Fully vaccinated individuals with confirmed antibodies are increasingly contracting measles, challenging assumptions about vaccine-induced immunity duration. Primary vaccine failures occur in immunocompromised patients and those with diabetes, while secondary failures result from declining antibody levels over time, particularly affecting infants in regions without circulating wild virus. T lymphocyte function appears critical for sustaining B cell immune responses. This pattern mirrors findings with other live-attenuated vaccines where boosting from natural exposure maintains immunity. The measles virus's extraordinary transmissibility (R0 of 12-18) combined with presymptomatic transmission creates vulnerability windows even in highly vaccinated populations. While concerning, breakthrough infections don't invalidate vaccination strategy—they highlight the complexity of achieving true herd immunity against highly contagious pathogens. The phenomenon underscores why elimination campaigns require near-universal coverage plus ongoing surveillance. For individual protection, adults in outbreak areas might consider titer testing, especially healthcare workers and travelers. The findings also suggest that measles eradication timelines may need revision, emphasizing sustained high coverage rather than assuming permanent population immunity after initial vaccination campaigns.