Effectiveness Analyses May Underestimate Protection of Infants after Group C Meningococcal Immunization
1Children’s Hospital Oakland Research Institute, Oakland, California; Departments of 2Paediatrics and 3Zoology, University of Oxford, and 4Vaccine Institute, St. George’s University of London, London, United Kingdom
Background.
Group C meningococcal conjugate–vaccine effectiveness in the United Kingdom declines from 
90% in the first year to 0% between 1 and 4 years after immunization in infants immunized at 2, 3, and 4 months of age and to 61% in toddlers given a single dose. Confidence intervals are wide, and the extent of protection is uncertain.
Methods.Serum samples were obtained from children 3–5 years of age who were participants in a preschool booster‐vaccine trial. Serum bactericidal activity was measured with human complement. Group C anticapsular antibody concentrations were measured by a radioantigen binding assay. Passive protection was analyzed in an infant rat bacteremia model.
Results.Serum samples from UK children who had been immunized 2–3 years earlier as infants or toddlers had higher levels of radioantigen binding, bactericidal activity, and passive protection than did historical control serum samples from unimmunized children (
). A higher proportion of children immunized as infants had serum bactericidal activity titers 
1:4 (considered to be protective) than those immunized as toddlers (61% vs. 24%; 
), but there were no significant differences in the proportion of serum samples conferring passive protection (50% and 41%, respectively; 
).
Conclusions.We found no evidence of lower immunity in children immunized as infants than as toddlers. On the basis of serum bactericidal activity and/or passive protection, 40%–50% of both age groups are protected at 2–3 years after immunization, which was significantly greater than in unimmunized historical controls (<5%).
Received 30 September 2005; accepted 18 January 2006; electronically published 13 June 2006.
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Presented in part: 45th Interscience Conference on Antimicrobial Agents and Chemotherapy, Washington DC, 16–19 December 2005 (abstract G‐407).
Potential conflicts of interest appear after the text.
Financial support: National Institutes of Allergy and Infectious Diseases, National Institutes of Health (NIH) (grants RO1 AI46464 and AI58122; Ruth L. Kirschstein National Research Service Award grant F32 AI056828 to D.M.V.) . The investigation was conducted in a facility constructed with support from the Research Facilities Improvement Program, National Center for Research Resources, NIH (grant number CO6 RR‐16226).