Indirect effect of 7-valent pneumococcal conjugate vaccine on pneumococcal colonization among unvaccinated household members

Background. Since the introduction of 7-valent pneumococcal conjugate vaccine (PCV7) in the United States, rates of invasive pneumococcal disease have decreased in both vaccinated and unvaccinated age groups. Reduction of invasive pneumococcal disease in unvaccinated groups has been attributed to reduced transmission of vaccine-type pneumococci in the community. Understanding the impact of PCV7 on carriage among vaccinated and unvaccinated community members is critical to interpreting, predicting, and understanding the impact of PCV7 on disease. Methods. A group-randomized, phase III efficacy trial of PCV7 was conducted among southwestern American Indian communities. Meningococcal conjugate vaccine against serogroup C was used as the control. After the trial was unblinded, we conducted a carriage study of participating communities to evaluate the impact of PCV7 on colonization among trial participants and their unvaccinated household members. Results. Adults and unvaccinated children aged <5 years living in households with a PCV7 vaccinee were less likely to be colonized with vaccine-type pneumococci (odds ratio [OR] for adults, 0.57; 95% confidence interval [CI], 0.33-0.99; OR for children, 0.57; 95% CI, 0.26-0.98) than were those living in a household with a control vaccinee. There was no difference for children aged 5-17 years. Decreases in vaccine-type carriage were offset by increases in carriage of nonvaccine types. Among adults living with a trial participant colonized with vaccine-type pneumococcus, those in the households randomized to receive PCV7 were less likely to be colonized with the same serotype than were those in the households randomized to receive the control vaccine (OR, 0.34; 95% CI, 0.11-0.99). Conclusions. Vaccine-type pneumococcal carriage was lower among adults and unvaccinated children living with a PCV7 vaccinee. This is attributable to reduced exposure and reduced transmission when exposure occurs.