ANTIBODY-RESPONSE OF LOW-BIRTH-WEIGHT INFANTS TO HAEMOPHILUS-INFLUENZAE TYPE-B POLYRIBOSYLRIBITOL PHOSPHATE-OUTER MEMBRANE-PROTEIN CONJUGATE VACCINE

Objective. To evaluate the effectiveness in low birth weight (LBW) inf ants of the currently recommended immunization schedule for conjugated Haemophilus influenzae type b (HIB) vaccine. Methods. We quantified a ntibody responses in 36 preterm infants with a mean birth weight of 10 60 g and a mean gestational age of 28 weeks. Infants were immunized wi th 0.5 mt of HIB vaccine at 2 and 4 months' postnatal age. Specific HI B antibodies were quantified on cord blood, immediately before each im munization and 2 months after the fast immunization. Results. Even tho ugh the geometric mean titers increased significantly during the study period, they were still markedly lower than values reported in term i nfants. After the second immunization only 24 infants (67%) attained a ntibody concentrations of more than 0.25 mu g/mL, defined as seroposit ivity. Also, only 53% of infants achieved antibody concentrations of m ore than 1.0 mu g/mL compared with 92% as reported in term infants. St epwise logistic regression identified gestational age of 27 weeks or l ess and the amount of intravenous immunoglobulin received as the signi ficant variables influencing the antibody response after the first imm unization. The incidence of side effects was negligible. Conclusions. We conclude that LBW infants, and especially those born at 27 or less weeks' gestation, do not respond as effectively to the HIB vaccine. We speculate that reevaluation of the current immunization schedule may be required for very LBW infants.