Antibody Persistence for up to 5 Years After a Fourth Dose of Haemophilus influenzae Type b and Neisseria Meningitidis Serogroups C and Y-Tetanus Toxoid Conjugate Vaccine (HibMenCY-TT) Given at 12-15 Months of Age

Background: A 4-dose series of recently licensed Haemophilus influenzae type b-meningococcal serogroups C and Y-tetanus toxoid conjugate vaccine (HibMenCY-TT) was immunogenic with a clinically acceptable safety profile in infants, with antibodies persisting in most participants for 1 year following dose 4. This study assessed antibody persistence up to 5 years after vaccination. Methods: Participants had received HibMenCY-TT or Hib-TT at 2, 4 and 6 months of age. At age 12-15 months, HibMenCY-TT vaccinees received a fourth HibMenCY-TT dose (HibMenCY x 4 group), whereas those who received Hib-TT received a fourth dose of either Hib-TT (Hib) or HibMenCY- TT (HibMenCY x 1). Blood samples were collected 1 month and 1, 3 and 5 years after the last dose for measurement of antipolyribosyl-ribitol phosphate (the Hib capsular polysaccharide) antibodies and serum bactericidal activity (human complement source) against meningococcal serogroups C and Y. Results: Five years after the fourth dose, the percentages of children with antipolyribosylribitol phosphate >= 0.15 mu g/mL in HibMenCY x 4, HibMenCY x 1 and Hib groups were 98.8% (95% confidence interval: 93.5%-100%), The percentages with human complement serum bactericidal activity >= 1:8 for meningococcal serogroup C were 82.9% (72.5%-90.6%), 73.5% (55.6%-87.1%) and 21.1% (9.6%-37.3%), respectively. The percentages with human complement serum bactericidal activity >= 1:8 for serogroup Y were 69.5% (58.4%-79.2%), 54.3% (36.6%-71.2%) and 18.4% (7.7%-34.3%), respectively. Conclusions: HibMenCY-TT given as a 4-dose series or as a single dose at 12-15 months of age induced immune responses for all 3 antigens that lasted for up to 5 years after vaccination in more than half of recipients.