Blood samples of 500 asymptomatic people in five Covid-19 containment zones in the city were collected on Tuesday as part of a nationwide survey to assess the spread of the disease, Calcutta Municipal Corporation officials said.

The survey — “sero-surveillance to monitor the trend of SARS-CoV2 infection transmission in India” — was done under the supervision of the Indian Council of Medical Research.

The samples were collected from people in Dhakuria’s Panchanantala, Chetla’s Pitambar Ghatak Lane, Hatibagan, Narkeldanga and Park Street-Royd Street area.

The first phase of the survey was undertaken on June 11 and June 12 in the non-containment zones in 60 districts across the country. In the second phase, blood samples are being collected from asymptomatic individuals in containment zones in 10 most affected cities of the country. “Calcutta, which was part of the first phase, too, is among the worst-hit cities. This survey will provide evidence on the extent of the spread of the virus among asymptomatic people,” said a public health specialist, who is helping the CMC conduct the sero-surveillance.

Asymptomatic suspects do not qualify to be tested according to the Covid-19 testing protocol. Only those with symptoms like fever, sore throat and runny nose can be tested.

As a consequence, there could be a significant number of asymptomatic people who are carrying the virus without anyone being aware of it and hence, are not quarantined.

“The survey will help the authorities gather an evidence base to formulate policies and strategies. It will also help us ascertain whether we need to focus on the asymptomatic population,” the public health specialist said.