Here’s a brief update, as Singapore remains on tenterhooks over a rise in community infections through April and May with clusters traced to a hospital ward, Changi airport and a tuition centre.

Between 1 April 2021 and 15 May 2021, there were seven cases of Covid-19 detected among dormitory-resident foreign workers.  April saw five cases with two more uncovered on 1 May.

Here’s the overall picture for the period under discussion, from Channel NewsAsia:

Three of the seven dormitory residents who tested positive in this period worked either as lashing specialists or stevedores at seaport terminals, and all three had been fully vaccinated in January and February 2021. All three were asymptomatic and their cases detected only through routine testing.

Another three worked at shipyards. Two of them worked at a shipyard in Sembawang and were also roommates with each other in a Woodlands dormitory. The third worked at a shipyard in Tuas; his dorm was also in Tuas.

One of the Sembawang shipyard workers had a runny nose. He received his second dose of the vaccine only three days before symptoms developed and so it might not have had sufficient time to take full effect.

Ther other two shipyard workers had been fully vaccinated, receiving their second dose at least three weeks before their tests came back positive. Both were asymptomatic.

Sole construction worker

The seventh worker is the sole construction worker in this group, employed by what appears to be a renovation contractor (going by the name of the company). He lived in the same Woodlands dorm as two cases among the shipyard trio though in a different housing block. After the first shipyard case was found, this construction worker was among the many who were placed in quarantine. Testing while in quarantine returned a positive result.

However — and this is where it gets interesting and cautions us against drawing conclusions hastily — MOH said that a second test came back negative.

Another test conducted by the National Public Health Laboratory on 25 April was negative for COVID-19 infection. His CT value was very high, and his serology test result has also come back positive. Based on his travel history, he was likely to have been infected while he was overseas, and is shedding minute fragments of the virus RNA, which are no longer transmissible and infective to others.

This worker was in Bangladesh from Febraury 2020 to December 2020. He was tested during his stay-home period after coming to Singapore in December and also underwent rostered routine testing after that; all results were negative.

MOH did not disclose exactly how high his CT value was to have come to the view that he was very likely only to be shedding minute gragments of viral RNA after an old infection.

But what is “CT value”?

Basically, how the Polymerase Chain Reaction (PCR) test works is to use a reverse transcriptase enzyme to convert any trace amount of viral RNA into DNA; then to amplify the amount of this complementary-DNA using another enzyme — DNA polymerase. See The Science Behind the Test for the COVID-19 Virus. By this method, minute fragments of complementary-DNA transcripted from virus RNA in a sample can be amplified into quantities sufficient for detection using fluorescing probes.

Swabbing samples can begin with highly varying amounts of virus, and so the amplification may need to go through few or many cycles before the complementary-DNA becomes detectable. Typically, if nothing is detected after 37 – 40 cycles, the result is considered negative.

“CT” stands for Cycle Threshold, i.e. the number of cycles needed to arrive at a positive result. A high CT value indicates the amount of viral genetic material in the original sample was very small, needing many cycles of amplification before it could be detected.

See also: One number could help reveal how infectious a COVID-19 patient is. Should test results include it?