INTRODUCTION:
As the COVID-19 pandemic transitions from pandemic to endemic, methods for detecting disease resurgence are essential. Wastewater-based epidemiology (WBE) is a powerful tool for population-level disease surveillance, offering a complementary approach to traditional case-based methods. This study evaluates the utility of WBE in monitoring SARS-CoV-2 viralload across three catchment areas in Cape Town, South Africa. It explores the correlation between wastewater signals and surveillance metrics (incident cases, hospital admissions, and fatalities).

METHODS:
This retrospective longitudinal study analysed wastewater influent samples collected bi-weekly from three wastewater treatment works in the Cape Town Metropolitan District between January and November 2021. SARS-CoV-2 RNA concentrations and Reverse Transcription quantitative Polymerase Chain Reaction (RT-qPCR) cycle-threshold (Ct) values were correlated with COVID-19 surveillance data, including incident cases, hospital admissions, and fatalities. Spearman rank correlations and multivariable linear regression models were used to assess the relationship between viral load and surveillance metrics, with time-lagged analyses conducted to evaluate WBE as an early warning system.

RESULTS:
There were strong correlations between SARS-CoV-2 RNA concentrations in wastewater and Covid-19 surveillance metrics across all three catchment areas. Log-transformed RT-qPCR Ctvalues demonstrated stronger associations with surveillance metrics compared to viral load measurements. Time-lagged analyses revealed that wastewater viral load and Ct values preceded case increases by up to 5 days, admissions by 3-10 days, and fatalities by 4-14 days. Adjusting for vaccination coverage, population and flow rates had minimal impact on the strength of correlations.

CONCLUSIONS:
WBE appears to be a cost-effective and reliable method for Covid-19 surveillance at sub-metropolitan level. The strong correlation between wastewater viral load, Ct values, and surveillance metrics underscores the potential of WBE as an early warning system for disease resurgence, particularly in resource-limited settings. Further investment in WBE are indicated to complement traditional surveillance methods, especially in regions with limited access to clinical testing.