The Covid-19 “sentinel survey”, which was announced by chief minister Mamata Banerjee and is to start from Friday, will cover more than 4,500 people in all districts of Bengal, health department officials said.

Samples will be collected from 200 people, split into high- and low-risk groups, in each district and the results will help the government track the spread of the disease.

“Patients coming to the outpatient department, including pregnant women, without any symptoms of chest disease will be in the low-risk group. Healthcare workers fall in the high-risk group,” an official said.

“The tests will be conducted on two days every week and the process will continue.”

The sentinel survey will cover Calcutta and Salt Lake, though the civic bodies in the two cities are conducting their own sample surveys.

“This survey is going to throw insight into the Covid-19 situation, which is likely to be useful in data-based planning of control strategies,” said Abhijit Chowdhury, physician and a member of the task force set up by Mamata on Covid-19.

Chowdhury said the state government’s global advisory board, chaired by Nobel laureate economist Abhijit Vinayak Banerjee, had suggested that the government undertake the initiative.

The large-scale movement of stranded people across the country has also necessitated such a survey

Public health experts said the survey, which will require scaling up of testing facilities, could be an effective tool to keep track of the spread of Covid-19 in a particular area.

“The data from the survey will help us develop a dynamic, real-time response to Covid-19 on the field,” a health department official said.

The Indian Council of Medical Research has announced a separate community-level study. The Telegraph has reported that the study, to start later this month, would cover 400 households in 60 districts across the country.

Six of the districts are in Bengal — Calcutta, South 24-Parganas, East Midnapore, Jhargram, Bankura and Alipurduar.