Indian pharmaceutical firm Bharat Biotech said Wednesday its domestically developed vaccine, criticised for being rolled out before final trials, was found in interim late-stage data to have almost 81 percent efficacy in preventing Covid-19 infection.
Questions had been raised by Indian medical experts about the effectiveness of Bharat Biotech's "Covaxin" after officials authorised it for emergency use in January even though its Phase 3 trials -- the last stage before regulatory approval -- were not complete.
"Covaxin (not only) demonstrates high clinical efficacy trend against Covid-19 but also significant immunogenicity against the rapidly emerging variants," Bharat Biotech chairman Krishna Ella said in a statement.
The company added that "severe, serious, and medically attended adverse events occurred at low levels and were balanced between vaccine and placebo groups".
It said interim phase 3 trial results for Covaxin -- co-developed with the scientific agency leading the government's response, the Indian Council for Medical Research -- were based on 43 cases of Covid-19 among 25,800 participants.
Of the 43 cases, 36 were recorded in participants who received a placebo and seven in those who were given Covaxin, pointing to an efficacy rate of 80.6 percent after two doses for those who had not previously been infected by the virus, the company added.
The firm said it would share further interim analysis after 87 cases, with a final analysis planned for 130 cases.
The data from the second and final studies would also be submitted to a peer-reviewed journal for publication, it added.
Covaxin, alongside the AstraZeneca/Oxford shot, is one of two vaccines approved for use in India.
The AstraZeneca/Oxford vaccine has been found to be about 62 percent effective in preventing infection. The Pfizer-BioNTech and Moderna vaccines were found to be more than 90 percent effective.
Fears about the lack of efficacy data for Covaxin as India's mass vaccination drive kicked off in mid-January saw some medical workers, including doctors, refuse to get the jab.
Activists had also accused the firm of cutting corners and breaching rules in conducting trials in India.
Prime Minister Narendra Modi took the Covaxin shot on Monday as the country opened up the process to those over 60 and any over-45s with serious illnesses.
Some 15 million shots have been administered so far in the vaccination programme, the health ministry said Wednesday.
The country has the world's second-highest caseload with more than 11.1 million infections.
The daily increase in cases has fallen sharply this year after peaking in September, although some regions have seen an uptick in infections in recent weeks.