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 a s measles vaccines. All effectively induce protective immunity, but many di fferences in immunogenicity and, potentially, in safety are seen between st rains in young infants. However, no system for assessing biologic differenc es between vaccine strains has been available. The SCID-hu thymus model sys tem 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 subs tantial thymocyte death; AIK-c replicated poorly but induced moderate thymo cyte death; Moraten replicated moderately well but did not affect thymic ar chitecture. Thus, live attenuated measles vaccines differ in replicative ca pacity and pathogenicity in vivo. These differences may account for strain- dependent variations in immunization.