The Delta variant is a highly contagious Sars-CoV-2 virus strain. Video / Paul Slater
A new ESR-led study suggests regular wastewater testing for Covid-19 – now being done across the country to guide our Delta outbreak response – could prove a nifty early warning system to pick up future flare-ups.
Because traces of Sars-Cov-2 can be isolated from what we flush down the toilet – and sometimes survive up to several days after leaving an infected person's body - wastewater-based epidemiology has played an increasingly large role in New Zealand's pandemic toolbox.
In the new study, released online ahead of peer review, ESR scientists spent four months sampling wastewater from an Auckland population of approximately 120,000, where cases were known and uniquely all located in a single managed isolation and quarantine facility.
The study involved the daily collection of 113 composite wastewater samples from the sewer outside the MIQF, and from the municipal wastewater treatment plant 5km downstream.
Over the period, the virus was detected in 54 per cent of cases at the treatment plant, compared with 95 per cent at the facility.
What's called logistic regression - or the process of modelling the probability of a discrete outcome given an input variable - was used to estimate the shedding of the virus into wastewater based on four infectious shedding models.
With a total of five and 10 Covid-19 infectious cases per 100,000 population, the predicated probability of detection at the treatment plant was estimated to be 28 per cent and 41 per cent, respectively.
However, when a more realistic proportional shedding model was used, this increased to 58 per cent and 87 per cent for five and 10 cases, respectively.
Some of the collected wastewater samples at ESR's Wellington laboratory. Photo / ESR
"In other words, when 10 individuals were actively shedding the virus in a catchment of 100,000 individuals, there was a high likelihood of detecting viral RNA in wastewater," said Dr Joanne Hewitt, an ESR senior scientist who co-led the project.
"The evidence provided confidence that the detections at the treatment plant were associated with increasing Covid-19 cases."
ESR chief scientist Dr Brett Cowan said the results reaffirmed the role of wastewater sampling as part of New Zealand's response to Covid-19, along with community testing and genome sequencing.
"Wastewater-based epidemiology is being used as a critical part of international public health responses to Covid-19 to monitor infections at the community level," Cowan said.
"The results of the study highlight that if the virus is in the community, then there is a strong probability we will pick it up.
Wastewater-based epidemiology has played an increasingly large role in New Zealand's pandemic toolbox. Photo / Supplied
"We are confident wastewater testing is going to become an even more important part of our public health efforts."
Dr Brent Gilpin, ESR's water and waste team manager, added the findings of the study were being borne out by the detections uncovered in the recent outbreak.
"The recent example of detecting SARS-CoV-2 in Warkworth, further demonstrates the benefit of the testing of wastewater samples.
A positive detection in the wastewater triggered health officials to specifically advise people in the area to get tested if they had any Covid-19 symptoms, despite there being no known cases.
The research was supported by MBIE's Covid-19 Innovation Acceleration Fund, and was a collaboration between ESR, Watercare and the University of Auckland.
