COVID-19, mass testing and the utility of wastewater epidemiology

To date, coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been responsible for over 170 million infections and over 3.54 million deaths. Diverse surveillance methods are employed to assess the scale of disease burden and population exposure; these include community testing, contact tracing and the monitoring of morbidity and mortality rates.

Currently, wastewater-based epidemiology (WBE) is being explored as a new tool to track the spread of COVID-19. Both symptomatic and asymptomatic individuals are reported to shed viral RNA in their feces, and many studies report viral detection in sewage and wastewaters across the world.

In a recent study, released as a preprint on the medRxiv* server, researchers developed a methodology to detect SARS-CoV-2 RNA in wastewater with the potential for finding an association between the wastewater treatment plant (WWTP) SARS-CoV-2 influent viral RNA load and COVID-19 cases in the community. This study indicates that a national scale wastewater-based epidemiology can play a role in COVID-19 surveillance.

The researchers demonstrated that the concentration and daily SARS-CoV-2 viral RNA load from the wastewater are associated with the COVID-19 cases. This can be used to predict the number of cases detected in the WWTP catchment area. “A clear statistically significant relationship is observed between these two variables above site-specific case thresholds,” the researchers noted.

Although proving effective as a surveillance tool, understanding the impact of viral shedding dynamics in faeces, viral persistence in wastewater and wastewater flow rates on viral detection remain serious challenges,” writes the team.

They developed a filtration-based methodology to concentrate the SARS-CoV-2 from the WWTP influent. Then they subsequently detected and quantified the viral load by optimizing RT-qPCR (quantitative reverse transcription-polymerase chain reaction).

This methodology, adopted by the Scottish Environment Protection Agency (SEPA), monitored 28 WWTPs across Scotland, serving 50% of the population, about 2.66 million people. Including large conurbations, as well as low-density rural and remote island communities, these sites cover rural and urban wastewater influent in the study.

Presently, the wastewater testing has been expanded to cover 75% of the population, with sub-catchment sampling being used to focus surge testing, the researchers informed.

For each WWTP catchment area, the researchers also collected data on COVID-19 cases and deaths. In the study, this is presented as quantified spatial and temporal relationships between SARS-CoV-2 RNA in wastewater and COVID-19 cases, the strength of which was strong in the larger WWTPs.

Significantly, the study reported that the identified threshold for detection is typically under 25 cases, whereas for some smaller WWTPs, a single detected community case was sufficient to yield a positive wastewater result.

On the low-level limits of detection, the researchers said, “Wastewater surveillance can be particularly valuable for areas reaching low prevalence and is therefore suitable as a logistically sustainable and cost-effective early warning system, making a targeted community testing strategy viable.”

Importantly, we demonstrate how WBE can be adopted across a range of catchments, from densely populated urban areas (Edinburgh and Glasgow) to smaller towns, rural areas and islands.”

It is known that COVID-19 patients shed SARS-CoV-2 RNA, with prolonged shedding observed up to 33 days after the initial onset of symptoms or hospitalizations. It is found even after full recovery. In this study, the researchers also estimated the level of viral shedding in feces and how this varies over time.

They observed a relatively short period over which infected individuals substantially contribute to the wastewater signal. Discussing the sensitivity, various factors and correlations that affect their study, the researchers presented a new model to calculate the daily WWTP SARS-CoV-2 influent viral RNA load using daily influent flow rates.

This study confirms the existence of a strong and measurable, statistically significant relationship between the SARS-CoV-2 daily WWTP viral RNA load and the number of detected COVID-19 cases in the week preceding wastewater sample collection.

The wastewater-based epidemiology (WBE) and SARS-CoV-2 variant detection, assessment of vaccination on community transmission and surveillance for other infectious diseases represent promising future applications. This study demonstrates the rapid inception, development, validation and operationalization of a national COVID-19 WBE program to provide highly cost-effective community surveillance during the pandemic, the researchers conclude.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Dr. Ramya Dwivedi

Ramya has a Ph.D. in Biotechnology from the National Chemical Laboratories (CSIR-NCL), in Pune. Her work consisted of functionalizing nanoparticles with different molecules of biological interest, studying the reaction system and establishing useful applications.