Wastewater surveillance reveals infection dynamics

SARS-CoV-2 viral load estimated higher number of cases than the actual number of reported cases 8-14 days in advance

Wastewater or sewage can tell us volumes about the health of a community. The idea here is very simple — most pathogens of interest, e.g., bacteria, virus or protozoa remain viable in the sewage environment for days.

Wastewater-based health surveillance or environmental surveillance is an excellent tool for tracking the presence of different pathogens in the environment. All it needs is a regular and consistent testing of wastewater. It is quite economical and provides real-time data, which makes it very useful for developing countries such as India where not everyone has access to healthcare. 

In India, tracking of COVID-19 pandemic relies heavily on testing symptomatic individuals for the presence of SARS-CoV-2 RNA and counting the positive tests over time. With high population density, many infected persons are likely to be asymptomatic or oligosymptomatic (few symptoms). They are generally not tested, leading to underestimation of COVID-19 cases. 

Furthermore, infected or asymptomatic individuals start to shed the virus via the faecal route four-seven days in advance of actual testing or symptoms showing up, which means the increase in viral load in sewage water ahead of reported cases works as an early warning system and complements the routine diagnostic surveillance by capturing near real-time virus circulation at the community level. 

Using this conceptual background, the Tata Institute for Genetics and Society (TIGS) in collaboration with Biome Environmental Trust and National Centre for Biological Sciences (NCBS), led a longitudinal study (January-June 2022) across 28 Bangalore sewer-shed sites capturing data from more than 11 million people. 

The study posted as a preprint in the medRxiv server (preprints are yet to be peer-reviewed) investigated the relationship between SARS-CoV-2 concentrations in wastewater and COVID-19 cases reported on a daily basis. Each sewer-shed site/sewage treatment plant has a specific catchment area i.e., wards served by the treatment plant. By normalising the viral load for population size of that catchment, water inflow etc, we could estimate the number of infected individuals in that catchment area which was estimated to be higher than the reported cases in the city. 

We were able to detect viral load copies in sewage in areas where there were few or no reported cases. Using exponentially weighted moving average (EWMA) algorithm (four days and seven days — after adjusting for Omicron and old SARS-CoV-2 lineage incubation period), we observed two major outbreaks in January and June 2022. The viral load estimated a higher number of cases than the number of reported cases 8-14 days in advance at the city-wide level. This further suggests that the viral load pattern mirrored with the clinical data, and COVID-19 positive cases appeared to remain underreported in the city.