Individuals who are converted to a seropositive state by the administration of adequate amounts of a killed measles antigen develop resistance to natural and attenuated measles infection.1,2 This immunity is not dependent upon the maintenance of an elevated antibody titer. It, therefore, seems reasonable to postulate that the level of clinical effectiveness of a specific lot of an inactivated measles vaccine would be correlated with its immunogenic potency in animals. The following report (in which the available data have been condensed because of limitations of time) describes a standard vaccine assay procedure and certain studies on measles virus and host factors which led to its development.
All of the antigenic activity of measles virus appears to be associated with the particle itself, i.e., there is no evidence of a soluble antigen. However, the estimation of virus concentration by infectivity titration alone provides only a rough index, because