At least 6.6 lakh people in Hyderabad could have been infected by Sars-Cov-2 with the majority of them being asymptomatic, according to a collaborative study conducted on the prevalence of Covid-19 infection in sewerage water in the city by Hyderabad-based Centre for Cellular and Molecular Biology (CCMB) and the Indian Institute of Chemical Technology (IICT).

The CCMB revealed the study findings on Wednesday. Both CCMB and IICT are institutions working under the Council for Scientific and Industrial Research (CSIR).

According to the study, Covid-19 infected individuals release virus not only through nasal and oral routes but also through faeces.

“This provides an opportunity to use sewage/wastewater samples to estimate the spread of the infection in a given locality or area. The SARS-CoV-2 in sewage samples is non-infectious, thus making sewage samples suitable for epidemiological studies,” a CCMB release said.

The team of scientists from CCMB and IICT harvested and processed sewage samples from major sewerage treatment plants (STPs) for detecting the presence of SARS-CoV-2 viral RNA to estimate the number of potentially infected individuals in the city.

The study covered about 80 percent of the STPs.

“Since only 40% of Hyderabad’s sewage reaches STPs, this data can be used to extrapolate the overall number of potentially infected people, which turned out to be approximately 6.6 lakhs, that is, around 6.6% of the city’s population. A conservative estimate of active cases shedding the virus may be around 2.6 lakh,” the CCMB release said.

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The cases include symptomatic, asymptomatic, and also recently recovered individuals in a time window of about 35 days. “The findings are posted on preprint server, MedRxiv, which is yet to be peer reviewed”, the CCMB release added.

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“Our finding clearly indicates that a large segment of the affected individuals is asymptomatic and did not need hospitalisation. This is also in agreement with the observation that hospitalisation rush or mortality (rate) is way lower than otherwise expected with such a large infection rate at a given time. It explains why our healthcare system has been able to handle the situation reasonably well during the pandemic,” CCMB director Dr Rakesh Mishra said.