Abstract: We identified 3745 citations through electronic searches and reviews of reference lists after deduplication of search results. We excluded 3619 records during title and abstract review and assessed 126 articles at full-text review for eligibility. We included one controlled study enrolling 46 participants. We rated this RCT at high risk for performance and detection bias due to a lack of blinding. The included RCT compared preventive use of mindfulness-based cognitive therapy (MBCT) with treatment as usual (TAU) in participants with a history of SAD. MBCT was administered in spring in eight weekly individual 45- to 60-minute sessions. In the TAU group participants did not receive any preventive treatment but were invited to start light therapy as first depressive symptoms occurred. Both groups were assessed weekly for occurrence of a new depressive episode measured with the Inventory of Depressive Syptomatology-Self-Report (IDS-SR, range 0-90) from September 2011 to mid-April 2012. The incidence of a new depressive episode in the upcoming winter was similar in both groups. In the MBCT group 65% of 23 participants developed depression (IDS-SR ≥ 20), compared to 74% of 23 people in the TAU group (risk ratio (RR) 0.88, 95% confidence interval (CI) 0.60 to 1.30; 46 participants; very low quality-evidence). For participants with depressive episodes, severity of depression was comparable between groups. Participants in the MBCT group had a mean score of 26.5 (SD 7.0) on the IDS-SR, and TAU participants a mean score of 25.3 (SD 6.3) (mean difference (MD) 1.20, 95% CI -3.44 to 5.84; 32 participants; very low quality-evidence). The overall discontinuation rate was similar too, with 17% discontinuing in the MBCT group and 13% in the TAU group (RR 1.33, 95% CI 0.34 to 5.30; 46 participants; very low quality-evidence). Reasons for downgrading the quality of evidence included high risk of bias of the included study and imprecision. Investigators provided no information on adverse events. We could not find any studies that compared psychological therapy with other interventions of interest such as second-generation antidepressants, light therapy, melatonin or agomelatine. The evidence on psychological therapies to prevent the onset of a new depressive episode in people with a history of SAD is inconclusive. We identified only one study including 46 participants focusing on one type of psychological therapy. Methodological limitations and the small sample size preclude us from drawing a conclusion on benefits and harms of MBCT as a preventive intervention for SAD. Given that there is no comparative evidence for psychological therapy versus other preventive options, the decision for or against initiating preventive treatment of SAD and the treatment selected should be strongly based on patient preferences and other preventive interventions that are supported by evidence.

Plain-language summary: The proportion of participants developing a depression in the upcoming winter was similar in both groups as well as the severity of these depressive episodes. However, the quality of the evidence was very low, so we can draw no valid conclusion if MBCT is really ineffective in preventing SAD or not. The included study reported no information on side effects of the intervention. Doctors need to discuss with patients the advantages and disadvantages of MBCT and other potentially preventive treatments for winter depression, such as other psychological therapies, drug treatments, or lifestyle interventions. As no available studies have compared these treatments, treatment selection should be strongly based on patient preferences and other preventive interventions that are supported by evidence. Review authors recommend that future studies should evaluate the efficacy of different psychological therapies in preventing SAD in larger study samples and should directly compare these interventions versus other preventive treatments, such as light therapy, antidepressants and agomelatine, to determine the best treatment option for prevention of SAD.