The phase-I human clinical trial of India's first indigenously-developed vaccine against novel coronavirus, Covaxin, began at Delhi AIIMS on Friday and the first dose of the injection was administered to a man who is in his 30s. Over 3,500 volunteers have registered themselves for the trial at AIIMS since last Saturday, of whom at least 22 are being screened, Dr Sanjay Rai, Professor at the Centre for Community Medicine at AIIMS and the principal investigator of the study, said.