Wastewater surveillance was hailed during the COVID-19 pandemic as a more equitable way to track disease. It provided a system that could monitor entire communities regardless of whether residents had access to a doctor or a test. But a new study published in the American Journal of Public Health finds wastewater surveillance carries its own built-in blind spots, and the communities bearing the brunt are the ones already most vulnerable.
Researchers at the Maxwell School worked with New York State's wastewater surveillance network and found that while the system does a reasonably fair job of including vulnerable populations, it struggles in larger populations when an outbreak is starting, which is when it matters most.
The reason is that vulnerable communities tend to be in cities and are connected to large wastewater treatment plants that serve hundreds of thousands of people. When a single infected person sheds a pathogen, it's diluted across an enormous volume of water. This makes early detection more difficult. Smaller, less vulnerable communities (where wealthier people tend to live) are served by smaller plants where a single case is easier to spot.
"Wastewater surveillance inherently has a high degree of equity in terms of inclusion," according to Professor David Larsen, lead author of the study. "But it also inherently has a high degree of inequity in outbreak detection."
As an example, more than 80% of people living in poverty in New York State lived in areas where an outbreak would surpass 10 infections before consistent detection in wastewater.
The findings arrive at a critical moment. The Centers for Disease Control and Prevention's National Wastewater Surveillance System now faces uncertain funding as emergency appropriations expire. Decisions about which treatment plants remain in surveillance networks are currently being made. The study warns that cutting smaller plants to reduce costs could make existing inequities considerably worse. "Outbreaks don't stay in one location. If they are problematic, they spread. Detecting an outbreak in a smaller community could signal a larger outbreak in a more vulnerable community nearby," says Professor Larsen.
The authors propose several strategies to close the gap. Taking wastewater samples upstream from large treatment plants (at the neighborhood level rather than at the plant itself) can detect outbreaks in smaller sub-communities before dilution makes it more difficult. Expanding plant participation in smaller communities and improving statistical modeling for low-population areas could also reduce inequities, though the researchers acknowledge these approaches carry significant cost.
