To measure the magnitude and persistence of the antibody response to a booster dose of Hib conjugate vaccine given between 9 months and 4 years of age to UK children who had completed primary immunisation with 3 doses of Hib vaccine given at 2/3/4 months of age either as a combined D/T/wholecellpertussis/Hib or D/T/acellularpertussis/Hib vaccine.

Inclusion Criteria:

• Written informed consent from parent or legal guardian of the infant or child
• Age >= 9 months and <4 years at recruitment
• Eligible to receive a single dose of Hib vaccine as per the UK schedule
• Receipt of three doses of Hib vaccine in infancy

Exclusion Criteria:

• History of Hib infection
• History of severe local reaction that can be confidently related to a prior Hib immunisation
• Deferral of vaccination if acute illness and/or temperature >38C on day of vaccination