Objective. To determine whether extremely premature infants have immunologic responses to tetanus toxoid, Haemophilus influenzae type b polysaccharide and polio vaccines similar to those of full-term infants.

Infants and Methods. Sixteen extremely premature (<29 weeks, <1000 g at birth) infants received separate diphtheria-tetanus-pertussis and H influenzae type b oligosaccharide-CRM,197-conjugated (HbOC) vaccines at 2, 4 and 6 months of chronologic age, enhanced potency inactivated polio vaccine at 2 months, and oral polio vaccine at 4 months. Serum was obtained for anti-tetanus toxoid (TT), anti-Haemophilus b polysaccharide (HbPs) and polio neutralizing antibody assays before the 2-month vaccination and 4 to 6 weeks after the 6-month vaccination. Comparison sera were obtained from full-term infants immunized with the same lots of diphtheria-tetanus-pertussis (n = 46) and HbOC (n 66) vaccines or the same sequence of polio vaccines (n = 10).

Results. Preterm and full-term infants had similar geometric mean titers of anti-TT antibodies, anti-HbPs antibodies, and neutralizing antibodies to polio serotypes 1, 2, and 3 after the completion of the primary series of vaccines. After vaccination, similar proportions of preterm and full-term infants had protective levels of antibody to TT (preterm 100% vs full-term 100% with levels >0.01 IU/mL), HbPS (82% vs 87%, >1.0 µg/mL), and polio serotypes 1 (85% vs 80%, ≥1:8) and 2 (100% vs 100%, ≥1:8). Preterm infants were less likely than full-term infants to have protective levels of neutralizing antibody to polio serotype 3 (31% vs 90%, ≥1:8).

Conclusions. Extremely premature infants have adequate antibody responses to tetanus and HbOC antigens but may have diminished responsiveness to serotype 3 polio vaccine.

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