COVID-19: Wastewater testing

Wastewater testing is a surveillance tool used to help monitor for COVID-19 in Aotearoa New Zealand. It is used alongside COVID-19 testing in the community, hospital surveillance and testing of international arrivals.

Increasing levels of virus in the wastewater are correlated with increasing transmission and numbers of cases in the community. Quantifying wastewater viral levels is a measure of transmission in the community that is not dependent on peoples’ testing behaviour, for example how likely they are to test or upload their results.

Wastewater is collected from sites that contain a mixture of the wastewater from the toilets, sinks and drains of hundreds of thousands of people in a community. A sample of the water is then tested to see if it contains fragments of SARS-CoV-2 – the virus that causes COVID-19. This allows testing of hundreds of thousands of people for COVID-19 at once.

How wastewater testing is being used

Wastewater testing can be used to provide:

• an early warning of COVID-19 cases in the community, to indicate need for increased local vigilance and clinical testing and help inform the public health response
• indication of how much virus is circulating in the community and show increasing and decreasing trends in the transmission rate
• detection of which SARS-CoV-2 variants are circulating in the community.

Wastewater surveillance

Wastewater is currently being sampled from many sites around the country. The Institute of Environmental Research and Science (ESR) tests these samples at their Wellington lab to determine if they can detect SARS-CoV-2, estimate the amount of virus circulating in that area. There are a smaller number of sites are sampled to monitor the variants of SARs-CoV-2.

The frequency of sampling will vary depending on the local population, access to wastewater collection points and risk factors.

ESR provides a weekly wastewater surveillance report and has a wastewater surveillance dashboard to help the public track potential COVID-19 risks in their local areas. Read more about ESR COVID-19 wastewater surveillance.

Detecting SARS-CoV-2 in wastewater

Detecting SARS-CoV-2 in wastewater depends on many different factors, including how many people are shedding the virus in the area, how many people’s wastewater goes into the sampling site and how much the wastewater is diluted.

The more people in the area that are shedding virus means, the better chance there is of detecting it. Even if we find no virus in the wastewater, this does not necessarily guarantee an absence of COVID-19 in the community.

People may shed fragments of the virus into the wastewater when they have an active COVID-19 infection. The quantity of viral fragments 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 on in their infection. Those who have recovered from COVID-19 and are no longer infectious may continue to shed small amounts of fragments of the virus into the wastewater for weeks after they recover.

The levels of virus detected in the wastewater tracked over time, tend to correlate with the number of active cases in the community. Viral levels in the wastewater are better at correlating with number of cases in larger populations, as there is more variability with smaller communities. Wastewater testing can be used to indicate increasing or decreasing trends in cases, rather than estimating the number of cases in the community.

Vaccines do not contain any of the virus that causes COVID-19, or any other live, dead or deactivated viruses so do not result in viral shedding and cannot be detected 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 the epidemic curve may be increasing or decreasing. If the levels of virus in the wastewater increase, then likely there is increasing transmission in the community.  Health advice may also be increased, encouraging those who are sick to stay home and get a test, reminders about hygiene, masks, ventilation, and other preventive measures.

If anyone has symptoms consistent with COVID-19, especially if they are in regions who have had COVID-19 detected in wastewater, they should stay at home, do a rapid antigen test and if you test positive follow the health advice.

Please keep up the hygiene measures that are critical to stopping the spread of the virus – wear a face mask,, wash your hands, and cough or sneeze into your elbow. 

The risk of infection with COVID-19 from wastewater

There is no risk of infection from the viral fragments in wastewater.