What if all those vaccines - those ones that work really well - all stopped working? Imagine if the viruses and bacteria from which they are trying to protect you against, evolved and adapted to life in a largely immune population? Those robust antibody and T cell responses generated within a person following vaccination supply the perfect breeding ground for the selection of resistant mutants where anibodies can no longer recognise and neutralize their targets and where T cells fail to eliminate infected cells. So, is it possible and is it happening?
Well, we already know this kind of phenomenon from influenza, right? Every year we have to change the strains that are put into your flu jab to match those viruses predicted to be circulating come winter. This is based on generating an antigenic match of vaccine to wild virus; specifically, their surface HA proteins must look the same. This is why there has been such a push to develop universal influenza vaccines capable of immunizing people against all flu strains. For viruses like measles and mumps however, we have our universal vaccine, or at least we thought we did.