To assess the efficacy of vortioxetine (10 to 20 mg/day) as adjunctive treatment to stable selective serotonin reuptake inhibitor (SSRI) dose versus stable SSRI monotherapy on cognitive performance (focusing on the aspect concerning speed of processing, executive functioning and attention) in patients who are in partial or full remission from their Major Depressive Episode (MDE).

Inclusion Criteria:

• The patient has achieved either partial (some symptoms of a MDE are present but full criteria are not met) or full remission of major depressive disorder (MDD), diagnosed according to DSM-IV-TR™.
• The patient has HAMD-17 total score ≤10.
• The patient has received SSRI monotherapy for the MDE from which the patient is currently in full or partial remission for ≥12 weeks at licensed doses and been on stable dose ≥8 weeks prior to Screening Visit.
• The patient has ≥50% response to current SSRI treatment (Antidepressant Treatment Response Questionnaire [ATRQ]).
• The patient has a PDQ-D total score >25.
• The patient is a man or woman, aged ≥18 and ≤65 years.

Exclusion Criteria:

• The patient has a score ≥70 on the DSST (numbers of correct symbols) at the Baseline Visit.
• The patient is, in the opinion of the investigator, not able to complete the neuropsychological tests validly at the Baseline Visit.
• The patient has physical, cognitive, or language impairment of such severity as to adversely affect the validity of the data derived from the neuropsychological tests.
• The patient is diagnosed with reading disability (dyslexia).
• The patient has a history of lack of response to previous adequate treatment with vortioxetine.
• The patient has any current psychiatric disorder or Axis I disorder (according to DSM-IV-TR™ criteria) other than MDD, as assessed using Mini International Neuropsychiatric Interview (MINI).
• The patient has a current or has had a diagnosis of dysthymic disorder within 3 months preceding the onset of the depressive episode from which the patient is currently in full or partial remission (DSM-IV-TR™ criteria).
• The patient has borderline, schizotypal, schizoid, paranoid, histrionic, antisocial personality disorders (axis II) as comorbid or primary diagnosis (DSM-IV-TR™ criteria).
• The patient suffers from personality disorders, mental retardation, pervasive development disorder, attention-deficit/hyperactivity disorder, organic mental disorders, or mental disorders due to a general medical condition (DSM-IV-TR™ criteria).
• The patient has a current diagnosis or history of manic or hypomanic episode, schizophrenia or any other psychotic disorder, including major depression with psychotic features (DSM-IV-TR™ criteria).

Other protocol-defined inclusion and exclusion criteria may apply.