefully give authorities a better sense of where transmissions were still occurring.
National development minister Lawrence Wong said on Tuesday that analysis of local infections after the circuit breaker found that many of the people involved were working in essential services or had family members working in essential services. Hence authorities shortened the list of essential services to cut the community workforce from the current 20 per cent to 15 per cent.
Authorities hope the extended circuit breaker will reduce both infections in general, and the number of unlinked cases.
A reduction in unlinked cases would suggest that contact tracing and case detection efforts were working well. Experts agree that the ability to swiftly track contacts of new patients and isolate them will determine how well countries can cope with any new outbreaks. This will help Singapore, and other countries determine their exit strategies from lockdowns and reopen their economies.
