To anticipate the potential long-term effects of the introduction of pneumococcal conjugate vaccination in sub-Saharan Africa, we have conducted a novel randomized, controlled trial in which a population of pneumococci was exposed to maximum immune pressure by vaccinating all residents of 11 Gambian villages with PCV-7. We measured the prevalence of pneumococcal carriage in these villages before and on three occasions after vaccination, and compared these findings with those obtained in control villages where only young children had been vaccinated with PCV-7. Time trends were investigated by comparing pneumococcal carriage within both groups of villages before and after vaccination.
By the time this study was ready to start, the results of the Gambian PCV-9 trial had become available 
, showing a high level of protection against IPD in children under the age of 30 mo, so it was considered unethical to proceed with a trial in which all children in this age group did not receive PCV, as had been originally intended. This design modification weakened the ability of the study to show the direct effect of vaccination on the community through differences between vaccinated and control villages, but serendipitously allowed demonstration of a potential herd effect resulting from immunization of children in control communities. Changes in the prevalence of carriage between the before and after vaccination surveys need to be interpreted with caution because of the potential influence of temporal trends unrelated to vaccination. However, methods used for sampling and isolation of pneumococci were consistent over the study period, including pre- and post-vaccination surveys. Furthermore, the investigators are not aware of any changes in the study villages that could have affected risk factors for carriage during the conduct of the study, apart from the administration of azithromycin at the time of the last CSS, an event which has been addressed in the analysis.
A time trend analysis showed a marked drop in the prevalence of carriage of VT pneumococci among all age groups in vaccinated and control villages following vaccination. The decline in the prevalence of VT carriage in older children and adults in the control villages strongly suggests a herd effect resulting from vaccination of infants and toddlers in those villages. This effect was apparent as early as 6 mo after vaccination and persisted for at least the next 2 y. This finding is in line with results from a pre-vaccination observational study conducted in the same villages that suggested that transmission occurred mainly from young children to older members of the family 
. Although the study villages were separated by at least 3 km, some mingling between communities may have contributed to the herd effect observed in the control villages, as some older children resident in these villages attended school in neighboring villages. Because of the high level of transmission of pneumococcal infection in Africa 
, there have been concerns that induction of herd immunity through vaccination of infants might be more difficult to achieve in this continent than elsewhere. We have shown here, to our knowledge for the first time in Africa, a herd effect on carriage of VT pneumococci, which might be translated into herd protection against IPD in adults, following routine immunization of infants and young children, as has been observed in the US 
. Our findings are relevant for other countries in Africa contemplating the introduction of PCVs where the pattern of pneumococcal infection is similar to that in The Gambia. Whether a similar effect will be observed in countries with a high prevalence of HIV (HIV-infected adults are more susceptible to being carriers of pediatric pneumococcal serotypes) remains unknown.
The prevalence of carriage of VT pneumococci was slightly lower, but statistically significantly so, in individuals from vaccinated than from control villages in most post-vaccination comparisons. Thus, vaccination of older children and adults with PCV-7 had some additional effect in decreasing rates of carriage of VT pneumococci, but this effect was not marked.
The prevalence of carriage of NVT pneumococci among adults was lower in the post-vaccination surveys than in the pre-vaccination survey in both control and vaccinated communities. This decrease was not observed in other age groups in either vaccinated or control villages, so this may have been a chance finding. There was a significant gap between the time that the pre-vaccination study was done and the first post-vaccination survey, and it is possible that there may have been some change in the pattern of carriage in the study villages during this time. However, surveys undertaken in The Gambia over a period of many years, including several conducted in the study area, have consistently shown very high levels of carriage, and it is unlikely that the overall level of carriage would have changed during this period, although there could have been some changes in serotype distribution 
. “Up and down” trends in the prevalence of different serotypes have been documented previously in the area over time 
. Another possible explanation includes a nonspecific effect on health produced by the intervention, but such an effect should have been observed in all age groups. Still, the observed decrease of NVT pneumococci prevalence from the pre-vaccination CSS to post-vaccination CSS-1 was significantly lower than the decrease of VT pneumococci prevalence observed for these two CSSs in both vaccinated and control communities.
The absence of a significant increase in the prevalence of carriage of NVT pneumococci in any age group, and the limited differences between control and vaccinated villages, are surprising, as many previous studies 
, including some conducted in The Gambia 
, have shown a marked increase in the prevalence of carriage of NVT pneumococci after vaccination. These previous carriage studies conducted in The Gambia focused on infants and younger children 
, an age group not sampled in our study. Lack of an increase in NVT pneumococci prevalence has previously been documented in a carriage study of household contacts among Native Americans whose children participated in a PCV-7 vaccine trial 
. The investigators observed no increase in carriage of NVT pneumococci among older children and adults within households of children vaccinated with PCV-7, although a reduction in carriage of VT pneumococci was found in the adult age group. An increase in carriage of NVT pneumococci was found only in non-vaccinated household members <5 y of age from households with PCV-7 vaccinees. Encouragingly, vaccination of older children and adults in our study did not add selection pressure towards an overall increase in carriage of NVT pneumococci.
We have also observed in our study a decrease of the overall pneumococcal carriage in the post-vaccination CSSs compared to the pre-vaccination CSS. Because there is little serotype replacement, PCV introduction might also have had an effect on the overall prevalence of pneumococcal carriage at least up to 2 y after vaccination. Pneumococcal prevalence reached its lowest levels in the post-vaccination CSS-2. As mentioned above, secular trends or the health effect of the intervention could partly explain such results.
An increase in carriage of serotype 5 pneumococci seen in vaccinated villages in CSS-3 was due largely to an outbreak in one village and may have been a chance finding unrelated to vaccination, an event similar to one observed in the study community before any intervention was performed 
. However, the possibility of an association with PCV vaccination is of concern as this serotype, along with serotype 1, is a major cause of IPD in The Gambia 
and elsewhere in Africa 
. Both serotype 5 and serotype 1 pneumococci also have the potential to cause epidemics 
. An increase in the prevalence of carriage of NVT serotypes such as 19A and 23B was observed during CSS-3 in the vaccinated villages (shown in both the arm comparison and the time trend analysis), probably driven by young children. An increase in carriage of serotype 19A after PCV vaccination has been documented previously in The Gambia 
and elsewhere 
. An increase has also been widely documented for IPD caused by this serotype 
, demonstrating lack of cross-protection of PCV-7 for serotype 19A. Serotype 6A was still present in all post-vaccination surveys but with lower prevalence in each of the post-vaccination CSSs compared to the pre-vaccination survey in both vaccinated and control communities, suggesting the expected vaccine protective effect in carriage, and potentially on IPD, a finding that justifies inclusion of this serotype in the VT group. However, the 6A decrease was statistically significant only among vaccinated communities. Serotype 3 was the most prevalent NVT serotype detected both before and after vaccination in this study, and serotype 19A was shown to be the most common at some point after vaccination. Both of these serotypes and serotype 6A are contained in the PCV-13, which has been shown to be immunogenic in infants, with potential protective effect against serotype-specific IPD 
. PCV-13 has recently replaced PCV-7 in routine childhood immunization programs in The Gambia and would be expected to provide coverage of these serotypes.
This study faced an additional challenge. The last post-vaccination survey was disrupted by the administration of azithromycin to individuals in several study villages as part of a national trachoma elimination program that had not been envisaged when the carriage study was planned. To avoid any direct or indirect effect resulting from this treatment, CSS-3 analyses include only a third of the target number of samples, all collected before treatment started in the area, and therefore, this survey lacked statistical power for comparison of vaccinated versus control villages.
Despite the challenges and limitations of this study, the main findings are robust and very encouraging. Vaccination of young Gambian children reduced carriage of VT pneumococci in vaccinated children but also in vaccinated and non-vaccinated older children and adults, revealing to our knowledge for the first time in Africa a potential herd effect from vaccination of young children. The immunological pressure induced by vaccinating whole communities did not lead to a community-wide increase in carriage of NVT pneumococci during a 2-y period after vaccination. Further long-term follow-up carriage studies in this community are planned.