Responses to a fourth dose of Haemophilus influenzae type B conjugate vaccine in early life.

OBJECTIVE

To describe the immune response of preterm infants, with a reduced response to primary Haemophilus influenzae type B (Hib) immunisation, to a fourth dose of Hib conjugate vaccine given in early life.

DESIGN

Prospective observational study.

SETTING

Five Wessex Neonatal Units.

PATIENTS

Infants born at < 32 weeks and immunised with three doses of combined acellular pertussis-Hib vaccine, with a Hib IgG geometric mean concentration (GMC) < 1.0 microg/ml after these primary immunisations.

INTERVENTIONS

An additional fourth dose of Hib conjugate vaccine given before 1 year of age. Blood taken to assess Hib IgG concentration and avidity after immunisation.

MAIN OUTCOME MEASURES

Hib IgG GMC and avidity index.

RESULTS

Ninety six infants (mean gestational age at birth 29.1 weeks) received a fourth dose of Hib at a mean age of 7.8 months. Hib IgG GMC after the primary immunisations was 0.17 microg/ml (95% confidence interval (CI) 0.14 to 0.20) rising to 4.68 microg/ml (95% CI 3.36 to 6.57) after the fourth dose (p < 0.0001). The IgG response to the fourth dose correlated positively with the response after the primary immunisations (p < 0.001). Hib IgG geometric mean avidity index (GMAI) after the primary immunisations was 30.87 (95% CI 20.40 to 46.73). This increased to 124.73 (95% CI 109.93 to 141.51) after the fourth dose (p < 0.0001).

CONCLUSION

Preterm infants with very low IgG responses to Hib after primary immunisations with a combined acellular pertussis-Hib vaccine mount a good response to a fourth dose of Hib. This study suggests that all infants will benefit from a fourth dose of Hib, regardless of the age at which it is given.