BACKGROUND: Prophylactic paracetamol (PP) was previously shown to reduce primary and booster antibody responses against the 10-valent pneumococcal non-typeable Haemophilus influenzae protein D conjugate vaccine (PHiD-CV). This study further evaluated the effect of PP on antibody persistence, immunological memory and nasopharyngeal carriage (NPC). METHODS: Two hundred and twenty children previously primed (3 doses, NCT00370318) and boosted (NCT00496015) with PHiD-CV with (PP group) or without (NPP group) prophylactic paracetamol administration received one PHiD-CV dose in their fourth year of life to assess the induction of immunological memory following previous immunisations. A control group of age-matched unprimed children enrolled in study NCT00496015 received an investigational tetravalent Neisseria meningitidis serogroups A, C, W-135, Y tetanus toxoid-conjugate vaccine, and thus remained unprimed for pneumococcal vaccination. Of these, 223 unprimed children received in the present study at least one PHiD-CV dose of a 2-dose catch-up regimen, which was relevant as control for assessment of immunological memory in PHiD-CV primed children. RESULTS: Induction of immunological memory was shown irrespective of PP administration at primary and booster vaccination. Antibody geometric mean concentrations were lower in the PP group for serotypes 1, 4, 7F and 9V. Opsonophagocytic titres did not differ significantly between PP and NPP groups. Previous use of PP seemed to have only a minor impact on kinetics of antibody persistence. Reduced NPC of vaccine pneumococcal serotypes and trends towards increased NPC of non-vaccine and non-cross-reactive serotypes were seen in primed groups versus the control group, with no obvious differences between PP and NPP groups. CONCLUSION: Regardless of whether previous PHiD-CV vaccination was given with or without PP, induction of immunological memory and persistence of PHiD-CV's impact on carriage was seen until at least 28 months post-booster vaccination. Our study results therefore suggest that the lower immune responses after primary and booster vaccination with PP are of transient nature.

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