Abstract: We included 83 trials in this review. The majority of trials (67) were carried out in school settings with eight in colleges or universities, four in clinical settings, three in the community and four in mixed settings. Twenty-nine trials were carried out in unselected populations and 53 in targeted populations. For the primary outcome of depression diagnosis at medium-term follow-up (up to 12 months), there were 32 trials with 5965 participants and the risk of having a diagnosis of depression was reduced for participants receiving an intervention compared to those receiving no intervention (risk difference (RD) -0.03, 95% confidence interval (CI) -0.05 to -0.01; P value = 0.01). We rated this evidence as moderate quality according to the GRADE criteria. There were 70 trials (73 trial arms) with 13,829 participants that contributed to the analysis for the primary outcome of depression symptoms (self-rated) at the post-intervention time point, with results showing a small but statistically significant effect (standardised mean difference (SMD) -0.21, 95% CI -0.27 to -0.15; P value < 0.0001). This effect persisted to the short-term assessment point (up to three months) (SMD -0.31, 95% CI -0.45 to -0.17; P value < 0.0001; 16 studies; 1558 participants) and medium-term (4 to 12 months) assessment point (SMD -0.12, 95% CI -0.18 to -0.05; P value = 0.0002; 53 studies; 11,913 participants); however, the effect was no longer evident at the long-term follow-up. We rated this evidence as low to moderate quality according to the GRADE criteria. The evidence from this review is unclear with regard to whether the type of population modified the overall effects; there was statistically significant moderation of the overall effect for depression symptoms (P value = 0.0002), but not for depressive disorder (P value = 0.08). For trials implemented in universal populations there was no effect for depression diagnosis (RD -0.01, 95% CI -0.03 to 0.01) and a small effect for depression symptoms (SMD -0.11, 95% CI -0.17 to -0.05). For trials implemented in targeted populations there was a statistically significantly beneficial effect of intervention (depression diagnosis RD -0.04, 95% CI -0.07 to -0.01; depression symptoms SMD -0.32, 95% CI -0.42 to -0.23). Of note were the lack of attention placebo-controlled trials in targeted populations (none for depression diagnosis and four for depression symptoms). Among trials implemented in universal populations a number used an attention placebo comparison in which the intervention consistently showed no effect. Overall the results show small positive benefits of depression prevention, for both the primary outcomes of self-rated depressive symptoms post-intervention and depression diagnosis up to 12 months (but not beyond). Estimates of numbers needed to treat to benefit (NNTB = 11) compare well with other public health interventions. However, the evidence was of moderate to low quality using the GRADE framework and the results were heterogeneous. Prevention programmes delivered to universal populations showed a sobering lack of effect when compared with an attention placebo control. Interventions delivered to targeted populations, particularly those selected on the basis of depression symptoms, had larger effect sizes, but these seldom used an attention placebo comparison and there are practical difficulties inherent in the implementation of targeted programmes. We conclude that there is still not enough evidence to support the implementation of depression prevention programmes. Future research should focus on current gaps in our knowledge. Given the relative lack of evidence for universal interventions compared with attention placebo controls and the poor results from well-conducted effectiveness trials of universal interventions, in our opinion any future such trials should test a depression prevention programme in an indicated targeted population using a credible attention placebo comparison group. Depressive disorder as the primary outcome should be measured over the longer term, as well as clinician-rated depression. Such a trial should consider scalability as well as the potential for the intervention to do harm.

Plain-language summary: We included 83 studies (in particular randomised controlled trials) of evidence-based psychotherapy interventions (cognitive behavioural therapy (CBT) and third wave CBT, interpersonal therapy) that had the specific aim of preventing the onset of depressive disorder. For the primary outcome of depression diagnosis at medium-term follow-up (up to 12 months), there were 32 trials with 5965 participants and for the primary outcome of depression symptoms (self-rated) there were 73 trials with 13,829 participants. What does the evidence from the review tell us? We found that, compared with any comparison group, psychological depression prevention programmes have small positive benefits on depression prevention. There were some problems with the way the trials were done and in particular the results showed that compared to an attention placebo comparison group (a control intervention that controls for non-specific factors like involvement in a trial and attention from researchers), these programmes had no effect. There is still not enough evidence to support the implementation of depression prevention programmes. However, based on the effects seen for targeted depression prevention programmes (albeit with inadequate control groups), we recommend that further research be undertaken to test the effectiveness of depression prevention programmes in populations of young people who already have some symptoms of depression. Such trials should compare the intervention to an attention placebo comparison group and measure whether depressive diagnosis is prevented in the long term. They also need to consider whether the approach is something that can be implemented in the real world. In addition, they should consider and measure whether the intervention produces harmful outcomes.