The second dose of an MMR vaccine is a catch up for persons who did not receive the first dose or for primary vaccine failures. Catch up doses can be scheduled according to convenience into the program of the country. The second MMR dose is often administered at the age of 5 years, before school entry. Some countries chose to implement the second dose at the age of 10-13 years, as is the case for Belgium. The here presented long-term follow-up of a cohort of children, set up originally to analyze maternal antibodies against vaccine preventable diseases, offers a unique opportunity to evaluate ad interim the current long-interval MMR vaccination schedule in Belgium. After 1 MMR dose at 12 months of age, rubella immunity is almost intact at 5 years of age (94.5 % is seropositive), measles seropositivity scores 86.8 %, and mumps 32 %, measured with ELISA. A seroneutralization (SN) test for mumps antibodies reveals much higher seropositivity rates (88 %). Using a regression model on the log (IgG) titer for all antigens, no influence was found from any of the studied variables, except for girls who had a significantly higher rubella IgG titer (p = 0.002) compared to boys. CONCLUSION: The data show considerable susceptibility to mumps and measles in 5-year-old children, confirming a previously conducted seroprevalence study (2006). Both advantages and disadvantages of shortening or enlarging the vaccine schedule are discussed