Measles-mumps-rubella revaccination; 18 months vs. 4-6 years of age: potential impacts of schedule changes.

OBJECTIVE The policy of administering the second dose of measles-mumps-rubella (MMR) vaccine (MMR(2)) has recently changed in Iran, at age 1.5 years instead of 4-6 years previously. The effects of such a change on the immune status of the individual are evaluated in this study. METHODS Totally 249 and 228 children aged 18 months and 4- to 6-year-olds, respectively, with a documented receipt of primary MMR vaccine at the age of ≥ 1 year were enrolled. Before, and 4-6 weeks after MMR(2) administration, anti-MMR IgG antibody levels were measured using ELISA method. IgM antibody levels were also assessed in measles-rubella seronegative children that responded to MMR(2). Collected data for each component from both age groups were compared by using Fischer's exact probability and chi-square tests. RESULTS Before revaccination, measles seroimmunity rate was similar between the two groups, but rates to mumps and rubella were significantly higher in younger children-measles: 74 vs. 78.3%; mumps: 82.3 vs. 68.4% and rubella: 75% vs. 67%, respectively. After administration of MMR(2), all seroimmune subjects were IgG boosted. Except for rubella, older seronegative children showed significantly higher seroconvertion rate to MMR(2) and seroprevalence rates increased in vaccinees--measles: 98.2 vs. 94%, mumps: 97 vs. 94.4% and rubella: 87 vs. 92.4%, respectively. Only few measles-rubella seronegative children showed IgM response to MMR(2). CONCLUSION This study showed that the majority of younger children were susceptible to MMR infection before revaccination. Earlier age policy provides more protection against MMR in preschool-aged children. Rubella strain seems to be less potent than reported.