In the 1960s, a formalin-inactivated measles vaccine (FIMV) predisposed recipients to atypical measles, an immune complex–mediated disease^,. To identify characteristics of the immune priming that leads to atypical measles, responses of monkeys to FIMV were compared with responses to live attenuated virus (LAV) and hemagglutinin (H-DNA) vaccines that do not prime for atypical measles. Antibodies induced by FIMV were transient and avidity did not mature. Antibodies induced by LAV and H-DNA vaccines were sustained and avidity matured over time. After challenge with measles virus, FIMV and H-DNA recipients developed high titers of complement-fixing antibodies. In FIMV recipients, the antibodies were of low avidity, whereas in H-DNA vaccine recipients, the antibodies were of high avidity. Neutralizing capacity in B958 cells correlated with avidity. Only FIMV recipients had immune complex deposition. Failure of FIMV to induce affinity maturation results in anamnestic production of nonprotective, complement-fixing antibodies, immune complex deposition and atypical measles.