Evaluation of wastewater SARS-CoV-2 surveillance for monitoring COVID-19 trends and early warning in Sweden, 2023-2024.

Wastewater surveillance (WWS) of SARS-CoV-2 RNA levels offers a way to monitor virus circulation independently of clinical testing. With COVID-19 now endemic, clarifying the role of WWS within routine surveillance is essential. The aim was to evaluate the potential of WWS as an early warning indicator for COVID-19 in Sweden, based on its correlation with reported case data. The study includes data from July 2023 to December 2024, from 19 wastewater treatment plants (WWTPs), covering 43% of Sweden's population, and surveillance data of reported COVID-19 cases. Analyses were performed by WWTP, separately for Stockholm (4 WWTPs) and at a national level (19 WWTPs). We compared wastewater SARS-CoV-2 levels to reported COVID-19 cases using cross-correlation and prediction metrics following trend classification. Correlations and classification metrics were highest without time shifts. SARS-CoV-2 levels in wastewater were strongly correlated with reported COVID-19 cases, with median cross-correlation values of 0.80 across all WWTPs and 0.87 at Stockholm and national levels. Positive predictive values of wastewater data to reported cases ranged from 63-74%, and negative predictive values from 85-95%. Sensitivity and specificity in Stockholm reached 78% and 87%; at national level, 72% and 84%, respectively. Under the evaluated weekly sampling and reporting system, WWS did not demonstrate a consistent early-warning advantage over reported cases but may support monitoring of trends when clinical testing is reduced.