Impact of heptavalent pneumococcal conjugate vaccine on nasopharyngeal carriage of penicillin-resistant Streptococcus pneumonide among day-care center attendees in central Greece

Background: In Greece, the heptavalent pneumococcal conjugate vaccine (PCV7) became available in October 2004 and it was incorporated into the national immunization schedule in January 2006. Methods: In February 2005, a yearly surveillance of the nasopharyngeal colonization with Streptococcus pneumoniae in children attending day-care centers in Central Greece began. Results: Between February 2005 and May 2007, nasopharyngeal cultures were obtained from 1829 children aged 13-76 months (median age, 47 months). The proportion of attendees vaccinated with >= 1 doses of PCV7 increased from 13% (2005) to 33% (2006) and to 70% (2007); 98% had been immunized on toddler catch-up schedules. Among vaccinated carriers, the proportion of PCV7 serotypes decreased from 33% (2005) to 29% (2006) and to 8.6% (2007) (chi(2) for trend, P < 0.001), the proportion of PCV7-related serotypes increased from 13% (2005) to 26% (2006) and to 28% (2007) (P = 0.16), whereas the proportion of non-PCV7 serotypes was 48% in 2005, 31% in 2006, and 55% in 2007 (P = 0.17). The proportion of PCV7 serotypes declined also among unvaccinated carriers. The carriage of serotype 19A did not increase. Among vaccinated carriers, the rate of highly penicillin-resistant isolates decreased from year I to year 3, respectively, 11%, 7.7%, and 0.6% (P = 0.001), whereas the proportion of penicillin-intermediate pneumococci was 13% in 2005, 23% in 2006, and 26% in 2007 (P = 0.22). Conclusions: In Central Greece, widespread PCV vaccination was followed by a significant reduction of carriage of highly penicillin-resistant pneumococci. The frequency of penicillin-intermediate isolates did not change significantly among vaccinated carriers.