Bharat Biotech's Covid-19 vaccine Covaxin (file photo). Photograph:( Others )
The Lancet study was conducted between April 15 and May 15 at AIIMS in New Delhi when India was hit with the second wave of the virus.
According to Lancet medical journal, Covaxin shots are 50 per cent effective against symptomatic COVID-19 patients.
The study was conducted between April 15 and May 15 at AIIMS in New Delhi. Those who had symptoms and had undergone an RT-PCR test for coronavirus were part of the study.
The Lancet research took place when India was hit by the second wave with the Delta variant surging across the country. Lancet in an earlier study had declared Covaxin to be 77.8 percent effective against symptomatic cases and 93.4 per cent against severe symptomatic cases.
India had rolled out its vaccination campaign on January 16 with Oxford's Covishield and Bharat Biotech's Covaxin with only healthcare workers and frontline staff given the vaccine jabs.
Lancet informed the participants were divided into "two groups" on the basis of their RT-PCR result - symptomatic test-positive cases and symptomatic test-negative cases (controls).
"Cases and controls were then matched (1:1) on the basis of age and gender. Vaccination status was compared between cases and controls," it said.
The study pointed out that "vaccine effectiveness takes a few weeks, at least 14 days to develop and requires full two doses to achieve maximum effectiveness" although face masks and physical distancing should be followed at all times.
The report said vaccine effectiveness remained stable over the seven-week follow-up period after two doses were administered. The World Health Organization (WHO) had approved Covaxin for emergency use this month.
(With inputs from Agencies)