Herd immunity and pneumococcal conjugate vaccine: A quantitative model

Highly significant declines in all the serotypes contained in PCV7 in all unvaccinated populations (5-19, 20-39, 40-64, and >64 years) from 2000 to 2003 were found under the model. No significant change in incidence was seen from 1994 to 1999, indicating rates were stable prior to vaccine introduction. Among unvaccinated persons 5+ years of age, the modeled incidence of disease caused by PCV7 serotypes as a group dropped 38.4%, 62.0%, and 76.6% for 1, 2, and 3 doses, respectively, received on average by the population of children by the time they are 15 months of age. Incidence of serotypes 14 and 23F had consistent significant declines in all unvaccinated age groups. In contrast, the herd immunity effects on vaccine-related serotype 6A incidence were inconsistent. Increasing trends of non-vaccine serotypes, in particular 19A, were noted in most unvaccinated age groups, but these increases were substantially smaller than the concurrent decreases among the vaccine serotypes. Also, the model estimated PCV7 to have a greater (p = 0.014) indirect impact on the incidence of invasive pneumococcal disease caused by all vaccine serotypes among African-Americans of all ages than for whites. Thus, conjugate vaccines may be able to induce herd effects even in situations where vaccine coverage is far from complete or with schedules using fewer than 3 or 4 doses. Because the model was based on incidence rates and PCV7 coverage in Atlanta, our findings should be validated in other geographic areas.