Bengaluru: Nothing alarming, say experts after sewage tests reveal low Covid viral load

'There are STPs that show positive RT-PCR results, but the viral load is quite low,' an expert said

Despite the government’s anxieties about spiking Covid cases in China, Bengaluru's sewage surveillance shows the viral load remains extremely low.

Sewage surveillance is considered a more reliable indicator of Covid cases in the community as it also includes data on those who don’t get tested or treated.

Rohan Pais, CEO, Infectious Disease Research Foundation (IDRF), said they are yet to see samples with Ct value lower than 25 since December. Samples with Ct value more than 25 are considered too mild and are usually not used for genomic sequencing.

IDRF has been sending surveillance reports to the state Covid TAC since December. It does weekly RT-PCR testing on two samples each from 16 sites in Bengaluru, including the airport, apartments, hospitals and manholes.

“Even a person who had Covid long ago may have a viral load with a Ct value above 30. So, if the Ct value is above 25 in a sample representing a large community, you can say there’s a very low infection or no active infection in the community,” Pais said.

Due to the low Ct values, IDRF has had no samples to send for genomic sequencing in November and December. “We sequenced only one sample in October and 1-2 samples each in August and September. We last saw high viral loads around July end only,” said Pais.

Weekly surveillance by the Tata Institute for Genetics and Society (TIGS) at the BWSSB’s 28 sewage treatment plants, which represent 11 million people in the city, has recently been showing an increase in samples that test RT-PCR positive. But here, too, the viral loads are low.

Farah Ishtiaq, the TIGS principal scientist, said the viral load has flattened out after September. “There are STPs that show positive RT-PCR results, but the viral load is quite low.”

As per TIGS’s Twitter handle, 50-71 per cent of the samples they tested between December 26 and January 7 tested RT-PCR positive. However, TIGS has been sequencing samples with a Ct value above 25, and the variants found are mostly the already common BA.2 sublineages.

“I have not seen viral loads this low since we started this surveillance. There has been no spike so far,” Farah said.