The Centre on Tuesday announced a plan to use mobile phones — believed held by over 90 per cent of the nation’s households — to check the prevalence of Covid-19 symptoms amid the country’s rising counts and fears that many infected persons remain undetected.

The National Informatics Network, a government agency, will conduct a telephone survey, calling up citizens across the country for feedback on the “prevalence and distribution” of Covid-19 symptoms, the health ministry said. The calls will arrive from the number 1921.

“People are informed that this is a genuine survey and are requested to participate in good measure when a call comes in from 1921… to enable proper feedback of prevalence and distribution of Covid-19 symptoms,” it said.

Health and technology experts have cautioned that the effort to build information about prevalence of Covid-19 symptoms would hinge on cooperation from citizens who receive the phone calls.

The health ministry had up to 5pm on Tuesday recorded 1,329 new cases, raising the total number of confirmed Covid-19 patients in the country to 18,985, of whom 3,260 have recovered and 603 have died.

Around 69 per cent of cases detected in the country during contact tracing did not have symptoms when tested positive, a health research official said, revealing the challenge of finding a virus that is deadly to some but relatively benign to most infected persons.

Public health experts and virologists have repeatedly cautioned over the past four weeks that the government needs to detect as many asymptomatic patients as it can and quarantine them to prevent them from spreading the infection.

“A mobile phone survey could be a start. It could help the government gain a better understanding of the ground reality — but there would be need for appropriate medical follow-up,” said Oommen John, a public health expert at The George Institute for Global Health, New Delhi.

The current Covid-19 testing criteria adopted by the government screens only persons with a recent travel history, contacts of positive cases, healthcare workers who show symptoms, and patients with severe acute respiratory illness or influenza-like symptoms.

However, observations in India and elsewhere show that around 80 per cent of Covid-19 patients have only mild symptoms that are unlikely to make them seek medical care.

The telephone survey might help identify locations with undetected clusters, John said. Dozens of districts across the country have not reported Covid-19 cases, but virologists have cautioned that in the absence of tests, no one can be sure.

“But such a survey would work only as long as citizens participate,” John said.

Technology experts have also underlined the need for the government to address apprehensions among people about providing information about symptoms.

“Many people are worried about what might happen to them if they report symptoms,” said Aaditeshwar Seth, associate professor of computer science at IIT Delhi who has established a social media platform through mobile phones for rural India.

Seth and his colleagues who have used the platform to track Covid-19-related incidents recently learned about a migrant worker at Jamui in Bihar who was taken by police to a quarantine centre on the basis of information provided by a local health worker.

“People don’t want to go to quarantine centres — in this case, the relatives of the migrant worker stoned the health worker’s home,” Seth said.

Maharashtra continues to have the largest number of Covid-19 patients in hospital, 3,862 on Tuesday night, followed by Gujarat (1,737), Delhi (1,603), Rajasthan (1,346) and Madhya Pradesh (1,284).