New Delhi: India recorded over 12,000 coronavirus cases at its peak in mid-April this year. The number wasn't big, though there was a scare of a new wave days before WHO declared Covid is no longer an international health emergency. A recent wastewater surveillance in two Indian cities, however, revealed that they might have witnessed an "invisible" wave of Covid that was far bigger than the third wave of 2022. The surveillance was conducted in Bengaluru and Hyderabad by the Tata Institute for Genetics and Society (TIGS).
Speaking to ABP Live, Dr Rakesh Mishra, director, TIGS, said this year the viral RNA load in the wastewater in Bengaluru was more significant than the third wave, during which India registered over 3 lakh cases at its peak in January 2022, driven primarily by the BA.1 Omicron sub-lineage.
"In our surveillance of Bengaluru wastewater data, we found the city witnessed an invisible wave of Covid-19 that was probably bigger than the third wave in January 2022. This probably was the situation across India as well as Bengaluru is not an isolated place," Dr Rakesh Mishra told ABP Live.
"The only difference from last year is that the virus is now less clinically potent and symptoms more moderate. Moreover, we are also more protected due to double dose vaccination and hybrid immunity," he further said.
READ | Covid No Longer A Global Public Health Emergency, Says WHO
Covid-19 Cases Peaked Around March End
This year, the TIGS director said, the Covid wave was "invisible" as people had milder Covid symptoms and did not require hospitalisation. This means official testing data could be missing a whole lot of infections throughout the country, and this is where wastewater surveillance helps fill the gap.
Wastewater surveillance, in collaboration with the Bengaluru Municipal Corporation (BBMP), is being conducted weekly for SARS-CoV-2 in sewage water samples from 28 sewage treatment plants (STPs) spread across the city since 2021.
Dr Mishra said these 28 STPs cover almost 80 per cent of the city population and the data helps gauge at the respective area level the extent of viral load.
"In the wastewater sample, we find only RNA fragments of the Covid virus and not the virus particle itself. We then quantify the viral load and estimate the infection patter as an infected individual sheds 1-10 million copies of the viral RNA per day. It gives an assay without going to any household, a city-level and ward-level viral load estimate," Dr Mishra said.
The viral RNA load found in sewage water revealed that Covid-19 cases peaked around March end, the TIGS study found. Similar trend was seen in Hyderabad where open drainage wastewater surveillance is carried under four city cluster project.
Why Wastewater Surveillance
Dr Mishra said protocols/method are being developed through which every municipal corporation can have wastewater surveillance of infectious diseases as a regular component of their system.
He said the biggest advantage of environmental surveillance was that it gives an accurate measure of pathogen load in sewage water, thereby informing civic authorities about the trend of increase in infections.
Even though the WHO has announced that Covid was no more a global health emergency, Dr Mishra said the virus was here to stay and there would be smaller "invisible" waves or blips in the future as well.
"Not only Covid, but other infectious diseases can be monitored for the pathogen load by this method as they will not be visible from case reporting point of view. The environmental surveillance will detect them. We also need to continue monitoring the virus through genome sequencing to detect any new variants as they emerge," he added.