Of the studies reporting the intention to use a condom, one study had a positive effect on this outcome among the whole population  and one reported a positive effect in females [23,24]. The third study did not report a significant effect [28,29].
The most common measure for sexual activity was ever having had sex. Eleven studies reported this outcome, of which 10 provided estimates disaggregated by sex. Magnani et al.  found a decrease in sexual activity for the whole population, whereas Shuey et al.  and Klepp et al.  found an increase. Overall, there is good evidence that interventions implemented to date do not increase sexual activity in youth (Table 3).
Nine studies presented information about the effects of interventions on multiple partners, five disaggregated by sex. There was pronounced heterogeneity, but little evidence of an increase in multiple partners (only detected in 1 of 21 reported outcomes) . In three of five studies, interventions had a larger effect in males than females [19,21,27,35].
HSV-2 incidence was lower in the intervention group in a cluster-randomized trial in South Africa [RR = 0.67 (0.47–0.96)], assessing the effects of 13 3-h sessions of participatory learning . No effects, however, were detected on HIV or pregnancy incidence. Similarly, no effect was noted on HIV or pregnancy incidence in a Tanzania trial with randomized communities . However, among female participants, the prevalence of N. gonorrhoea was higher in the intervention arm, though this difference was of borderline significance. Meta-analysis of HIV incidence data in these two trials showed an overall RR of 0.91 (95% CI = 0.66–1.26; I 2 = 0.0%) (Table 4).
Surprisingly little information was available on youth interventions in sub-Saharan Africa: only 28 studies were identified with as few as two studies collecting biological endpoints, and many studies had suboptimal study designs. This is particularly concerning given the extent of the vulnerability to HIV infection faced by the 125 million young people in sub-Saharan Africa, and the presence of numerous HIV-prevention initiatives and funding opportunities in this region. The paucity of high-quality studies confirms findings of previous reviews on this topic [8,11,36,37]. Field experiences of the authors of this study, however, indicate that considerably more interventions are implemented, but are generally not designed for robust evaluation, or evaluation data are not analysed and disseminated.
There were very few commonalities in study design and interventions tested, perhaps suggesting that there is little consensus on the optimal approach to these interventions and that few studies have built upon previous knowledge in a linear fashion. In addition, no two studies used the same methods of analysing or reporting data, and outcome indicators very markedly diverse. Oftentimes odds ratios and CIs were not provided, and multivariate analysis was not universal. There should be more studies that use a strong evaluation design and measure biological outcomes. Moreover, evaluators should provide more transparency in their multivariable analysis and reporting.
The same outcomes are often operationalized in slightly different ways. Several attempts have been made to standardize indicators and make them easily available online (e.g. by the Centre for HIV Identification, Prevention and Treatment Services – http://chipts.ucla.edu/assessment/index.asp), which has not yet translated into widespread and systematic use of these indicators. The development of standardized methods and indicators does not guarantee the use thereof, and therefore organizations and evaluators should be informed and sensitized on the importance of using standardized indicators and scales.
It is encouraging to note that taken together the evidence indicates that sex education and condom promotion activities among youth does not increase sexual activity, nor promote risky sexual behaviour. However, we could not observe large positive changes either. Youth did not significantly reduce sexual activity, and condom use at last sex only increased notably among males. Only one study reported a positive impact on a biological outcome (HSV-2 ). This finding corresponds with other reviews, who find significant changes in knowledge and attitudes, but a small degree of risk reduction [5,7,8,36,38,39].
Studies ascribed the limited impact of interventions to poor implementation of the intervention [22,27,31,40–42]. Several authors explicitly reported a reluctance of teachers and health professionals to discuss condom use with youth [22,31,41]. Resource constraints and general disorganization in schools often hampered implementation of the planned activities, such that time assigned for the scheduled activities was often curtailed or cancelled altogether [27,42]. Since interventions which reported implementation barriers did not generally have fewer positive outcomes, it is possible that other studies also experienced implementation difficulties, but did not report them.
Limited effectiveness might also stem from flaws in the assumptions underlying HIV risk reduction interventions. Although the interventions varied markedly in the setting and delivery strategies they adopted, they predominantly focused on HIV/AIDS as a means of changing sexual risk behaviour. However, the existence of a direct causal link between sexual behaviour and HIV infection does not mean that the converse is true. From an ecological perspective, HIV/AIDS is only one factor among a great number of interacting factors which operate on different levels to influence sexual behaviour . Seen from an ecological viewpoint, it is quite logical that interventions focusing on knowledge or attitudes to HIV/AIDS can only result in relatively small changes in sexual behaviour.
Several indicators showed a larger impact on males than females (condom use at last sex, ever had sex, and number of partners). This might suggest that women still experience marked difficulties in negotiating condom use or assuming full control over their sexual activity [44,45]. Also, several evaluations drew a distinction between moderate and high exposure to the intervention, reporting more impact among the highly exposed group [22,26,32–34,46]. In order to increase programme effectiveness, it is crucial to analyse the determinants of the level of intervention exposure, and how to optimally reach underexposed groups. Furthermore, differences in impact according to sexual history were present in several evaluations: participants who were virgins at the time of exposure to the intervention reported higher rates of abstinence after the intervention , less sexual intercourse in the past months  and higher intentions to use a condom . This highlights the importance of HIV prevention interventions for children and early adolescents.
Though no publication bias was detected with funnel plots, it is highly likely that organizations will generally be less likely to publish negative or neutral results, especially regarding subgroups. However, it is also possible that those research groups who do publish their findings are more conscious of the importance of rigorous evaluation and publication, and therefore perhaps more cognisant of the importance of meticulous intervention development and implementation. All but two studies relied exclusively on self-reported data on sexual behaviour, which is subject to several biases and unsupported by biological outcomes. We selected behavioural interventions to reduce HIV/STI infections, whereas other interventions also may have an impact on sexual behaviour among young people, such as interventions to reduce alcohol or drug use.
There remains a stark mismatch between the burden of HIV in youth and efforts to conceptualize putative interventions and subject them to trial conditions with robust endpoints. The effectiveness of HIV-prevention interventions on sexual behaviour overall, to date, however, appears relatively small and perhaps confined to particular subgroups such as males. More attention is required to comprehend difficulties with implementation, differences in response to interventions by sex, determinants of exposure to interventions and promoting the inclusion of factors other than HIV which determine sexual behaviour.
Sources of support: K.M. was funded by the Research Foundation Flanders (FWO) Belgium.
Description of the role of each of the authors in the study: K.M.: initiation of the study, literature search, data extraction, writing.
M.F.C.: data extraction, statistical analysis, support in writing.
S.L.: data extraction, support in statistical analysis and writing.
P.deK.: data extraction, support in writing.
R.vanR.: data extraction, support in writing.
M.T.: promoter of the study, support in writing.
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