This study aims to evaluate the feasibility and effectiveness of a web-based self-help intervention in adolescents with acute or remitted major depressive disorder. We will examine whether this intervention improves positive affect, reduces stress and alleviates negative affect and depressive symptoms in adolescents aged 12 to 18 years. We will also investigate the rates of adherence among the adolescents who use this web-based intervention and the acceptability of the intervention with adolescents.

Youth with major depressive disorder often have little knowledge about depression, and do not receive an appropriate treatment. Besides little knowledge, concerns about social stigma, confidentiality and limited access to mental health services are some of the main barriers to seek help. Therefore, highly acceptable and easily accessible information and services as a complementary strategy to mental health treatment, such as knowledge about depression and self-help strategies, are urgently needed for young people seeking help for depression.

We developed a website, which provides evidence-based information about depression (e.g., identification, etiology, treatment, and prevention of depression) and will be launched in autumn 2021. Furthermore, the website provides information about self-help strategies (e.g., reducing stress, doing exercise, undertaking positive activities), which are meant to serve as an addition to professional treatments of depression or to promote mental health in adolescents. Target groups of the website are adolescents aged 12 to 18 years seeking help for depression, as well as healthy adolescents seeking information about mental health promotion or depression.

To increase the acceptability of the website and the engagement of young people, the website will integrate continuously updating content consisting of short exercises based on principles of positive psychology. This web-based self-help intervention is thought to provide a mode of delivery (the combination: "online" & "positive psychology"), which is acceptable and engaging to youth. In addition, the provided self-help exercises might be a complementary approach to professional treatments of depression.

Since the website targets two different groups, we will evaluate the web-based self-help intervention accordingly:

Target group 1: Adolescents with a major depressive disorder (acute or remitted) Target group 2: Healthy Adolescents (no mental health condition)

The current study will focus on target group 1. A study focusing on target group 2 can be found in a separately registered clinical trial on clinicaltrials.gov.

The primary aim of the study is to evaluate the feasibility and effectiveness of this web-based self-help intervention to improve positive affect, reduce stress and alleviate negative affect and depressive symptoms in adolescents with acute or remitted depressive disorder.

Participating young people will be randomized to either the web-based intervention group or the web-based control group (i.e. sham intervention / comparator). All participants will be evaluated at pre- and post-intervention, and at a two-week follow-up.

Inclusion Criteria:

• Current or remitted diagnosis of MD (ICD-10 diagnoses F32.0; F32.1; F32.2, F33.0, F33.1, F.33.2)
• Intelligence quotient (IQ) of ≥ 80

Exclusion Criteria:

• Acute suicidality
• Schizophrenic disorder
• Pervasive developmental disorder
• Bipolar disorder
• Borderline personality disorder
• Substance dependence disorder
• Insufficient knowledge of German

Data in our study contains sensitive patient information, such as sociodemographic information and comorbidities. Since patients could possibly be identified by making our raw data publicly available, ethical principles of protecting patient confidentiality would be breached. Aggregated group data can be made available upon request.

Participants are not aware of the assigned intervention group