Do sewage alerts flush out Covid-19 outbreaks?

It’s a familiar headline in the Covid-19 era – thousands of residents “on alert” after virus fragments are found in sewage, suggesting undetected cases could be lurking in the community.

Every few days or so, health authorities across Australia issue fresh warnings – 20 suburbs on the Sunshine Coast here, 100,000 residents of Western Sydney there – urging people to come forward and get tested.

The sewage surveillance program, which analyses samples of wastewater for fragments of SARS-CoV-2, started in NSW last July and was soon replicated in other states.

In theory, the goal is to provide “early warning” of Covid-19.

Of roughly 18,000 combined samples tested to date – 4488 in NSW, 11,057 in Victoria and 2441 in Queensland – hundreds have returned positive results for SARS-CoV-2, the virus which causes Covid-19.

A large number of those positive tests are expected, as they come from sewage treatment facilities in catchment areas which serve quarantine hotels or hospitals.

But when an unexpected positive result is discovered, suburbs go “on alert”.

Sky News host Paul Murray took aim at the trend earlier this year, claiming the “familiar” headlines were “there to scare everyone”.

“If I seem like I’m not taking this that seriously, it’s not because I don’t take covid seriously – it’s just we’ve seen this dance quite a few times,” he said.

The wastewater alerts drive thousands of residents in the affected areas to get tested. Naturally, some might then wonder – do they ever amount to anything? posed that question to health departments in the three biggest states.

But out of NSW, Victoria and Queensland, only Victoria’s Department of Health and Human Services was able to cite an example where wastewater surveillance led to the detection of previously unknown infections in the community.

“Wastewater testing is an important element of Victoria’s Covid-19 response, providing an early warning for communities which complements individual testing,” a DHHS spokeswoman said.

“Wastewater testing has provided early warning of Covid-19, with viral fragments previously detected in wastewater on four occasions for the current outbreak response – in the Epping area, Altona, Kensington and Ivanhoe area call outs.”

That included the first case diagnosed in the City of Whittlesea outbreak, after virus fragments were detected in wastewater from the Epping area.

“The confirmed case was diagnosed after residents of Epping and Wollert were encouraged to get tested if symptomatic following the wastewater detection,” she said.

In February, NSW Health and Sydney Water were awarded the Research and Development Excellence Award for the surveillance program at the NSW Water Awards.

“The work being done by both the NSW Health Water Unit and Sydney Water cannot be understated – it’s been a key element of our response to the global pandemic,” NSW Health’s Director of Health Protection, Dr Richard Broome, said in a press release, which stated that the program had “proved a vital tool in efforts to contain the spread of Covid-19”.

But a NSW Health spokeswoman could not cite any examples of a wastewater call-out leading to the detection of unknown outbreaks, saying only that “on several occasions” it had indicated the “possibility of positive cases in a community”.

“The sewage testing program provides a crucial extra level of surveillance at a population level,” she said.

“The program has detected results from sewage that align closely with known cases of Covid-19, giving confidence in its reliability and sensitivity. On several occasions, the program has provided an extra signal indicating the possibility of positive cases in a community, which has proved helpful in encouraging local populations to get tested – assisting another key goal of public health efforts.”

She added, “Covid-19 remains a constant threat and this testing can provide an early warning in places without recent known cases.”

A spokeswoman for Queensland Health was also unable to cite any examples.

“Queensland Health uses a range of information sources to monitor for Covid-19 risk in the community and support the state’s pandemic response,” she said.

“A detection of SARS-CoV-2 in wastewater is used in combination with or to support other information available to Queensland Health, such as recent known cases of Covid-19 in that area, evidence from contact tracing, clinical testing (including monitoring of trend data) and local intelligence regarding vulnerable groups (such as within aged care) or high transmission risk facilities.”

The spokeswoman said wastewater surveillance provides “information at the community level on shedding of viral particles into the sewer from everyone”.

“This means that, as new cases and clinical testing rates decline, wastewater surveillance can provide valuable data on any continuing presence of SARS-CoV-2 within communities,” she said.

“However, a detection of viral particles in wastewater does not always mean that there are active (infectious) cases in the community. Detections can be related to a recovered case who is still shedding but is no longer infectious. We know some Covid-19 cases can shed viral fragments for a couple of months after they are no longer infectious.”

Cobus Gerber, associate professor of chemistry at the University of South Australia, has argued that sewage testing can be helpful but it’s no “magic bullet”.

He notes there is the risk both of false positives and of false negatives.

“If Covid-19 virus fragments were detected in the wastewater in my area, I would be alert for symptoms and I would also take necessary precautions,” he wrote in The Conversation last month.

“I’d be ultra careful about sharing surfaces, physical distancing, wearing a mask and using hand sanitiser. It may be nothing but, at the same time, I would just follow the health advice.”

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