Association between confirmed COVID-19 cases at hospitals and SARS-CoV-2 levels in municipal wastewater during the pandemic and endemic phases

COVID-19 is now considered endemic in many countries. On May 8, 2023, Japan reclassified COVID-19 from a pandemic to an endemic status, shifting surveillance from universal to sentinel reporting and transitioning the testing and treatment cost of COVID-19 from public funding to individual health insurance coverage. Restrictions on movement, events, and business hours were lifted, potentially increasing cases and complicating tracking. Monitoring hospital cases remains essential to protect high-risk inpatients from nosocomial infections. In this study, 13,812 COVID-19 cases in 12 hospitals were analyzed and the results revealed a strong correlation between SARS-CoV-2 levels in municipal wastewater and weekly new cases during both the pandemic period (February 15, 2021 - February 26, 2023; Pearson's r = 0.8321) and the endemic period (May 8, 2023 - October 1, 2023; Pearson's r = 0.7501). SARS-CoV-2 RNA levels in wastewater from municipal catchment areas showed a stronger correlation with the number of COVID-19 cases at hospitals than did RNA levels in wastewater from the catchment area where the hospitals are located. The difference in correlations was more pronounced during the endemic period. During the endemic period, measurements of SARS-CoV-2 RNA levels in samples obtained from larger sewersheds may be more effective in capturing the overall trends of COVID-19 cases in a region. In other words, during the endemic period, municipal wastewater surveillance may reflect the number of COVID-19 cases in hospitals. Even for facilities that do not monitor SARS-CoV-2 in their own hospital wastewater, publicly available municipal wastewater data can be used to estimate the number of COVID-19 cases in hospitals. Furthermore, COVID-19 infection control measures within hospitals can be evaluated by comparing the number of nosocomial infection patients based on the concentration of SARS-CoV-2 in municipal wastewater.