Our data confirm information indicating higher neverless effectiveness of new vaccines [39]. An encouraging finding was that a majority of PNSSP and MDR-SP belonged to serotypes included in PCV13 and PCV10, but there is no spectacular difference between covering of drug-resistant serotypes belonged to PCV7 and PCV10/PCV13.Even though 25.4% children were colonized twice and 8.4% children were colonized thrice during our study, in most cases, pneumococci isolated from the same child in 2 different seasons were unrelated. Two children carried the same pneumococcal strain for more than 6 months (in three seasons of our study), and two children were colonized by identical strain in autumn and spring (without pneumococcal isolation in winter) probably due to too small number of bacteria in the throat/nose made isolation impossible.

Moreover, 28 children were colonized by identical strain for 3 months. This confirmed high dynamics of pneumococcal strain replacement in short time among healthy preschool children attending DCC [31]. Prolonged pneumococcal colonization was observed by other authors [31, 32], and it has been inversely correlated with age but also depends on genetic backgrounds of both host and bacteria [40]. We did not observe correlation between prolonged colonization and age. However, 75.8% strains colonizing tested children up to 6 months belonged to 6B (24.2%), 14 (18.2%), and 19F (33.3%) serotypes which are known to be weak inducers of immunity [1, 27, 31, 32].5. ConclusionThe present study demonstrated that upper respiratory colonization with S.

pneumoniae in preschool children is a dynamic process. We found seasonal variations in the rate of pneumococcal colonization as well as in the factors affecting colonization. Autumn appears to be exceptional season when children meet each other in large groups after summer holidays and they have more RTIs due to seasonal collapsing of immunity and frequently received antibiotic therapy. In this season, DCC attendance and antibiotic therapy (number of courses and type of antibiotic) were the factors predisposing to SP colonization. In winter, continuation of antibiotic therapy causing cumulation of antibiotic courses probably combined with children absence in DCC due to infections were the factors which limited frequency of pneumococcal colonization in this season.

In spring, children come back to DCC after illnesses and frequency of colonization increases. Our observation clearly indicated that each DCC is unique setting, which creates conditions conducive to the transmission of pneumococci, including drug-resistant strains, and the appearance and efficient spread of endemic/epidemic genotypes are strongly associated with greater antibiotic Entinostat pressure and presence of greater group of children.