A "low-level" detection of Covid-19 was found in a wastewater sample taken on Thursday in Tauranga. But how does wastewater testing work and what does this mean? Institute of Environmental Science and Research (ESR) senior scientist Brent Gilpin explains.
Wastewater is used water from toilets, showers, baths, basins, sinks and laundries that pass through the sewerage system.
ESR tests wastewater because the presence of viral fragments shows the virus that causes Covid-19 has been found in the sewerage system catchment, he said.
A recent ESR study showed when 10 people were "actively shedding" the virus out of 100,000 people, there was a "high likelihood" of detecting viral RNA [a molecule] in wastewater.
"With less cases - potentially from just a single case - it may still be possible to detect the virus," Gilpin said.
There were "several possible explanations" for this detection, including "post-infectious shedding" by someone released from MIQ or from an individual in the community.
The Ministry of Health and Ministry of Business, Innovation and Employment has been approached for comment about how many people were released from the quarantine facility and travelled to Tauranga between September 9 and 23.
Institute of Environmental Science and Research senior scientist Brent Gilpin. Photo / Supplied
What happens now?
Gilpin said the institute's agreed protocol when low levels were detected, as in this case, was to progress with a second round of testing.
"If a second test is positive, it would signal the probability of community cases and potentially transmission (even if only within a household)," he said.
"However, we have also had previous initial detections in samples (New Plymouth and Wellington) without the presentation of any cases."
A detection of the virus can provide an "early warning" of the need for increased local vigilance, clinical testing and allow health authorities to issue public health advice to prevent transmission.
Background
The wastewater sample was received by ESR on Friday and the result was reported in less than two working days. This included time for "repeat analysis" of the samples due to the initial "low-level detection of the virus", Gilpin said.
A sample from Tauranga was analysed on September 21, and no virus was detected. Wastewater in Tauranga is tested twice per week, he said.
Director-general of health Dr Ashley Bloomfield. Photo / NZME
In the 1pm briefing yesterday, director-general of health Dr Ashley Bloomfield said a wastewater sample taken on September 23 in Tauranga had tested positive.
Further samples were taken from the wider nearby areas and results are expected on Thursday, Dr Bloomfield said.
Bloomfield said people in Tauranga should not be worried about the wastewater testing results as unexpected testing results could be due to recovering cases.
The positive test was "too far down the track" to be connected to the truck driver who tested positive and visited Tauranga, Cambridge and Hamilton two weeks ago, Bloomfield said.
Workers travelling from Auckland to Tauranga should check they are up to date with testing, and people in Tauranga were also advised to get tested if symptomatic.
