In 1999, Engelen and coworkers investigated colonization in Amsterdam among
259 children attending 16 day-care centers (DCCs) and among 276 children w
ho did not attend day-care centers (NDCCs). A 1.6- to 3.4-fold increased ri
sk for nasopharyngeal colonization was observed in children attending DCCs
compared with NDCC children, while no difference in antibiotic resistance w
as found between groups. The serotype and genotype distributions of 305 nas
opharyngeal Streptococcus pneumoniae isolates of the latter study were inve
stigated. The predominant serotypes in both the DCC and the NDCC groups inc
luded 19F (19 and 18%, respectively), 6B (14 and 16%, respectively), 6A (13
and 7%, respectively), 23F (9 and 7%, respectively), and 9V (7 and 7%, res
pectively). The theoretical vaccine coverage of the 7-valent conjugate vacc
ine was 59% for the DCC children and 56%, for the NDCC group. Genetic analy
sis of the pneumococcal isolates revealed 75% clustering among pneumococci
isolated from DCC attendees versus 50% among the NDCC children. The average
pneumococcal cluster size in the DCC group was 3.8 and 4.6 isolates for tw
o respective sample dates (range, 2 to 13 isolates per cluster), while the
average cluster size for the NDCC group was 3.0 (range, 2 to 6 isolates per
cluster). Similar to observations made in other countries, these results i
ndicate a higher risk for horizontal spread of pneumococci in Dutch DCCs th
an in the general population. This study emphasizes the importance of molec
ular epidemiological monitoring before, during, and after implementation of
pneumococcal conjugate vaccination in national vaccination programs for ch
ildren.