Indirect effect of conjugate vaccine on adult carriage of Streptococcus pneumoniae:: An explanation of trends in invasive pneumococcal disease

Background. Use of heptavalent protein-polysaccharide pneumococcal conjugate vaccine (PCV7) has been associated with decreases in PCV7-type invasive pneumococcal disease and nasopharyngeal (NP) carriage in children. Vaccine use has also indirectly decreased the rate of invasive disease in adults, presumably through decreased transmission of pneumococci from vaccinated children to adults. Methods. We conducted NP carriage surveys in 8 villages in Alaska in 1998 - 2004. Streptococcus pneumoniae isolates were characterized by serotype and antimicrobial susceptibility. We analyzed trends in serotype distribution, antibiotic resistance, and factors associated with adult carriage of PCV7-serotype pneumococci before and after the introduction of PCV7 in 2001. Results. We collected 15,598 NP swabs; overall, 52% of adults living in the villages surveyed participated in the colonization study. The proportion of adult carriers with PCV7-type pneumococcal carriage decreased from 28% of carriers in 1998 - 2000 to 4.5% of carriers in 2004 (P < .0001). Among adults, the proportion of colonizing isolates that were resistant to penicillin decreased from 13% in 1998 - 2000 to 6% in 2004 (P < .01), whereas the percentage of isolates with intermediate susceptibility to penicillin increased from 12% in 1998 - 2000 to 19% in 2004 (P < .01). Adults were more likely to carry PCV7-type pneumococci if they lived with a child <= 5 years old or if they lived with a child who had not been age- appropriately vaccinated with PCV7. Conclusions. Pediatric vaccination with PCV7 has resulted in decreased PCV7-type pneumococcal carriage among adults and helps to explain recent decreases in the rate of PCV7-type invasive pneumococcal disease among adults.