Pneumococcus exists only in the nasopharynx of humans, and it has no natural reservoir in animals. Humans are the exclusive targets of pneumococcal diseases, with transmission only coming from other human carriers. Thus, carriage is essential for the propagation of pneumococcal diseases, and its epidemiology is important. Pneumococcal colonization may be influenced by multiple factors. Although these factors are not entirely clear, there is no doubt that the local host immune response plays an important regulatory role in the trafficking of pathogens in the nasopharynx. Poorly immunogenic serotypes tend to colonize for longer periods, and the low carriage rates in adults suggest the existence of immunological protection after previous exposure (16
Pneumococcal carriage occurs early in life, usually with a prevalence of about 30%-60% in infants and 1%-10% in adults (17
). In some cases, more than 95% of children can be colonized with up to six different serotypes by the age of two (19
). The prevalence of co-colonization by multiple serotypes was estimated to range from 1.3% to 48.8% (20
). Geographical factors, study population, and different detection techniques may be responsible for this variation (21
). The epidemiology and clinical significance of co-colonization needs better clarification. As children grow older, the prevalence of pneumococcal carriage decreases, and the distribution of colonizing serotypes changes to those found in adults. Before the age of nine, the carriage rate is maintained above 30%-40%, but it declines progressively afterward () (22
). However, the carriage rate is quite variable according to the local epidemiology, trending toward higher carriage rates in impoverished communities with low vaccination rates. Studies by Mackenzie et al. and Hill et al. showed remarkably high carriage rates (≥ 30%), even in young adults () (25
). In Korea, the pneumococcal carriage rate was estimated to be 34.3% among children aged 5 yr or younger before the introduction of PCV7 (27
), and it was 16.5% among those aged 18 yr or less in the era of PCV7 (28
). Considering the difference in study populations, there is a limitation to comparing the results of these two studies (), and data on Korean adults is not available.
Pneumococcal carriage rate by age group
Common colonizing serotypes differ between young children (< 5 yr) and adolescents/adults. Before the introduction of PCV7, serotypes 19F, 6A, 6B, and 23F were prevalent in young children aged less than 5 yr, while serotypes 3 and 23F were relatively common in adolescents and adults (22
). After the introduction of PCV7, these were replaced by non-vaccine serotypes, including 19A, 6C, 11A, 15A, and 15B/C (28
The duration of colonization can range from days to months and depends on the serotype (31
). Serotype 1 is rarely found to colonize the nasopharynx, and serotypes 4, 5, and 7F also colonize to lesser degrees (32
). These serotypes are able to initiate colonization of the nasopharynx, but colonization may be much shorter in duration compared to other serotypes. Sleeman et al. (33
) estimated the duration of pneumococcal carriage among common colonizing serotypes. The duration ranged from 5.9 weeks for serotype 15C to 19.9 weeks for serotype 6B.
Causative mechanisms of carriage rate remain unclear. Several mechanisms have been postulated, most of which hinge on interactions between the bacteria and host immune defenses. Serotype-specific polysaccharide capsules protect against immune-mediated clearance in several ways, including blocking the deposition and function of opsonins (34
), trapping by neutrophil extracellular traps (NETs), and clearance by mucus (35
). Both the prevalence of carriage and virulence appear to be directly related to the degree of encapsulation. In a murine study, Weinberger et al. (37
) demonstrated higher carriage rates among serotypes with larger capsules. Furthermore, those serotype capsules containing more carbon molecules per polysaccharide repeat unit tend to be smaller and more prone to nonopsonic killing. Authors from this study suggest that low-carbon polysaccharide repeats would require less energy to manufacture and afford a lower "metabolic cost," allowing a larger polysaccharide bulk and therefore improved fitness for carriage.