Population structure of invasive Streptococcus pneumoniae isolates in Italy prior to the implementation of the 7-valent conjugate vaccine (1999-2003)

A total of 773 pneumococcal isolates were collected from a nation-wide surveillance of invasive pneumococcal diseases during 1999-2003 prior to the implementation of the 7-valent conjugate vaccine (PCV7) in Italy. Isolates included vaccine serotypes (VS, 393 isolates), vaccine-related serotypes (VRS, 93), non vaccine serotypes (NVS, 279). The 10 most prevalent serotypes were: 14 (16.4%), 3 (8.4%), 23F (8%), 19F (7.4%), 4 (5.9%), 7F (5.8%), 9V (5.3%), 6B (4.9%), 19A (4.7%), and 1 (3.7%). VRS or NVS isolates showed a lower rate of penicillin or drug resistance than VS. Representative isolates of the major VS, VRS, and NVS were genotyped by PFGE and MLST. The isolates examined were found to belong to 18 international clones and to 8 newly described clones. VS isolates sharing clonal groups with VRS or NVS were also detected. Evidence of a past history of capsular switching events was observed in 5 clones.