Sampling and testing of wastewater is helping governments across the world to track COVID-19 infections on a large scale.
Viruses are some of the simplest biological structures on earth. They are not ‘alive’ and cannot replicate outside of their host. How do you track infectious disease outbreaks at the scale of countries or continents? Lack of available testing in most countries, either in the form of quantitative methods or medical tests, prevent a public health response to the ‘rapidly rising tide’ of cases that occurs in easily transmitted infectious diseases. This includes the current pandemic virus – SARS-CoV-2. Testing is invasive, difficult to perform and requires repeat testing of individuals who are susceptible to the disease, so other approaches are needed.
It is common for people infected with viruses to release them in poo and wee. Therefore, one popular approach to monitor outbreaks is wastewater-based surveillance (WBS). Before COVID-19, it was used for monitoring polio and drug use. The value of WBS is that decision-makers can estimate the amounts of disease circulating in the population with only a few samples. In total, tens of thousands of researchers in over 50 countries are undertaking WBS for SARS-CoV-2. Alongside the vaccination roll-out, it is one of the biggest mobilisations of scientists since the Second World War for a single vision.
Grab samples or a dedicated automatic sampling device are set up at various points in the sewage/wastewater infrastructure. This includes discharge points from large buildings like schools and hospitals, sewage pipes, and at water recycling centres (sewage works). Some countries like China have also started monitoring toilets and the swabs and poo obtained directly from individuals. Thankfully, the UK is not sampling ‘at source’ yet… Collecting wastewater with respect to COVID-19 is safe; the World Health Organization suggested in 2020 that wastewater does not represent a friendly environment for this virus and so is unlikely to infect people.
However, many challenges remain in the collection, analysis and interpretation of this data. On the wastewater side, municipal sewage is often changed by industry inputs and rainfall from roads and buildings. These inputs to the sewage system do not contain the ‘signal’, which is the virus fragments (RNA), but contain lots of dilution in the best case or ‘noise’ in the worst case. On the medical side, one aspect we do not understand well with SARS-CoV-2 is faecal shedding. This is the number of viruses each infected individual will shed in the faeces (in this case) or nose and throat for mass testing. At present we don’t fully understand how long people will shed the virus and how long it will be detectable in wastewater. Another question is whether children or vaccinated individuals shed viruses to the same extent. Or how poo shedding changes in people infected with different variants (mutant strains of the virus). These factors are important to interpretation of the data for wastewater surveillance. One further challenge is how to monitor in ‘low-prevalence’ scenarios – this is where the number of infected individuals drops due to lockdowns, vaccination or natural immunity. Wastewater surveillance has been credited as one of the big success stories of the pandemic, helping governments respond in a rapid way without infringing the ethics and data protection rights of individuals. A major challenge is communicating the results of these surveys to the public in an appropriate way.
Dr Francis Hassard is a lecturer in public health microbiology at Cranfield University.