Millennium Cohort; the Largest Population-Based US Military Cohort; 2001 to 2022

Military service confers unique occupational exposures, and sometimes intensive stressors, that may have profound impact on long-term health. Most studies of military-related exposures are limited by retrospective and cross-sectional design, convenience sampling, and/or short-term follow-up. The Millennium Cohort Study was established in 2000 by the US Department of Defense (DoD), in collaboration with the US Veterans Administration (VA), to follow the health of nearly 150,000 participants for up to 21 years. The Cohort consists of three separate panels, which are each surveyed at three year intervals from the start of enrollment through 2022. Enrollment of all three panels is nearly complete. Participants of the first two panels have completed a baseline questionnaire; >70% of Panel 1 participants have provided their first follow-up data. More than 40% of the Cohort has deployed in support of the wars in Iraq and Afghanistan. Extensive reliability and validity analyses have been completed and published. The Millennium Cohort Study is setting a new standard for prospective evaluation of the long-term health consequences of military occupational exposures, both among active military personnel as well as among the growing number of Reserve and National Guard (citizen soldiers) and Cohort members who have separated or retired from military service. The prospective design will allow complex issues of national public health importance to be addressed.