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These studies assessed the immunogenicity and reactogenicity of booster vaccination with diphtheria-tetanus-acellular pertussis-hepatitis B-inactivated poliovirus-adsorbed conjugated Haemophilus influenzae type b (DTaP-HBV-IPV/Hib) at 18–20 months, and with DTaP during the fifth year of life in children who had been born prematurely (<37 weeks gestation).Open-label, parallel group studies in which preterm and full-term subjects primed with DTaP-HBV-IPV/Hib received booster vaccination with DTaP-HBV-IPV/Hib (Infanrix hexa) at 18–20 months and DTaP (Infanrix) at 4 years of age. Immunogenicity was assessed before and 1 month after DTaP-HBV-IPV/Hib dose and 1 month after DTaP administration. Local and general symptoms were recorded for 4 days, unsolicited symptoms for 31 days after each dose.Before the second year booster, Hib, hepatitis-B, and polio type 3 seroprotection rates were higher in the full-term group (antipolyribosyl ribitol phosphate ≥0.15 μg/mL observed in 76.2%/83.6% preterm/full term respectively, anti-HBs ≥10 mIU/mL in 75.0%/80.6% respectively). One month after the DTaP-HBV-IPV/Hib booster, ≥98% in both groups were seroprotected/seropositive for all vaccine antigens, except hepatitis-B in preterms (seroprotection rate 91.6%). By the fifth year hepatitis-B seroprotection rates were 85.3%/70.5% (preterm/full term) in subjects who had previously responded to hepatitis-B vaccination, and seroprotection rates for polio and polyribosyl ribitol phosphate were >95%. No differences between groups were observed after the DTaP booster. Both booster doses were generally well tolerated with minimal differences observed between groups. Local symptoms occurred more frequently after the fifth vaccination at 4 years of age.Despite trends for lower immune responses to some vaccine antigens in preterm subjects, these findings support undelayed primary and booster vaccination in infants and children born before term. Booster vaccinations with DTaP-HBV-IPV/Hib and DTaP were well tolerated in this susceptible group.