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Depressive disorders are highly prevalent mental disorders associated with an enormous individual and societal burden. The efficacy of both; treatment and prevention of depression have been meta-analytically demonstrated. Over the past two decades, an increasing number of internet-based interventions for depression has been developed and their efficacy was also metaanalytically shown. However, the uptake of such interventions – despite all the suggested advantages of internet-based interventions – is still rather low. The stigma still associated with “depression” may be one major barrier also to internet-based interventions. To overcome this barrier and potentially increase uptake, the paradigm of indirect interventions has been proposed recently. Indirect interventions primarily address common mental health problems, which are presumed to be less stigmatizing, and are suggested to reduce depressive symptoms indirectly. Targeting common mental health problems that are transdiagnostic risk factors for depression and other mental disorders – such as stress or repetitive negative thinking – seems especially promising.
This dissertation evaluated the efficacy of three different internet-based interventions that can be regarded as indirect interventions to reduce depression since they primarily targeted risk factors for depression. For this purpose three registered randomized controlled trials were conducted. In addition to assessing the efficacy of the interventions regarding the primary outcomes, the efficacy to reduce depression and further secondary outcomes was studied. In Study I (N = 200) the efficacy of an internet-based stress management intervention (iSMI), which was adapted and tailored to career starting teachers, was compared to a waitlist control group (WLG). The participants of the intervention group (IG) reported significant reductions on the primary outcome perceived stress at post-intervention (T2), ΔWLG-IG = 3.5, d = 0.52, 95% CI [0.24, 0.80], and threemonth follow-up (3-MFU), d = 0.49, 95% CI [0.21, 0.77]. Furthermore, it was shown that the intervention indirectly also reduced depression at T2, d = 0.66, 95% CI [0.38, 0.94], and 3-MFU, d = 0.47, 95% CI [0.19, 0.75], nad produced significant clinically meaningful reductions of depression with a number needed to treat (NNT) of 3.9 at T2. The effects were sustained at an extended 6-MFU. Besides efficacy, the feasibility to complement the iSMI with a newly developnedte rinet-based classroom management training was shown. Moreover, mediation analyses corroborated the role of problem- and emotion-focused coping skills in the intervention’s effect on stress and the indirect effect of the intervention on depression through stress.
Study II (N = 262) demonstrated the efficacy of an internet- and app-based gratitude intervention on the reduction of primary assessed repetitive negative thoughts at T2, ΔWLG-IG = 6.6, d = 0.61, 95% CI [0.36, 0.86], and 3-MFU, d = 0.75, 95% CI [0.50, 1.00], as compared to a WLG. The participants of the IG also reported significantly reduced depressive symptoms at T2, d = 0.38, 95% CI [0.13, 0.62], and 3-MFU, d = 0.40, 95% CI [0.15, 0.64], with significant clinically meaningful effects with an NNT of 4.3 at T2. The effects were sustained at an extended 6-MFU. Besides efficacy, mediation analyses showed that repetitive negative thinking mediated the gratitude intervention’s effect on depression.
Finally, Study III N( = 351) showed that an interneta-sbed intervention, tackling worries at the beginning of the COVID-19 pandemic, was effective as compared to an active mental health advice group. At T2, two weeks after randomization, the IG reported significantly reduecveedl sl on the primary outcome worry as compared to controΔlsW, LG-IG = 5.0, d = 0.38, 95% CI [0.17, 0.59]. Participants of the IG also reported significantly reduced levels of depression at T2, d = 0.47, 95% CI [0.26, 0.68], with significant clinically meaningful reductions with an NNT of 3.6. The extended follow-ups in the IG indicated that the improvements from baseline were sustained until the 2-MFU and the 6-MFU. In a mediation analysis, worry was shown to mediate the intervention’s effect on depression.
Across all three studies a reliable deterioration of depression was occasionally observed ranging from 3% to 5% in the IGs and from 5% to 12% in the control groups at T2. In summary, the studies in this dissertation demonstrated the efficacy of various indirect interventions focusing on rather common psychological problems to indirectly reduce depressive symptoms. The extent to which depression severity could be reduced is comparable to reductions found within participants with comparable baseline depression severity, in internetbased interventions directly addressing depressive symptoms. Indirect interventions are suggested to increase the uptake of interventions that reduce depressive symptoms, since they might be perceived as less stigmatizing and might broaden the range of interventions to choose from. Future research needs to compare indirect interventions for depression with direct interventions in head-to-head studies regarding uptake, efficacy and potential harmful effects. The indirect interventions examined in this dissertation could then complement the existing range of care for depression and thereby contribute to a reduction of the treatment gap and the burden of disease associated with depression.
