Last week on Monday (14 December), the Singapore Ministry of Health (MOH) revealed in a statement that 98,289 migrant workers living in dormitories had tested positive for COVID-19 in serology tests.

Serology test results show whether a person has previously been infected with COVID-19. This means that those 98,289 workers had been infected with the virus at some point before but have since recovered.

This is different from those who test positive for the virus via a polymerase chain reaction (PCR) test, which shows if a person is currently infected.

This revelation by MOH shot up the total tally of migrant workers who have had COVID-19 to 152,749 — about 47 per cent of all migrant workers living in dormitories in Singapore.

This includes the original figure of 54,505 that was already public knowledge, which is the figure that the MOH updates daily as it continues its contact tracing efforts.

The MOH situation report still only includes the figures based on the PCR tests.

TOC questioned in a previous report why the number of positive serology tested cases were not included in the total tally of migrant workers who have tested positive for COVID-19, given that we know they have had the virus before.

Now, the MOH might say that they are merely following the World Health Organisation (WHO) diagnostic guidelines, which state that a suspected case of COVID-19 is confirmed when:

  • a person is tested positive on a Nucleic Acid Amplification Test—such as the PCR used in Singapore
  • a person is tested positive on an antigen rapid diagnostic test AND meets the case definition for probable or suspected cases; and
  • an asymptomatic person who tested positive on an antigen rapid diagnostic test who is a contact of a probable or confirmed case.

The WHO diagnostic guidelines also stated: “Serology should not be used as a standalone diagnostic to identify acute cases in clinical care or for contact tracing purposes.”

This means a positive serology test has to be considered with other factors such as the timing of the disease clinical morbidity and prevalence of the virus within the setting the patient is in before it is classified as a case of COVID-19.

What changed in MOH’s diagnostic criteria?

The thing is, we know that the MOH has classified COVID-19 cases which came up positive on a serology test before.

For example, back in March when contact tracing efforts revealed that two people — Cases 83 and 91 — who were identified as links to other cases were given serological tests since they have already recovered by the time MOH traced the infection chain back to them.

An article on Gov.sg noted, “MOH arranged for Cases 83 and 91 to be tested at NCID. However, as both were already well, the current test to confirm COVID-19 infection would not work.”

It continued, “However, knowing that the immune system produces antibodies in response to the infection – which can stay with the recovered patient for several years – MOH decided to test them with a new type of test, known as the serological test.”

The results of the serology tests were positive for both cases. This, combined with links found between other cases eventually led to the MOH classifying them as COVID-19 cases and adding them to the total tally of COVID-19 cases in Singapore.

So, what changed between then and now?

Why did the MOH classify these two cases as COVID-19 cases that were identified via serological testing but not the 98,289 cases of migrant workers in dormitories which were positive on serological tests as well?

Further, can the MOH simply change its diagnostic criteria along the way?

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