AIIMS Director Dr Randeep Guleria said the challenges ahead for developing a COVID-19 vaccine is not as big as distribution. He says research is not the biggest challenge, but it is buying and distributing the vaccine and ensuring it reaches remote corners. An effective strategy needs to be worked out to prioritise who get it first, how many dozes will be needed and could a combination of vaccines be better than just one, Dr Guleria told NDTV.