How wastewater testing works
Wastewater testing is a surveillance tool used to help track COVID-19 in Aotearoa New Zealand. We use it alongside community testing of people with COVID-19 symptoms.
The Institute of Environmental Research and Science (ESR) collects wastewater samples for testing. Wastewater testing sites have a mixture of wastewater from toilets, sinks and drains of hundreds of thousands of people.
They test these samples to see if it contains fragments of SARS-CoV-2, the virus that causes COVID-19. This allows for widescale testing for COVID-19 in the community at one time.
If there are increasing levels of the virus in the wastewater, it suggests cases are growing in the community.
Wastewater testing gives us an idea of transmission in the community that is not dependent on peoples' testing behaviour.
How we use wastewater testing
Wastewater testing provides:
- an early warning of COVID-19 cases in the community, to guide our behaviours and help inform the public health response
- a sign of how much virus is circulating in the community and shows increasing and decreasing trends in the transmission rate
- detection of which SARS-CoV-2 variants are circulating in the community.
Wastewater is currently sampled from many sites around the country. ESR tests these samples for SARS-CoV-2, to estimate the amount of virus circulating in that area. ESR takes samples from fewer sites to check for variants of SARS-CoV-2.
The frequency of sampling depends on:
- the local population
- access to wastewater collection points
- risk factors.
ESR provides a weekly wastewater surveillance report and dashboard to help you track potential COVID-19 risk in your area.
Detecting SARS-CoV-2 in wastewater
Detecting SARS-CoV-2 in wastewater depends on:
- how many people are shedding the virus in the area
- how many people’s wastewater goes into the sampling site
- how diluted the wastewater is.
The more people in the area that are shedding the virus, the better the chance is of detecting it. Even if ESR find no virus in the wastewater, this does not mean there is no COVID-19 in the community.
Shedding of the virus
People may shed fragments of the virus into wastewater when they have an active COVID-19 infection.
The amount of virus shed varies between people and over the course of a person’s infection and recovery. People tend to shed many times more virus while they are infectious, compared to later in their infection.
Those who have recovered from COVID-19 and are no longer infectious may continue to shed small amounts for weeks after they recover.
The levels of virus detected in wastewater tracked over time, tend to match with the number of active cases in the community. It is more accurate in larger populations than in smaller communities, which can have more variability.
Wastewater testing can show increasing or decreasing trends in cases.
Vaccines do not contain any of the live virus, so do not cause viral shedding and are not found as viral fragments in wastewater.
How we respond to SARS-CoV-2 detection in wastewater
The levels of virus in the wastewater give us information on whether cases are increasing or decreasing. If the levels of virus in the wastewater increase, then likely cases in the community are growing.
As cases grow, we encourage you to stay home if you are sick, get a test, and wear a mask in public indoor spaces.
Infection risk of COVID-19 from wastewater
There is no risk of infection from the viral fragments in wastewater.
Last updated: at