The
utility and interpretation of SARS-CoV-2 wastewater monitoring
data as a predictor of community health can be confounded by variables
such as wastewater system complexity, viral variants, and human behavior,
including vaccination status and use of at-home tests. Here, we explored
the relationship between the COVID caseload and SARS-CoV-2 concentration
in wastewater for 23 locations over a 22-month period as the pandemic
evolved. Spearman’s rank analysis showed strong correlations
(ρ > 0.7) for most facilities, independent of normalization.
While correlations remained strong throughout the pandemic, application
of change point analysis (CPA) identified shifts in the relationship
between reported clinical cases and the wastewater signal over time.
These shifts did not generally coincide with known pandemic milestones,
suggesting the involvement of multiple interacting or unknown variables.
Models accounting for these shifts in the pandemic phase showed significantly
improved predictions of reported caseloads. Additionally, the existence
of change points highlights the increased reliability of wastewater
data over clinical data when changes in the ratio of cases to wastewater
concentrations are due to changes in human immunity and behaviors.
In future pandemics, public health professionals will ideally be aware
that the case-to-copy ratio can change unpredictably as pandemics
evolve, and CPA can support public health decision-making.