A single dose of the homegrown Covaxin in people already infected with Covid-19 is equivalent to two doses in uninfected persons, the country’s health research agency said on Saturday after a preliminary study.

The Indian Council of Medical Research (ICMR) said if these findings are confirmed through large population studies, a single dose of Covaxin may be recommended to previously infected people so that uninfected individuals may benefit under limited vaccine supplies.

A natural infection with SARS-CoV-2, the virus that causes Covid-19, triggers a range of immune responses, including virus-neutralising antibodies, that last for many months. The ICMR research is part of efforts to understand how natural immune responses might influence response to vaccines.

Studies in the UK and the US had earlier this year found elevated immune responses in previously infected recipients of the Pfizer-BioNTech and Moderna vaccines and comparable to the levels seen after two doses in people without prior infections. Similarly, the AstraZeneca vaccine — Covishield in India — also elicits increased levels of neutralising antibodies in people with a history of Covid-19. But there was no similar data on Covaxin until now.

The ICMR study has indicated that a single jab of Covaxin in previously infected persons serves as a powerful immune booster dose. Although based on a small sample of recipients, the study has found that a single dose in prior-infected individuals generates an even stronger response than two doses in uninfected individuals.

The levels of one set of antibodies, for instance, increased in prior infected recipients from an average of 48 units before the first dose to 167 units after the first dose and to 211 after the second dose. In persons without prior infections, the levels rose to 2 units after the first and to 86 units after the second dose. Another set of antibodies increased to 76 units after the first dose in previously infected recipients and to 56 units after the second dose in uninfected recipients.

The ICMR study examined immune responses in subsets of 30 Covaxin recipients with a history of prior Covid-19 infection and 80 recipients who had not had infections.

“The number of participants in the study is small. One should be guarded in extrapolating its inferences at the population level,” said Samiran Panda, the chief of epidemiology at the ICMR and a senior author of the study published on Saturday in the Indian Journal of Medical Research.

The findings, however, bolster evidence for suggestions that the Covid-19 vaccination policy could be tweaked to offer only a single dose to Covid-19-recovered individuals as this would free up vaccine doses to prioritise two shots to all uninfected individuals.

“Our study provides scientific data,” Panda told The Telegraph. “A policy change now will be a too quick expectation and premature at this stage,” he added, saying the results would need to be corroborated through large population studies.

India’s Covid-19 vaccination campaign has administered more than 620 million doses, but only about 142 million (or 15 per cent) of the estimated 943 million adults have received two doses required for full immunisation.

The campaign, whose pace has been constrained by limited supplies of vaccine doses, on Friday for the first time achieved a record 1 crore doses administered in a single day. Health experts have said the campaign needs to administer 1 crore doses every day to fully immunise all adults by December-end.