Are wastewater surveillance metrics associated with high community case and hospitalization rates of COVID-19 across US counties?

By Neha MathurReviewed by Lily Ramsey, LLMAug 1 2023

In a recent article published in JAMA Network Open, researchers completed an observational cohort study in 268 counties in 22 United States (US) states between January and September 2022 to understand whether the use of at-home COVID-19 tests impacted the determination of community-level COVID-19 incidence via county-level wastewater metrics.

Study: Use of Wastewater Metrics to Track COVID-19 in the US. Image Credit: Bilanol/Shutterstock.com

Background

The national wastewater surveillance system (NWSS) documents and reports wastewater surveillance metrics, including sewershed location identifiers of all 22 participant US states, to the US Centers for Disease Control and Prevention (CDC).

They use this public health department–monitored sewer sheds data serving 3,000 people (minimum) to estimate county-level COVID-19 incidence.

Availability, uptake, and affordability of at-home COVID-19 tests made them the preferred mode of testing, which led to substantial underestimation of COVID-19 incidence, including new cases and hospitalizations across the US.

Likewise, COVID-19 vaccines and medications that reduce disease severity decreased the reliability of the estimates of COVID-19 incidence. The lack of reliable data acquired through wastewater surveillance increases the risk of contracting COVID-19 for all, especially more vulnerable patients in the community.

Working in an agnostic manner towards symptomatic, diagnosed, or reported COVID-19 cases and hospitalizations, high-res wastewater sequencing helps accurately identify emerging SARS-CoV-2 variants of concern (VOCs) and determine the effective reproductive number (Re), a predictor of viral transmission. 

Yet, the use and adoption of wastewater surveillance in the US are geographically limited due to challenges in interpreting findings with shifting detection methods, wastewater dynamics, and populations served.

Per recent estimates, over 70 counties covering >3,500 sites are investing in wastewater surveillance.

About the study

In the present study, researchers used publically available NWSS data for January-September 2022, when the Omicron VOC was prevalent, to evaluate whether this wastewater SARS-CoV-2 measures data matched with temporally increasing SARS-CoV-2 transmission and formally documented new COVID-19 cases and hospitalizations in the US, assuming that widespread at-home COVID-19 testing would attenuate this association.

Additionally, they determined whether it is feasible to operationalize wastewater surveillance metrics for future integration with other diseases. It could help make apt policy decisions about resource allocation to counties with high COVID-19 incidence.

Regardless of sample type and quantification method, the sewer sheds quantify severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in unconcentrated wastewater/sludge.

At 672 and 545 sewer shed sites, they use reverse-transcription quantitative polymerase chain reaction (qRT-PCR) and digital RT-PCR, respectively. Eventually, they presented SARS-CoV-2 concentrations per volume of wastewater.

The team analyzed NWSS-collated metrics encompassing wastewater percentile for a facility, i.e., the current SARS-CoV-2 concentrations in ascending/descending order relative to peak and the all-time lowest point at that facility, the wastewater percentage change in virus concentration, and wastewater samples with detectable virus concentrations in the preceding 15 days but with focus on the first two NWSS-collated measures.

All included US counties submitted at least one week of January 2022 data and data for >50% of the coming weeks, which decreased fluctuations in percentile metrics by assessment date.

The team aggregated data from each sewer shed under a county

creating a weighted average; the sewer shed serving the highest population had the most weight to the average county-level COVID-19 incidence estimate.

After primary analysis, the team reported new COVID-19 cases and inpatient admissions per 100,000. They evaluated the association of two wastewater metrics (the test of interest) with clinical case metrics for 2022.

Moreover, they used logistic regression to determine whether combining two wastewater metrics was associated with a high incidence of COVID-19 at the community level, accounting for wastewater percentile, change, and variable interactions. 

In sensitivity analyses, the team assessed wastewater percentile performance in small and large counties, defined by criteria outlined by US Census Bureau. Importantly, the number of counties analyzed varied every week.

Results

During the study period, 268 of 730 wastewater treatment counties with an average population of 95,938 across the US submitted their wastewater metrics.

Consistent with national data, new COVID-19 cases and hospitalization rates were lower in the second quarter but higher in the first and third quarters, with the highest incidence of new COVID-19 cases in the first quarter of 2022 since the start of the pandemic.

Furthermore, the authors noted a direct correlation between wastewater percentile and SARS-CoV-2 concentrations, even though it became less evident in the third quarter despite high levels of SARS-CoV-2 in wastewater as measured by facility percentile, indicating an increasing dissociation between community viral prevalence and reports of infection to health departments due to increasing at-home COVID-19 testing.

Moreover, as population immunity increased due to prior infection and vaccination, COVID-19–related hospitalizations decreased; thus, the authors observed an increasing dissociation between wastewater and new COVID-19-related hospitalization rates.

Conclusions

The current time-series analysis of SARS-CoV-2 wastewater metrics incorporating NWSS-monitored sewer sheds in the US reported to the CDC showed that an aggregate measure of the percentage of maximum wastewater SARS-CoV-2 concentration could help estimate county-level COVID-19 prevalence.

Intriguingly, US counties documenting lengthier historical data from January 2022 provided the most reliable estimates. 

The study findings rationalize the operationalization of wastewater surveillance data to inform public policy actions targeted to protect COVID-19 vulnerable populations. In addition, policymakers could integrate wastewater metrics with school attendance or emergency department visits for other respiratory or viral illnesses, e.g., influenza.

The CDC continues to update NWSS data, which they could use during a future pandemic as an early signal of a local disease outbreak and monitoring circulating viral variants.