Phase-III trials of Covaxin, made by Hyderabad's Bharat Biotech showed it was 93.4% effective against severe symptomatic Covid-19, the firm said on Saturday.
The data demonstrated 65.2% protection against the Delta variant, first identified in India.
The Delta variant had led to a surge in infections in April and May, and the world's highest daily death tolls.
It further showed effectiveness of 77.8% against symptomatic Covid-19 in the trial.
Last month, vaccine maker AstraZeneca also said its vaccine was effective against the Delta and Kappa variants, citing a study.
The country has been administering the AstraZeneca vaccine, made domestically by the Serum Institute of India, which plans to step up monthly production from July to nearly 100 million doses.
Bharat Biotech now estimates it will make 23 million doses a month.
The Phase-III data came as Ocugen Inc, which is co-developing Covaxin with Bharat Biotech for the US market, prepares to file a request for full US approval.
India, with a tally of 30.45 million infections, is the second most affected nation after the United States, with 33 million. The south Asian nation's death toll has now crossed 400,000.
