[Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] [Tag] This is called strain replacement. It does occur and usually by strains that are less pathogenic and thus not included in the vaccine (pneumococcal vaccine is an example). It is something that is monitored closely after a vaccine is introduced.