Vortioxetine, 5, 10, and 20 mg, Relapse Prevention Study in Adults With Major Depressive Disorder (MDD)

The drug being tested in this study is called vortioxetine. Vortioxetine is being tested for the prevention of relapse in adults with major depressive disorder (MDD) who respond to daily treatment with vortioxetine. This study will look at relapse rates of MDD in people who take vortioxetine. The study will enroll approximately 1100 participants. All participants will receive vortioxetine 10 mg open-label for the first 16 weeks of the study. Participants who meet the appropriate MDD response criteria from the Week 8 Visit through Week 16 Visit will be eligible for randomization into the double-blind treatment period. Participants will be randomly assigned (by chance, like flipping a coin) to one of the four treatment groups-which will remain undisclosed to the patient and study doctor during the study (unless there is an urgent medical need): * Vortioxetine 5 mg * Vortioxetine 10 mg * Vortioxetine 20 mg * Placebo (dummy inactive pill) - this is a capsule that looks like the study drug but has no active ingredient All participants will be asked to take one capsule at the same time each day throughout the study. This multi-center trial will be conducted in the United States. The overall time to participate in this study is up to 55 weeks. Participants will make 19 visits to the clinic, and will be contacted by telephone 4 weeks after last dose of study drug for a follow-up assessment.