Objective.To determine the efficacy, immunogenicity and safety of the heptavalent CRM197pneumococcal conjugate vaccine (PCV) in low birth weight (LBW) and preterm (PT) infants against invasive pneumococcal disease caused by vaccine types.Methods.In a randomized double blind trial of 37 868 infants given either PCV or meningococcal type C conjugate vaccine (MCV), 1756 infants <750 g <2500 g (LBW) and 4340 infants from 32 to <38 weeks old (PT) were identified. Risk of invasive pneumococcal disease in LBW and PT infants was compared with risk in normal birth weight (NBW) and full term (FT) infants. Local and systemic events observed within 48 h of recent vaccine were assessed by telephone interviews and similar comparisons made. Premature infant Emergency Department visits and hospitalization were also identified and compared with FT and NBW infants.Results.Initiation of immunization and intervals between doses were similar for all groups. The risk ratio for invasive pneumococcal diseases for LBW infants compared with NBW infants was 2.6 (P= 0.03), and for PT compared with FT infants the risk ratio was 1.6 (P= 0.06). Vaccine efficacy for both groups was 100%. PCV was as immunogenic in LBW and PT as in NBW and FT infants. Fever and local events after PCV vaccination were similar when adjusted for clustering among multiple doses per child. When stratified for individual doses there was more redness and swelling for LBW infants and more swelling for PT infants after Dose 3. Isolated local and systemic reactions were more commonly seen with PCV than with MCV, a pattern similar to that in NBW and FT infants. Hospitalization rates were similar for PCV and MCV recipients.Conclusion.These data support the use of PCV in LBW infants and PT infants.