OBJECTIVES: To estimate the incremental cost-effectiveness ratio (ICER) of routine meningococcal serogroup C conjugate vaccination of children at 2, 3, and 4, or at 5 and 6 months instead of at 14 months in the Netherlands and to determine the impact of including herd-immunity. METHODS: The analysis was performed from a societal perspective and in accordance to the Dutch guidelines for pharmacoeconomic research, using a cohort model. Direct and indirect (friction cost method) costs were considered. Future costs and effects were discounted at 4% and 2000 was chosen as baseline year. A vaccine effectiveness of 90% (based on UK data) and a vaccine protection duration of 20 years (based on the experience with Haemophilus influenzae type b) were used. Also on the basis of UK data, we assumed that 70% of all unvaccinated persons aged 0-14 months are protected because of the herd-immunity effect caused by the recent meningococcal C conjugate vaccination of all persons aged 14 months to 18 years. RESULTS: For immunization of children at 2, 3, and 4 (5+6) months, the herd-immunity effect decreases the yearly number of additional life years gained from 51 (36) to 15 (11) but leads to a strong increase of the ICER from €149,000 (€105,000) per life year gained to €497,000 (€349,000) per life year gained. Even if only a herd-immunity effect of 50% is assumed, the ICER still increases to about €298,000 (€209,000) per life year gained. These results are sensitive to the protection duration and effectiveness of the vaccine and the meningococcal C incidence. CONCLUSIONS: The inclusion of herd-immunity decreases the rendered small health gain and thus leads to even less favorable cost-effectiveness ratios for vaccination of children aged less than one year. Hence, the current Dutch vaccination strategy of routine vaccinating children at 14 months should not be changed.