Insights from wastewater surveillance into testing-related underreporting and hospital-acquired SARS-CoV-2 infections

Wastewater surveillance for monitoring infections faces challenges due to discrepancies with sentinel-confirmed cases. This study aimed to clarify how SARS-CoV-2 RNA concentrations in wastewater can reflect COVID-19 dynamics within a hospital. We first examined whether testing rates were associated with the discrepancy between RNA concentrations in City of Sapporo wastewater treatment plants and the infection cases at Hokkaido University Hospital over approximately four years. Then, we analyzed associations between wastewater RNA concentrations and incidences of new cases among hospital-acquired infections. Linear regression was performed using RNA concentrations as the main explanatory variables, with air temperature and variant strain types included as covariates; infected cases with and without correction for testing rates were deemed the outcome. Poisson regression was also conducted, with new hospital-acquired cases as another outcome. After COVID-19 was legally reclassified in Japan to the same category as seasonal influenza, hospital testing rates declined significantly, even though wastewater RNA remained high. Corrected community-acquired infection cases showed higher R² values (0.80 [95% confidence interval: 0.77-0.84]) than non-correction (0.73 [0.65-0.80]). Hospital-acquired infection incidence was positively associated with RNA concentrations from one week prior (incidence risk rate: 2.45 [1.37-4.40]), and a log₁₀ RNA concentration [copies/L] of 4.51 (4.03-4.99) corresponded to a 25% probability of new incidence. These findings emphasized that SARS-CoV-2 wastewater surveillance provides an objective and useful indicator of COVID-19 dynamics in a hospital independent of testing rates.