The majority of infants will not be protected by maternal antibodies until their first measles vaccination between 12-15 months of age. This provides incentive to reduce the age of measles vaccination, but immunological consequences are insufficiently understood and long-term effects are largely unknown. Infants who received early measles vaccination between 6-12 months and a second dose at 14 months of age (n=79) were compared with a control group who received one dose at 14 months of age (n=44). Measles-neutralizing antibody concentrations and avidity were determined up to 4 years of age. Infants with a first measles vaccination administered before 12 months of age show long-term reduced measles-neutralizing antibody concentrations and avidity compared to the control group. For 11.1% of children with a first dose before 9 months of age, antibody levels had dropped below the cutoff for clinical protection at 4 years of age. Early measles vaccination provides immediate protection in the majority of infants, but long-term neutralizing antibody responses are reduced compared to infants vaccinated at a later age. Additional vaccination at 14 months of age does not improve this. Long-term, this may result in an increasing number of children susceptible to measles.