Wastewater Surveillance Reveals Testing-Related Underreporting and Hospital-Acquired SARS-CoV-2 Infections

Publication date: Sep 17, 2025

Wastewater surveillance to monitor the incidence of infections faces challenges, in terms of discrepancies with sentinel-confirmed cases. We examined whether testing rates could explain the discrepancy between SARS-CoV-2 RNA found in a City of Sapporo wastewater treatment plant and the number of infections recorded at Hokkaido University Hospital over a period of approximately four years. Then, we analyzed the association between wastewater RNA concentrations with incidences of new cases among hospital-acquired infections. Linear regression analyses were performed using wastewater RNA concentrations as the independent variable and infected cases with and without correction for the testing rate as the dependent variable. In addition, modified Poisson regression analyses were performed, with the incidence of new cases among hospital-acquired infections as the dependent variable. After the legal reclassification of COVID-19 in Japan was changed to the same category as seasonal influenza, the rate of hospital testing declined significantly, though wastewater RNA concentrations remained high. Compared to non-correction for testing rates, corrected community-acquired infection cases showed a stronger association with wastewater RNA concentrations (R2 = 0.54 and 0.75, respectively). The incidence of hospital-acquired infections was positively associated with wastewater RNA concentrations (incidence risk rate: 2.24 [95% confidence interval: 1.36-3.71]), and a log10 wastewater RNA concentration [copies/L] of 4.57 (4.10-5.03) was suggested as a 25% probability of new incidence. This study emphasized that SARS-CoV-2 wastewater surveillance is an objective and useful indicator reflecting infection incidence independent of testing rates.

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