The Centre hopes to immunise up to 250 million people across India with a vaccine against the coronavirus disease by July 2021, Union health minister Harsh Vardhan said on Sunday, announcing a “rough” estimated target and timeline.

The minister said the health ministry was working with state governments to develop lists of “priority population groups” such as frontline healthcare workers in the government and private sectors.

A top health researcher and member of a national task force on Covid-19 told The Telegraph that it would be “premature” to plan any rollout without knowledge about which vaccine will be available.

“At this point, we’re looking at possible scenarios — scenario 1, scenario 2, scenario 3. But things could change,” the researcher said, requesting anonymity.

The researcher added: “Manufacturers have different capacities; the mathematics changes depending on which candidate becomes available.”

Multiple candidate Covid-19 vaccines are under various stages of clinical trials worldwide – including three candidates in India – to be assessed for safety and efficacy. It is unclear which of these India’s drug regulatory authority might approve.

“Our government is working round the clock to ensure there is a fair and equitable distribution of vaccines once they are ready,” Vardhan said, interacting with citizens on a social media platform.

“Just like everywhere else in the world, our government’s utmost priority is how to ensure vaccines for each and everybody in the country.”

Vardhan said the ministry had asked the states to submit lists of priority population groups to receive the Covid-19 vaccine. These lists are expected to include doctors, nurses, paramedical staff, surveillance officers, sanitary staff and others involved in testing, tracing and treating patients.

“We have placed a realistic target of end-October to complete this humungous exercise,” Vardhan said, adding that the Centre would also help build capacity in human resources, supervision, training and the infrastructure required to distribute the vaccine across the country.

“Our rough estimate and target would be to receive and utilise 400 to 500 million doses covering approximately 20 crore to 25 crore people by July 2021,” the minister said. “All this is still under various stages of finalisation.”

Vardhan said the government was monitoring immunity against Covid-19 which, health researchers said, would be available through antibody surveys that provide information if a person has been previously infected by the coronavirus.

In routine viral infections, previous exposure to an infection generates an immune response that protects the person from future infections by the same virus. This principle might be taken into account to decide who among the general population gets the vaccine on priority, two top health researchers advising the government have told this newspaper.

Antibody surveys carried out between July and September suggested that 17 to 29 per cent of the populations sampled in several large cities – Ahmedabad, Chennai, Delhi, Mumbai and Puducherry – had already been infected.

Amid the growing epidemic, health experts say, the proportion of infected people would have risen to even higher levels by the time a vaccine becomes available.

“If 30 to 40 per cent of people are already exposed, we can prioritise the vaccine for those without antibodies,” one researcher said.

The Indian Council of Medical Research has already conducted two nationwide surveys showing a 10-fold increase in the prevalence of the infection over a 3-month period.

Survey findings have indicated that over 92 million people had already been infected by August.

“If the natural infection itself can do the job (providing immunity), a vaccine becomes superfluous,” Samiran Panda, head of the ICMR’s epidemiology division, had said last week.

India is testing two home-grown candidate vaccines – one developed by the Hyderabad-based Bharat Biotech, the other by the Ahmedabad-based Zydus Cadila – and the Oxford University candidate. The results of their trials are expected later this year.