When Testing Went Quiet, the Sewers Kept Talking: A Five-Year SARS-CoV-2 Wastewater Surveillance from Munich

Wastewater-based epidemiology (WBE) was introduced early in the COVID-19 pandemic to provide continuous, anonymous, population-level information on SARS-CoV-2 circulation at a time when individual testing was constrained by shortages of reagents, staff, logistics, and funding. It has been difficult to assess whether wastewater data retain their epidemiological value once clinical testing declines, because most published records cover only the first one or two pandemic years. We monitored SARS-CoV-2 in Munich wastewater continuously from April 2020 to April 2025, spanning all lockdown and post-lockdown phases, the vaccination campaigns, and the first post-pandemic years. Samples were collected once or twice weekly as qualified morning grab samples from six residential catchments, and as 24-h composite samples from the wastewater treatment plant (WWTP) Gut Grosslappen. A total of 1100 samples were analyzed by RT-qPCR or droplet digital PCR (ddPCR), and compared with official notification data for the corresponding catchment areas. Weekly viral loads correlated significantly with reported cases at every site (Spearman ρ 0.47 to 0.79), with no consistent lead or lag at weekly resolution. Correlations were higher for the residential catchments (ρ 0.62 to 0.79) than for the WWTP (ρ 0.47 and 0.58), although this difference did not persist when the comparison was restricted to an interval in which both site types were sampled concurrently. The ratio of wastewater viral load to reported cases remained near baseline while testing was mandatory, and rose markedly after mandatory testing ended in March 2023. This shows that notification data increasingly underestimated infection numbers while wastewater continued to track circulation. Wastewater surveillance therefore becomes more informative, not less, as clinical surveillance contracts.