The Institute of Environmental Science and Research (ESR) has developed a number of new techniques to use in wastewater testing in an attempt to detect Covid-19.
A machine sorts positive Covid-19 samples for genome testing. Photo: RNZ / Dom Thomas
Public health professor Philip Hill yesterday raised concerns about the testing, saying there there were doubts about its sensitivity and its ability to distinguish between viable and non-viable virus.
He said the sensitivity of wastewater testing is not absolutely clear.
"It can't distinguish basically unviable virus from viable virus, in other words when you're no longer infectious and you're still getting rid of still some viral particles that have in parts of the genome are still detectible, then it'll still be positive in the wastewater."
ESR chief scientist Brett Cowan agreed and said people can continue to shed the virus for some time, even for weeks after they are infectious, but the amount of the virus that they are shedding is generally falling during this time.
But he said the sensitivity of wastewater testing is now much better understood.
"We know if we test outside Jet Park, Auckland's quarantine facility, that the water is always positive, but that's a very easy case. If we add 100,000 people to that wastewater stream and we test at the end of that stream we know that perhaps if we have five or six people positive in that catchment that we're probably going to detect it."
He said wastewater testing is just one of the tools in the kit and at the moment the most important tool is contact tracing.
Cowan said wastewater testing gives a broad picture akin to one taken by a satellite.
"We can see a huge amount of New Zealand, up to 80 percent of it but we see it at a slightly lower resolution."
Cowan said tests tend to be done at the city level and an unexpected positive result would require further investigation.
He said firstly they would test more frequently, perhaps moving to daily testing.
"The second thing that we would do is if a particular town had four main pipes coming into that treatment station we would test those individually. If we found one of those was positive we could then work our way up the piping system to get more and more specific."
Cowan said how much of the virus that people with Covid-19 shed is quite variable with some people shedding a few thousand viral particles while others shed billions.
Cowan said ESR scientists have been looking for viruses such as norovirus in wastewater for a few years now.
"What's new however is the focus on Covid, there are a number of new techniques and approaches that we're using for Covid that have allowed us to improve the performance of this testing."
