Between-strain competition in acquisition and clearance of pneumococcal carriage--epidemiologic evidence from a longitudinal study of day-care children

Abstract

The state of pneumococcal carriage-that is, pneumococcal colonization in the nasopharynx of healthy persons-represents a reservoir for the spread of pneumococci among individuals. In light of the introduction of new pneumococcal conjugate vaccines, further knowledge on the dynamics of pneumococcal carriage is important. Different serotypes (strains) of pneumococcus are known to compete with each other in colonizing human hosts. Understanding the strength and mode of between-serotype competition is important because of its implications for vaccine-induced changes in the ecology of pneumococcal carriage. Competition may work through reduced acquisition of new serotypes, due to concurrent carriage in the individual, or through enhanced clearance of serotypes in carriers who harbor more than 1 serotype simultaneously. The authors employed longitudinal data (1999-2001) on pneumococcal carriage in Danish day-care children to analyze between-serotype competition. The data included observations of carriage in children who had not been vaccinated against pneumococcus, and the level of pneumococcal antibiotic resistance and antibiotic usage in the community was very low. Clearance of any single serotype was not affected by simultaneous carriage of other serotypes. In contrast, acquisition of other serotypes in already-colonized hosts was weak (relative rate of acquisition = 0.09, 95% credible interval: 0.05, 0.15).

Figure 1.

Epidemiologic “states” (compartments) and “transitions” for acquisition and clearance of 2 Streptococcus pneumoniae serotypes, A and B. State “0” denotes noncarriage, and state “AB” denotes simultaneous carriage of types A and B. The actual analysis accommodated the corresponding states: “noncarriage,” “single carriage” for each of the 8 serotypes and the deidentified group (9 states altogether), and “double carriage” for all pairwise combinations of states of single carriage (9 × 8/2 = 36 states). For more details, see the Web Appendix (http://aje.oxfordjournals.org/).