Strains of measles vaccine differ in their ability to replicate in and damage human thymus

A number of strains of live attenuated measles virus are in use worldwide as measles vaccines. All effectively induce protective immunity, but many differences in immunogenicity and, potentially, in safety are seen between strains in young infants. However, no system for assessing biologic differences between vaccine strains has been available. The SCID-hu thymus model system was used to compare AIK-c, Edmonston-Zagreb (EZ), and Moraten - 3 common vaccine strains derived from the Edmonston prototype virus - for replication and pathologic changes in human thymus. EZ replicated best and induced substantial thymocyte death; AIK-c replicated poorly but induced moderate thymocyte death; Moraten replicated moderately well but did not affect thymic architecture. Thus, live attenuated measles vaccines differ in replicative capacity and pathogenicity in vivo. These differences may account for strain-dependent variations in immunization.