Victoria keeps finding COVID-19 in its wastewater. What’s going on?

COVID-19 has been detected in Victorian wastewater at least 13 times in the past month despite there not having been a single case of coronavirus in the community for 63 days – but experts say the testing program is working well.

Just yesterday, the Health Department contacted hundreds of Victorians and urged them to undergo a coronavirus test after “strong and unexpected” COVID-19 fragments in the city’s north and west, and there are currently active wastewater warnings for at least 80 towns and suburbs across the state.

Wastewater testing is used all over the world to check for the presence of coronavirus and other diseases.Credit:AP

Wastewater testing is a powerful but inexact early warning system, potentially signalling the start of a new infectious cluster before people even know they are unwell and is sensitive enough to pick up one sick person in a catchment of 100,000 people.

The Health Department say it has spotted cases early in Lakes Entrance and Roxburgh Park.

“The detection of COVID-19 viral fragments in wastewater are not ‘false positives’. They are legitimate positive findings of viral fragments which are verified through independent testing,” a department spokesman said.

But it is unable to tell if the viral fragments it picks up come from someone who is infectious, or are from someone who has recovered and is just shedding dead virus, such as a person released from hotel quarantine.

Dr Warish Ahmed, team leader of the CSIRO’s wastewater testing program.Credit:CSIRO

Instead, the signal is used – along with other data, such as the known location of hospitals and hotel quarantines – by the Health Department to work out if an outbreak is possible, and if it is to launch targeted wastewater testing of a suburb, to try to find out if there really is a cluster.

“The tests we are doing are very specific,” said Dr Warish Ahmed, team leader of the CSIRO’s wastewater testing program.

“It does not produce any false positive results at all. When we detect the virus in a sample, it means someone in a catchment is shedding.

“Our point is better safe than sorry. If there is an outbreak, everything shuts down and we are losing a lot of money, our job is down the drain. We are extra careful and take it as a precaution.”

People who are infected with COVID-19 shed microscopic droplets containing the virus’s genetic material. Fragments of SARS-CoV-2’s genetic code end up in wastewater when infected people flush used tissues and wash their hands. The genetic code can also be found in some infected people’s faeces.

In many cases, this shedding starts happening a few days before a person develops symptoms

“That makes the power of wastewater monitoring unique”, said associate professor David McCarthy of Monash University.

A study by the CSIRO suggested cases in Brisbane in early 2020 could have been picked up weeks earlier by wastewater detection.

Eventually, an infected person’s immune system gets on top of the infection, they stop coughing and sneezing, and are no longer infectious. But while they are no longer contagious, they will continue to shed virus – both live and dead – potentially for up to eight weeks after they have cleared the virus.

“What’s clear is it can be for quite a long time, many weeks perhaps, and certainly well beyond the period they are unwell or infectious,” said University of Queensland microbiologist Associate Professor Paul Griffin.

Typically, people shed more virus from their nose and mouth in the early stages of their infection, and more through their stool in the later stages, said Dr Dan Deere, a microbiologist at Water Futures who heads the national wastewater tracking project.

“Hence a person can leave quarantine with a negative test from their nose and throat swab but still be shedding virus from their gut,” he said.

Genetic information from each sample is filtered out and tested using a similar PCR test to that used in humans. The amount of genetic material found can roughly suggest how many people are shedding virus.

“You could very nicely see the concentrations of SARS2 in the wastewater follow a very similar pattern to the clinical cases appearing in Melbourne [during our second wave],” said Professor McCarthy, who works closely with the government on wastewater testing.

“The shape of the curve was the same.”

Once a signal has been detected, the Health Department follows up with targeted wastewater testing to try to home in on where the case might be, and release public health notices to the community asking people with symptoms to go get tested.

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