A comparative study of PENTA vaccine booster doses given at 12, 15, or 18 months of age.

For infants immunized with Haemophilus influenzae type b conjugate vaccines, booster immunization is usually recommended in the second year of life, typically between 12 and 18 months. This study assessed the effect of age at booster immunization on pre-immunization antibody trough levels and on subsequent responses, for a PRP-T conjugate vaccine. Subjects were healthy children who had received PENTA vaccine (DPT-IPV/PRP-T combination) as infants. They were enrolled and given measles-mumps-rubella vaccine (MMR) at 12 months of age, then randomly assigned to receive PENTA vaccine concurrently with MMR or at 15 or 18 months of age. Parents were asked to note any adverse effects after PENTA vaccination. Blood samples were obtained prior to PENTA vaccination and 4 weeks later, and tested for antibodies to each antigen. In total 253 children received PENTA vaccine: 86 at 12 months, 85 at 15 months and 82 at 18 months. Injection site redness and swelling were least extensive in the youngest group (p < 0.001) but their rates of occurrence did not differ with age. Anti-PRP levels were similar in each age group prior to immunization; post-booster geometric mean concentrations (GMCs) ranged from 13.0 microg/ml in the youngest to 33.9 microg/ml in the oldest subjects (p < 0.0001). For each of the other antigens examined, booster responses were strongest at 18 months. We conclude that anti-PRP levels are stable between 12 and 18 months in children previously given PRP-T vaccine. PENTA boosters given at 12 months appear to cause milder injection site morbidity whereas those given at 18 months result in stronger responses to virtually every constituent antigen, although each age group responded satisfactorily.