Bengal chief secretary Rajiva Sinha on Thursday said that Calcutta had emerged as a Covid-19 challenge, accounting for “not less than 80 per cent” of the 58 new cases reported in the state overnight.

“(The emergence of fresh cases in) North 24-Parganas and Howrah has been contained to some extent. Calcutta is a challenge for us,” Sinha told a news conference at Nabanna. “We are taking all the required measures so that it does not spread further.”

Sinha said that of Howrah’s about 70 containment zones — places that have witnessed Covid-19 cases — 10 had reported no infections since April 12.

Ten of North 24-Parganas’ 40 containment zones had reported no fresh patients since April 14, and 4 of East Midnapore’s 5 containment zones had reported none since April 8.

“Calcutta, which has 150-odd containment zones, has seen no positive cases reported from 5 zones in the past 14 days,” the chief secretary said.

What this means is that 145-odd containment zones in the city have been reporting fresh cases over the past two weeks. The figure of 145 suggests a large number of people are at risk, given Calcutta’s population density.

Nabanna sources said that Calcutta remained a challenge primarily for three reasons.

One, the novel coronavirus has already spread across a large area, as indicated by the large number of containment zones.

Two, the city has more congested localities and slums than any other place in the state. “It’s difficult to contain the spread in a densely populated place,” a source said.

Three, the movement of people during the lockdown because of their involvement in essential services is higher in the city than elsewhere.

Sinha, however, said that it was tough to pinpoint whether Covid-19 was spreading through local transmission in the city or whether those newly infected had all come in contact with coronavirus-positive patients.

“About 70 per cent of the cases in India are asymptomatic. It’s very difficult to pinpoint whether it was local transmission or whether the (infected) had come in touch with a positive (case),” the chief secretary said.

He said the Bengal government was initiating special measures for Calcutta to contain the spread of Covid-19 in the city.

These include immediate barricading of the areas that report positive cases, sanitising the entire area, shifting the markets, implementing social-distancing measures strictly, and arranging for home delivery of essential items and the health-screening of residents.

Government sources said the authorities may be forced to bring more areas under the restrictions as positive cases were being reported from fresh areas, that is, from outside the declared containment zones.

Sinha said the containment plans were being tweaked depending on factors like population density and the nature of the areas.