Prospective analysis of depression and deployment in support of the wars in Iraq and Afghanistan

Background: Previous studies have reported an association between deployment and depression; however, these studies have been limited by small sample size or lack of longitudinal design.

Objective: To prospectively investigate the association between deployment and new-onset depression among US service members.

Methods: Designed to investigate the long-term effects associated with military service, the Millennium Cohort is the largest prospective study of military personnel. Of 77,047 members who completed a baseline questionnaire between 2001 and 2003, 55,021 (71.4%) submitted a follow-up questionnaire between 2004 and 2006. This study included 40,226 Millennium Cohort members who completed both questionnaires and met study criteria.

Main Outcomes: Using nine items from the Primary Care Evaluation of Mental Disorders Patient Health Questionnaire (PHQ-9), participants were identified with new-onset depression if they met the PHQ-9 criteria at follow-up but were symptom-free at baseline.

Results: Deployed men and women with combat exposures had the highest onset of depression at follow-up (5.6% and 15.7%, respectively), followed by those not deployed (3.9% and 7.7%, respectively), and those deployed without combat exposures (2.3% and 5.1%, respectively). After adjusting for demographic, behavioral, and military characteristics, combat-deployed men and women were at increased odds for new-onset depression compared with nondeployed men and women. Conversely, deployed men and women without combat exposures were at decreased odds for new-onset depression.

Conclusions: To our knowledge, this is the first large, longitudinal study to examine the relationship between deployment and depression. Deployment with combat exposures is a significant risk factor for new-onset depression among US service members.