The novel coronavirus has spread undetected among people in at least 36 districts across India, government medical researchers said on Thursday, releasing the strongest evidence yet for community-level transmission of the virus.

A nationwide surveillance network coordinated by the Indian Council of Medical Research (ICMR) has found 40 patients admitted in government hospitals for severe acute respiratory illness (SARI) positive for the coronavirus although they had no contact with a positive case or any history of international travel.

Testing for the coronavirus disease (Covid-19) in India has primarily focused on people with a recent international travel history and contacts of positive cases. The new findings imply that the virus has silently slipped into at least 36 districts over the past two weeks and spread there below the radar of disease surveillance systems.

“The districts with Covid-19 positive SARI patients should be considered as hotspots and prioritised for containment efforts,” said Manoj Murhekar, director of the National Institute of Epidemiology, Chennai.

In part prompted by the findings, the ICMR on Thursday expanded its testing criteria to recommend that all patients with influenza-like symptoms such as fever, cough, sore throat, or even a runny nose should be tested for Covid-19 in hotspots and other areas with Covid-19 clusters.

Public health experts have welcomed the ICMR’s decision to expand testing criteria in hotspots.

“This is a brilliant strategy to ensure that all persons who have suspicious respiratory symptoms are considered as positive unless proven otherwise,” said Oomen John, a public health specialist at The George Institute of Global Health, New Delhi.

The ICMR-coordinated surveillance exercise that screened throat swab samples from 5,911 patients with SARI or influenza-like illness found 104 (1.8 per cent) Covid-19 positive scattered across 51 districts in 20 states, sampled between February 15 and April 2.

This hospital-based exercise was conducted in parallel with the daily Covid-19 laboratory testing exercise that primarily focused on people with international travel history and the contacts of positive cases that generates the daily Covid-19 count.

The ICMR study found Covid-19 positive SARI patients in eight districts in Maharashtra, six districts in Bengal, five in Delhi, five in Tamil Nadu and four in Gujarat.

Among the 256 SARI patients tested in Bengal, nine (3.5 per cent) were Covid-19 positive.

A senior Bengal government official said in Calcutta on Thursday night that extra attention was being paid to districts on either side of the Hooghly river with a considerable migrant population who have returned from other states. “Bengal will intensify tests in these districts based on studies conducted by the state government,” the official said.

The 40 patients among the 104 who had no history of contact with a positive case or international travel were in 36 of these 51 districts. They were in hospital for severe respiratory illness but had picked up the virus in their own localities.

“What this means is that people in these 36 districts have been exposed to the virus for some time,” said Tarun Bhatnagar, a senior epidemiologist at the NIE and study team member.

While the study does not name the 36 districts, senior ICMR officials said, their list has been provided to the states. Containment measures are implemented by district authorities relying on guidelines set by the Union health ministry.

Public health experts say the absence of any apparent contact or travel history would imply community-level transmission of the virus. The ICMR had last month said it had detected early signs of community transmission.

Health experts said the detection of Covid-19 cases among SARI patients raises the question of what personal protection precautions had their contacts, including the healthcare workers who had treated them, taken during their care.

The health ministry on Thursday recorded 591 new cases with Covid-19, raising the country’s total number of cases to 5,865. More than 470 patients have recovered and 169 have died.

John, among other medical experts, has for weeks been calling for expanded testing to help detect patients with mild symptoms who could without knowing it spread the virus to others.

The rising counts despite the 21-day lockdown now in its final week are not surprising, health experts said. “Over the past three weeks, we’re also testing more and more — this is likely to contribute to the higher numbers detected each day,” an epidemiologist said.

Maharashtra on Thursday had the highest number of “active” patients, 946 still in hospital, followed by Tamil Nadu with 709 in hospital and Delhi with 639.