To our knowledge, this is the largest report documenting the prevalence of pneumococcal serotypes in nasopharyngeal carriage in pre-school children before the introduction of PCV7 into the Danish Childhood Immunization Programme. We found a high prevalence of pneumococcal carriage, highest in children aged 12–23 months, where nearly 70% were carriers. Serotypes included in PCV7 were not commonly represented, only in about 30% of carried isolates.
We found that both the carrier rates and the serotype distribution of carried isolates varied markedly with age. The high prevalence of carriage among the youngest children coincides with the observation that the incidence of IPD is highest in this age group [8
]. Paediatric serotypes/groups (6, 14, 19 and 23) were found frequently both in carriers and as a cause of IPD in the youngest children in the pre-PCV7 period. These serotypes/groups are all represented in PCV7, explaining the higher coverage of the PCV7 among IPD cases in children <23 months, for whom vaccination is attempted (Fig.
). The predominance of serotypes included in PCV7 declined significantly with increasing age, both among carriers and IPD cases, in accordance with findings reported from other populations [2
]. Serotype 5 is not a frequent cause of paediatric IPD in Denmark, while serotypes 1 and 7F become more important in IPD in older children and adults [8
]; all these factors may determine a relatively low coverage of PCV10 among carried isolates in our study population. The higher serotype coverage of PCV13 was mainly related to the presence of serotype 6A in both carriage and in IPD (Fig.
). These findings are comparable to those reported from a number of carriage studies from different geographic locations [7
]. Also, the analysis of risk factors for the carriage of vaccine serotypes showed that age, gender, size of DCC, type of DCC, and month of sampling were not found to be significantly associated with an increased risk of carrying an isolate belonging to a serotype included in PCV7, PCV10 or PCV13 (data not shown).
The pneumococcal capsular polysaccharide has been described as a major determinant of the duration of carriage, relative invasiveness, disease presentation and mortality related to IPD [13
]. It also appears that the biochemical structure of the capsule determines the success of a given serotype in nasopharyngeal carriage [24
]. In spite of differences in study design and population, a number of studies have identified a few serotypes as having the greatest invasive potential. Similar to our findings, a study from Gambia reported that serotypes/groups 1, 12 and 14 were more frequently found in patients with severe pneumococcal disease than in carriers, while serogroups 19 and 23 were often isolated from healthy controls [5
]. In a study from India, serotypes/groups 1, 5, 7, 12 were prevalent among invasive isolates, but not among carriers [25
]. Also, a study from Papua New Guinea [26
] reported that serogroups 11, 15 and 22 were of low invasiveness, similar to what we found in our study. A study from Oxford reported that PCV7 serotypes showed to be associated with high invasiveness [27
]. The consistency between these reports supports the fact that the invasiveness of capsular serotypes is a serotype-specific characteristic. However, even if capsular serotype is a factor that determines the invasiveness of a particular strain, the study population may determine the prevalence of host-related factors (such as genetic predisposition, the prevalence of co-morbid conditions and socioeconomic factors), which influence the invasiveness of serotypes.
In conclusion, pneumococcal colonization in children attending DCCs is frequent, highest in the youngest infants, and represents a heterogeneous population of pneumococcal serotypes. Studies investigating the nasopharyngeal pneumococcal carriage in the coming years would be of major interest in order to evaluate the dynamics of the pneumococcal population in the post-PCV7 period.