Serum Institute of India CEO Adar Poonawalla has said that the effectiveness of the Covishield vaccine rises to as much as 90 per cent if the two shots are separated by about 2.5-3 months. Interestingly, a study in The Lancet published earlier this year had claimed that the vaccine, developed by AstraZeneca in collaboration with Oxford University, had an efficacy of 70 per cent with one month gap between the doses.