Seroconversion after measles vaccination at nine and fifteen months of age.

Background: Despite high vaccination coverage, single dose measles immunization programs have been unsuccessful in eliminating the disease. Because seroconversion rates are lower in infants vaccinated before 12 months of age, a second dose of measles vaccine is recommended at 15 months. The aim of this study was to determine the seroconversion rates in children after the first and second doses of measles vaccinations at 9 and 15 months of age.

Methods: Study population comprised 116 infants attending the Well Baby Clinic of Istanbul University, Faculty of Medicine. Serum specimens were obtained from children before and 1 month after the first measles (Rouvax, Schwarz strain 1000 TCID(50)) vaccine given at 9 months. A second dose was given to 72 children at 15 months of age as measles-mumps-rubella (Trimovax, Schwarz measles strain, 1000 TCID(50); Urabe Am 9 mumps strain, 5000 TCID(50); Wister RA 27/3 rubella strain, 1000 TCID(50)). Third blood samples were collected 20 months after the second vaccine.

Results: Passive antibody positivity rate was 5.2% at the age of 9 months. Seroconversion rate was 77.6% after the first dose and 81.9% after the second dose of measles vaccine. Of 15 children who were seronegative, 13 (86.7%) became seropositive after the immunization at 15 months. Eleven children (19.2%) seroconverted from positive to negative after the second vaccine.

Conclusion: The two dose schedule seems to increase the seropositivity rate. Our findings also indicate that increasing vaccination coverage and revaccination at 6 years of age are important even with the early two dose schedule.