This study determines the effectiveness of telephone-delivered cognitive-behavioral continuation therapy (T-CT) in comparison to usual care in people with recurrent or chronic depression. The primary research question is whether participating in T-CT reduces depressive relapses. The continuation therapy comprises eight therapy sessions delivered over the telephone by a trained therapist over a period of approximately six months following acute-phase psychotherapy.

Major depression is a serious mental disorder that often takes a recurrent or chronic course causing enduring individual suffering as well as immense direct and indirect health costs. Research indicates that psychological continuation interventions following successful acute-phase therapy are effective in preventing depressive relapse and recurrence but access to these interventions is limited. Systematic psychological continuation interventions are hardly implemented in health care yet, and research shows that there are obstacles concerning access to and compliance for these interventions in a face-to-face setting underlining the need for alternative ways of delivery. The present study ("NaTel study") aims to investigate the effectiveness of telephone-administered cognitive-behavioral continuation therapy (T-CT) following acute-phase psychotherapy. The primary research question is whether participating in T-CT reduces depressive relapses within an observation period of 18 months compared with usual care alone. T-CT comprises eight therapy sessions delivered over the telephone by a trained therapist over a period of approximately six months after acute-phase therapy. Focus of the structured intervention is to train and foster relapse prevention strategies and to facilitate the transfer of skills acquired during acute-phase therapy to daily life. The effectiveness of T-CT as add-on to usual care is tested in a two-parallel group, multicenter, evaluator-blind clinical trial in patients with chronic/persistent or recurrent depressive disorder. Upon acute-phase therapy termination patients who have responded to cognitive behavioral therapy are randomized either to T-CT or usual care alone. Primary outcome of this study is relapse of a depressive episode. Relapse is determined by investigators blind to the study conditions based on clinical interviews conducted at months 6, 12, and 18 of follow-up. Further secondary outcome criteria are assessed with interviews and self-report questionnaires at various time points during follow-up. Overall, the study lasts approximately 48 months.

Inclusion Criteria:

• Recurrent major depressive disorder or chronic/persistent depressive disorder based on the criteria of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), or first major depressive episode with an elevated risk for relapse (defined as the presence of at least of the following clinical characteristics: index episode duration ≥ six months; severity of the index episode at least moderate; DSM-5 comorbidity; residual symptoms at the end of index treatment)
• Having regularly terminated acute-phase CBT for depression (index treatment)
• Having achieved therapeutic response during index therapy defined as at least 25%-improvement in depressive symptoms between start and end of acute-phase therapy based on a standardized symptom measure (e.g. PHQ-9, or Beck Depression Inventory; BDI)
• Having experienced partial or full remission at the end of the index treatment based on DSM-5 criteria for major depressive disorder
• Sufficient command of German language
• Having given written informed consent

Exclusion Criteria:

• Unstable psychopharmacological medication regimen (either with or without antidepressant (AD) medication) at the end of the index treatment, i.e. change in type or dosage of medication envisaged at the end of index treatment
• Acute risk for suicide based on clinical practice guidelines; patients with self-reported suicidal ideation are eligible as long as the treatment is deemed safe by the clinician's judgment
• A history of or acute psychotic symptoms, bipolar disorder, or organic brain disorder
• Severe cognitive impairment based on clinical evaluation during index treatment