The weekly sewage surveillance for SARS-CoV-2 in 28 STPs of Bengaluru, being jointly conducted by Tata Institute for Genetics and Society (TIGS), NCBS, and Biome Trust in collaboration with BWSSB has shown consistent COVID positivity which has increased slightly, from nine to 13 STPs, for the week ending December 24.
According to the surveillance report, the city-wide wastewater positivity rate for SARS-CoV-2 increased to 46% last week (ending December 24), albeit with a low viral load. This increase in positivity rate is significant compared to 32% reported the previous week (ending December 17) when COVID positivity was found in nine STPs. “This could mean that several thousand persons may actually be shedding the virus in Bengaluru,” TIGS director Rakesh Mishra told The Hinduon Friday.
TIGS, NCBS, and Biome Trust have been conducting weekly sewage surveillance for SARS-CoV-2 in 28 sewersheds of Bengaluru covering much of its 11 million people since August 2021.
Viral RNA load
“Although the remaining STPs show that the viral RNA load is below the threshold, the possibility of them having viral RNA below the limit of detection cannot be ruled out,” said Dr. Mishra, who is also coordinating a multi-city centre consortium for environmental surveillance, supported by Rockefeller Foundation.
According to the latest report, while STPs in Sarakki, Hulimavu, Lalbagh, Mallasandra Phase 1, Mallathahalli, Lalbagh and Agaram were found to have a higher viral load, Hebbal STP had relatively lower. In the previous week, the highest viral load was detected in Yelahanka Phase 1, Nagasandra Phase 1 and Rajacanal STPs while the lowest was found in Hebbal. These observations indicate that the current viral load is low, close to the detection limit that keeps fluctuating and needs to be closely monitored, said Dr Mishra.
“Last week, the viral load was lowest ever with a mixture of BA.2 sub-lineages. Nonetheless, it shows that there are people infected but not enough to show up in the wastewater. Such environmental surveillance helps to keep track of emerging patterns when used as a complementary tool to clinical testing,” he said.
No need to panic
Elaborating on the report, Farah Ishtiaq, senior scientist, heading the wastewater surveillance programme at TIGS, said the real-time SARS-CoV-2 viral load in STP sites and reported COVID-19 cases show a similar trend in Bengaluru.
A look at the daily caseload distribution in the State shows that nearly 85% of the total cases reported are from Bengaluru Urban. This trend has been uniform in the last two months. On Friday, of the total 45 cases reported, 38 were from Bengaluru Urban. Nearly 96% of the total 1,324 current active cases are from Bengaluru.
Asserting that there is no need to panic, she said the viral load in most of the STPs is within the limit of detection. “The intensity of viral load is almost stable so far and unless there is a further increase (in viral load, there is no need to panic,” she said.
Dr. Mishra said it is possible to estimate the infection through wastewater surveillance as infected individuals shed 1-10 million copies of the viral RNA. “With a majority of cases being asymptomatic, the number of cases officially reported through COVID tests is an under-estimation. This is mainly because it is not possible to test every individual. In such a scenario, wastewater surveillance is a very useful tool to follow the trend (increase/decrease), approximate level of infection and also very importantly keep a watch on the variant distribution,” he explained.