Capsular serotype-specific attack rates and duration of carriage of Streptococcus pneumoniae in a population of children

Background. The relative invasiveness rates (attack rates) of Streptococcus pneumoniae of different capsular serotypes in children are not known. Estimates of capsular serotype invasiveness (designated "invasive odds ratios") that are based on cross-sectional prevalence carriage data have been published, but these estimates could be biased by variation in the duration of carriage. Methods. The relative attack rates of invasive pneumococci were measured using national UK surveillance data on invasive pneumococcal disease (IPD) incidence and data on incidence of pneumococcal acquisition from longitudinal studies of nasopharyngeal pneumococcal carriage. Results. We found significant differences in capsular serotype-specific attack rates. For example, capsular serotypes 4, 14, 7F, 9V, and 18C were associated with rates of > 20 lPD cases/ 100,000 acquisitions, whereas capsular serotypes 23F, 6A, 19F, 16F, 613, and 15B/C were associated with < 10 IPD cases/100,000 acquisitions. There was an inverse relationship between duration of carriage and attack rate by capsular serotype (P <.0001). Attack rates were significantly correlated with invasive odds ratios (P <.0001). Conclusions. The capsular serotype is a major determinant of both pneumococcal duration of carriage and attack rate. Published invasive odds ratios are a reliable and practical method of determining capsular serotype invasiveness and will be valuable for investigating and characterizing emerging capsular serotypes in the context of conjugate vaccination.