Vaccines for the unvaccinated: protecting the herd.

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Abstract
Interrupting the transmission of infectious agents by introducing new vaccines into a population and creating herd immunity has proven to be a remarkable hidden genie for vaccine effectiveness. For example, one- to two-thirds of the effectiveness of the bacterial conjugate vaccines is linked to protection of the unvaccinated. However, the mechanisms of herd immunity are often not well understood, it is poorly predicted and/or considered in licensure or implementation strategies for new vaccines, and the longterm consequences of preventing natural exposure to agents covered by vaccine are not known. Defining, quantifying, and monitoring the impact of herd immunity is becoming increasingly important for analyses of vaccine effectiveness and cost effectiveness and for decisions about vaccine introduction and use. In this issue of the Journal, Maiden et al. [1] report on the impact of meningococcal serogroup C polysaccharide conjugate vaccines on the prevalence of meningococcal carriage, herd immunity, and meningococcal disease in the United Kingdom. In one of the largest multicenter meningococcal carriage studies ever performed, 48,309 samples from students were obtained during a 2-year period after vaccine was introduced (2000 and 2001). A total of 8599 meningococcal isolates were recovered and characterized by genotyping and phenotyping. A significant and specific reduction in the prevalence of serogroup C meningococcal carriage was noted that lasted at least during the 2 years of the study, without evidence of new meningococcal serogroup replacement. Vaccine efficacy against serogroup C carriage was at least 75%, with a high impact on the virulent clonal complex of serogroup C meningococci associated with invasive meningococcal disease. The decreases in the prevalence of carriage correlated with reductions in serogroup C meningococcal disease in unvaccinated individuals; at least 50% of the effectiveness of the meningococcal serogroup C conjugate vaccines is due to herd immunity [2], and the impact of herd immunity against this serogroup has lasted for years [3].