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Long-term Immunogenicity of Measles Vaccine: An Italian Retrospective Cohort Study. BACKGROUND: Levels of antibodies induced by the measles virus-containing vaccine have been shown to decline over time, but there is no formal recommendation about testing immunized subjects (in particular, healthcare workers [HCWs]) to investigate the persistence of measles immunoglobulin G (IgG). METHODS: This study aims to evaluate the long-term immunogenicity of measles vaccine in a sample of medical students and residents of the University of Bari who attended the Hygiene Department for a biological risk assessment (April 2014-June 2018). RESULTS: Two thousand immunized (2 doses of measles-mumps-rubella [MMR] vaccine) students and residents were tested; 305 of these (15%) did not show protective anti-measles IgG. This proportion was higher among subjects who received vaccination at ≤15 months (20%) than in those who received vaccination at 16-23 months (17%) and at ≥24 months (10%) (P < .0001). After an MMR vaccine booster dose, we noted a seroconversion of 74% of seronegative HCWs. The overall seroconversion rate after a second dose (booster) was 93%. No serious adverse events were noted after the booster doses. CONCLUSIONS: An important proportion of subjects immunized for measles do not show a protective IgG titer in the 10 years after vaccination. Our management strategy seems consistent with the purpose of evidencing immunological memory.