"Many measures of Covid-19 and other public health surveillance rely on people to seek out clinical testing or medical treatment, and those behaviors have changed over the past few years. Wastewater surveillance offers a more consistent approach by monitoring the amount of virus shed in sewage systems. But interpreting that data can be complicated — and with Covid, wastewater levels can’t be directly translated to case counts. The amount of virus that an infected person sheds depends on a many factors, including the presence of antibodies from a vaccine or previous infection and the severity of the current infection. Data from Biobot Analytics, a biotechnology firm that has partnered with the CDC, shows that wastewater concentrations of the coronavirus are similar to what they were at the start of the first winter surge in 2020. But now that the vast majority of people in the US have some immunity to Covid-19 through vaccination, infection or both, those same viral concentrations could translate to a larger number of infected individuals with milder — but still contagious — infections."