A Covid-19 wastewater testing pilot's being rolled out around the country as a safety net for catching the virus in the community.
The eight-week ESR pilot will see sewage sampled at managed isolation and quarantine facilities as well as in communities that do not have MIQ hotels - including Gisborne, Dunedin, Queenstown, Taupō and Whangārei.
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People with Covid-19 can excrete the virus for weeks, so a positive wastewater test would mean health authorities could target testing in a community to contain the spread.
"When the work first started there were dreams that you'd be able to look at the sewage and say, 'this relates to five people or 500, or a few people'," ESR's Dr Brent Gilpin told Checkpoint.
"I think as we're realising, people can shed the virus anywhere from just a few hundred per ml of whatever you're looking at, through to potentially trillions of viruses.
"What you can really say from detecting in sewage is that there's at least one person excreting virus sometime in the previous period. Some of the things we're really learning is around how long people shed the virus.
"People typically shed the most virus around the time that they show symptoms and two to five days afterwards. But people may shed virus for up to five weeks afterwards, in their faeces or from other forms.
"When we're looking at sewage we're also looking at what you spit. So everything that goes into the sewer system, you brush your teeth, you spit, you clean your nostrils out, you hoick the shower, as well as what comes out through the number ones and twos.
"So both faecal deposits as well as what you might be spitting are both going to contribute into the sewer.
"Detections in the sewage of viral fragments - the response to this is to prompt additional testing of people. In Australia they've had a number of detections in wastewater, and in most of those they haven't actually found people in the community who are actively shedding. So there certainly hasn't been an outbreak, which may mean that either someone who was previously infected and is no longer infectious was shedding some virus, and then when they went back to look at them they'd stopped shedding.
"Or else it may in some of those situations have been a case where someone was passing through - the truck driver who stopped in one of these locations, left a deposit in the sewer system and then carried on.
"It's unlikely that you can really get down to definitively saying this is one person or five people.
"What I think we will be able to say as it is someone or a number of people who are more likely to be infectious based on the levels of virus present.
"And then the other bit that we're looking to develop is the genotyping approach - being able to take what we detect in the search and match it up with the known strains of the SARS-COV2 virus.
"It's amazing when you think about the hundreds of millions of litres of sewage a day. We're taking 24 composite samples, we'll be taking a few ml of water every 15 minutes, combine that together into a sample.
"And then there's quite a complicated extraction process of concentrating that sewage down, pulling out the viral fragments of everything that's there. Then we use the same PCR-type detection, to look for these fragments.
"We're looking really at the extremes of the detection limits, when you get into sewage, but we are able to pick up virus that could be from just one person. But that one person could be shedding trillions of viruses, if they're a highly infectious person."