Study is done on mice not Humans. Do study on humans over a broad scope of population who had previous infection without vaccination. Here in Africa is lot of people infected again but because of previous infections infections now much less severe because of natural immunity. Vaccination rate very low due to lack of trust in government and big Pharma. Medical fraternity need a more balanced scientific view. Môre studie needs to be done on natural immunity but I doubt if it will be done because of lack of funding and there is no money to be made out of these studies.