All 323,000 foreign workers staying in dormitories will be tested for Covid-19 to ensure they are free of the virus before they resume work in the community and return to their dorms, said National Development Minister Lawrence Wong.
This will be done through the use of mass polymerase chain reaction (PCR) tests and mass serological tests, said Mr Wong, as he outlined the Government's "systematic" approach to ensure that dormitories are clear of the coronavirus.
First, serology tests, which can detect if an individual has had Covid-19 in the past, will be applied to dorms with high infection rates, he said at a virtual press conference yesterday.
The Ministry of Health (MOH) said workers with a positive serological test would have been infected in the past - at least 10 to 14 days ago - and would no longer be infectious after a period of isolation.
"After a period of isolation, we can assume they've recovered from the virus," said Mr Wong, who co-chairs the multi-ministry task force tackling the pandemic.
Serological tests detect the presence of antibodies to the virus in the bloodstream. Antibodies are evidence of the body's reaction to an infection, and show that a person was previously infected. Their presence might also suggest the person is now immune to the virus.
PCR tests will be done on workers who test negative in the serology tests, and for those in other dormitories without high infection rates.
PCR technology detects the presence of viral genetic material in patient samples. Such tests will be applied individually or in batches.
MOH said such pooled tests involve combining swabs of up to five individuals in one laboratory test, which does not affect the sensitivity of the tests.
Where a pooled test is positive, the original five individuals could be retested individually to identify the infected person. This is an effective strategy if used where infection prevalence rates are likely to be low, said MOH.
Since such dormitories are not likely to have many infected, this approach should reduce the number of tests required.
As a PCR test cannot detect the virus when it is in incubation, a worker who tests negative the first time will be subjected to a 14-day isolation period, said Mr Wong.
The worker will need to have a second negative result after the isolation period to be confirmed clear of the virus.
Two different tests for Covid-19
POLYMERASE CHAIN REACTION (PCR) TEST
The test detects genetic sequences specific to Covid-19, and is more complex to perform than the serology test as it involves expensive machinery that requires trained technicians to operate.
Samples are taken by means of a swab test from the nose or the back of throat, or from sputum - mucus from the lower part of the lungs.
A positive result indicates a high likelihood that someone is currently infected with Covid-19.
But a negative test does not mean a person is not infected, as the virus could still be in incubation and a subsequent test could be positive.
The test, which is done by taking a blood sample, looks for antibodies produced by the immune system against the virus.
It can be done in satellite labs such as polyclinics.
Those with a positive serology test would have been infected in the past - at least 10 to 14 days back.
But some positive cases may still be infectious, and may still require extended isolation or additional testing.
While the presence of antibodies may show that a person has recovered from Covid-19, it is not clear if they indicate that one has immunity against re-infection.
Serological studies can help uncover the true prevalence of the virus in the population or population segments.
About 3,000 tests are now being done daily in the dorms, and that number will be stepped up in the coming weeks, said the minister.
So far, more than 32,000 workers in dorms have been tested - about 10 per cent of such workers.
It could take several weeks - going into next month or July - to complete the testing process, said Mr Wong.
How quickly the testing can be completed hinges on several factors, including the extent to which Singapore can ramp up its testing capacity and whether the workers need to be isolated or quarantined.
This testing is being done dormitory by dormitory, rather than by sectors, Mr Wong added.
Asked for an estimate of the percentage of workers in dorms who would have tested positive for Covid-19 by the time the testing process is completed, Mr Wong said the true underlying prevalence rate will be known only when all tests have been completed.
If the true prevalence rate is low, the dormitories can be cleared of the virus faster, he said.
But if it is high, more steps will need to be taken to isolate and quarantine workers.
When the workers in dorms resume work, the Government will put in place a regime to test them on a regular basis, Mr Wong added.
This testing regime will also apply to workers in the construction sector who are living outside dorms and serving stay-home notices, he said.
"(This is) because we do not want to have a recurrence of clusters forming among construction workers in particular, now that we have identified this as an activity that could potentially result in large clusters forming," he said.