To the best of our knowledge, this is the first extensive community study to measure the density of pneumococcal nasopharyngeal carriage. The data were collected as part of a CRT conducted in rural Gambia, an area with a high prevalence of pneumococcal nasopharyngeal carriage [14
]. We evaluated the density of pneumococcal carriage in different age groups before and after PCV-7 vaccination. Our main finding was that the density of pneumococcal carriage decreased with increasing age. We also showed in a before-after analysis that the density of pneumococcal carriage was lower after vaccination.
We have shown a strong inverse correlation between pneumococcal nasopharyngeal density and age, which is in agreement with the fact that the overall prevalence of pneumococcal carriage falls with increasing age [14
]. Younger individuals have a higher prevalence of nasopharyngeal pneumococcal carriage and, at the same time, a higher density of pneumococci in the nasopharynx. This may be due in part to less close contact between adults than between young children, but it is also likely to be due to the progressive acquisition of immunity as a result of prior pneumococcal carriage. Immunity acquired by prior pneumococcal exposure may prevent colonization and also bring a quick decrease in density of infection that, in turn, leads to accelerated clearance, as observed in mouse models [24
]. Several mechanisms might be responsible for the naturally acquired immune response that protects against carriage and reduces carriage density. Some studies have shown a negative association between the presence of antibodies to individual pneumococcal polysaccharides and the risk of subsequent acquisition of pneumococci of that serotype [25
]. In our study, we were not able to match antibody concentrations to the prevalence of carriage at the individual level. However, antibody concentrations reached a plateau in the study villages at approximately 10 years of age (Ota et al, unpublished data), while the decrease in the prevalence of pneumococcal carriage [12
] and the pneumococcal density continued beyond this age, suggesting that other non–serotype-specific mechanisms, such as acquisition of humoral or cellular immune responses (ie, interleukin 17 release [17
]) to conserved antigens, are involved in this process [27
]. The occurrence of the highest density of carriage in young children might explain, at least in part, why child carriers transmit pneumococcal infection more efficiently than adults, as we have shown in our study community [30
] and as has been detected elsewhere [4
]. Reduction in high-density carriage among young children by vaccination may contribute to the vaccine's herd effect.
The usefulness of measurement of the density of pneumococcal carriage in the nasopharynx as a tool for the diagnosis of pneumococcal pneumonia is of current interest [6
]. Bacterial densities are increased among pneumonic individuals, compared with healthy controls [6
]. Our findings indicate that if the ratio of carriage density between a case of suspected pneumonia and a healthy asymptomatic carrier is to be used as a diagnostic test, the impact of age on carriage density will have to be taken into account in setting “normal” values.
We have shown previously that vaccination with PCV-7 lowered the prevalence of nasopharyngeal VT carriage in both fully vaccinated communities and in those in which only children younger than 30 months received PCV-7, suggesting that young children are the main drivers of carriage in the community as a whole [12
]. The results of the current study, which showed reductions in the density of VT carriage in vaccinated and control communities (in which only part of the population was vaccinated), are in keeping with this hypothesis and in agreement with the findings from a CRT conducted in a Navajo population, in which the density of pneumococcal carriage (also measured as a semiquantitative variable) among vaccinated children was almost 40% lower than that among unvaccinated children [33
]. Reduction in density is also in agreement with the hypothesis stated above, which asserted that accelerated clearance (that would lead to a shift to a lower density of carriage) rather than inhibition of colonization may be an important mechanism of protection against pneumococcal carriage acquired either through natural exposure or through vaccination [24
]. If reduction in the density of carriage results in less effective transmission, as seem likely, this could be an important mechanism through which PCVs exert an indirect protective herd effect. We did not find any significant difference in carriage density between subjects from full or partially vaccinated communities, contrasting with the findings in relation to the prevalence of VT carriage, in which differences between study arms were small but statistically significant in some of the comparisons [12
An anomalous finding was the detection of a reduction in the density of NVT carriage following vaccination. Because we do not expect any direct effect of PCV-7 vaccination on serotypes not included in the vaccine, alternative explanations are needed. One possibility is that the decrease in density of NVT was a consequence of the unmasking of low-density NVT after the removal VT as a result of vaccination. Some investigators have suggested that the observed increase of NVT nasopharyngeal carriage after vaccine introduction is an artifact of unmasking, in which the reduction in the prevalence of VT makes it easier to detect NVT present in the population that were undetected in the absence of vaccination [6
]. Because the commonly used serotyping methods evaluate only a limited number of colonies, investigators may fail to detect cocolonization with serotypes present at low density. In the CRT discussed above conducted in a Navajo population [33
], investigators found a decrease in the prevalence and density of VT but not of NVT in the vaccinated arm. In that study, the serotyping method was more sensitive than ours, making unmasking less likely. Another possible explanation for our findings of a reduction in the density of NVT carriage after vaccination is that there were some secular trends unrelated to vaccination. However, it is difficult to envisage what changes could have occurred in the study villages that could have affected the density of carriage during the conduct of the study, because there were no significant climatic or social variations during the study period or changes in medical practice, apart from the administration of azithromycin at the time of the last CSS, an event that has been addressed in the analysis. The methods by which samples were collected, stored, and tested were identical in all CSSs, and quality controls were in place through the study. Any secular change affecting density should probably have had the same impact on the density of NVT and VT.
The analysis presented here has additional limitations. Density was measured as a semiquantitative and subjective variable, but technicians were blind as to which study group a sample belonged. Newer methods (eg, real-time polymerase chain reaction) are able to provide fully quantitative measurements and are likely to make measurement of bacterial density a more feasible end point for vaccine trials than has been the case in the past. Because of the heterogeneity of the results of the different postvaccination surveys, we averaged the results for the main analysis. In addition, analysis was oversimplified by grouping serotypes in 2 groups (VT and NVT). Density differs by serotype and serotype distribution within VT and NVT varied in the pre vs the postvaccination CSSs. However, such variation has not significantly altered the mean densities in the comparisons (data not shown).
In summary, our analysis has given insights into why children are more efficient than adults in pneumococcal transmission when they are nasopharyngeal carriers. Our time-trends analysis also shows that vaccination may lower the density of pneumococcal carriage in wholly and partially vaccinated communities. Further studies to support or disprove our findings of an effect of vaccination on NVT are needed.