SINGAPORE - A pilot programme launched here in February could detect the level of coronavirus spread through tests of wastewater, an approach that has implications for how policies like individual testing and isolation are effected.
A team led by the National Environment Agency (NEA) is currently monitoring wastewater in 20 large foreign worker dormitories as part of the Government's gradual clearance of dormitories of Covid-19 to allow workers to return to work.
The presence of viral material in samples of wastewater from some dorms has led to more targeted swab tests on workers there, leading to more detection of cases and earlier isolation.
On the other hand, the absence of viral material at other dorms has given the authorities added confidence that they remain free of the virus when no cases have been reported.
This detection method is based on the understanding that even asymptomatic individuals can shed virus in their stool, registering their viral load when an infection could otherwise have been missed.
Research in the Netherlands has recently explored the potential of using sewage surveillance as an early warning alarm for disease outbreaks. Australia has also rolled out a programme of sewage testing to detect virus clusters.
In a statement on Friday (June 19), the NEA, together with national water agency PUB and Home Team Science and Technology Agency (HTX), said the programme is still in its early stages here, although there are plans to expand it.
More information on the gleaned results will be shared later, they said.
The unusual method has been in development and put to gradual use since February, after the first case of the coronavirus was reported here on Jan 23.
Trial results have found a correlation between the concentration of coronavirus material in wastewater and the prevalence of Covid-19 cases in dormitories.
"The trending of Sars-CoV-2 (the virus that causes Covid-19) over time can determine if infection control measures taken have been effective," the agencies said.
The method, however, has its limitations. In particular, it seems to be less effective when there are few cases, a shortcoming which means more research is needed to understand its use as an early warning system.
When Singapore had 160 Covid-19 cases nationwide on March 9, viral ribonucleic acid (RNA) was not detected in the wastewater samples that the agencies pooled from the water reclamation plants where they had also been trialling the programme.
Viral RNA became detectable only later, in late March, which correlated with the increase in cases in migrant worker dormitories across the island.
"Several reports overseas have shown that wastewater testing at treatment plants could be useful for early detection of Covid-19 transmission in the community. However, at low level transmission, wastewater surveillance at the treatment plant appears to be less sensitive than clinical surveillance of cases in Singapore," said director of NEA's environmental health institute, Associate Professor Ng Lee Ching.
She explained that this was because Singapore's clinical testing is intensive and picks up more cases. According to statistics compilation website Worldometer, Singapore conducts about 98,514 tests per million of its population, ranking 22 in the world.
The NEA also said that viral material in wastewater "does not suggest the presence of viable or infectious virus".
"Without a host, the virus will not be able to propagate over time in wastewater. As an added preventive measure, wastewater from locations with Covid-19 cases, such as hospitals, isolation facilities and dormitories, (is) disinfected with chlorine at the premises before discharge into the public sewers."
The method was jointly developed by scientists from the NEA, with input from the Singapore Centre for Environmental Life Sciences Engineering at the Nanyang Technological University, the Singapore-Massachusetts Institute of Technology Alliance for Research and Technology, and the National University of Singapore.