Posttraumatic stress disorder (PTSD) is one of the most common health conditions among U.S. service members, with an estimated prevalence between 5% and 20% among the 2.7 million who have deployed to Iraq and Afghanistan since 2001, depending primarily on level of combat exposure [1, 2]. PTSD has been associated with a nearly 200% increase in hospitalizations among active duty service members between 2006 and 2012, and is a leading diagnosis in Department of Veterans Affairs medical settings [3]. However, these statistics underestimate the impact of PTSD since many service members in need of treatment do not seek care [1].

The course of PTSD varies, but symptoms can become persistent and last years or even a lifetime [4–7]. Persistent PTSD has been associated with long-term neurobiological changes and comorbidities that can have profound effects on physical and mental health functioning [8–11]. These changes may affect the person’s psychological, behavioral, and physical health [12–14], decreasing one’s quality of life and manifesting in the form of increased rates of cardiovascular disease [12, 15], hypertension, obesity [12, 16], immune-mediated disorders [17], and mortality. In addition, PTSD often co-occurs with depression, anxiety, and substance abuse [1, 18], all of which have been linked to suicidal behaviors [19]. As a direct effect of PTSD or an indirect result of comorbid health effects, important social interactions involving intimate relationships, such as marriage, are also often negatively affected, leading to decreased support and further worsening of symptoms [20]. Ongoing PTSD has been associated with increased disability, decreased productivity, and decreased fitness, thereby limiting one’s ability to continue to serve in the military or function in other occupational settings [21]. The longer PTSD remains untreated, the greater the probability that this stress-related disorder will lead to a health complication [22]. Therefore, understanding the mechanisms related to the persistence of PTSD symptoms is critical to the life and well-being of service members, veterans, and their families.

Limited research has examined factors associated with persistent PTSD among U.S. veterans of the Vietnam conflict [23–25] and World War II [26], as well as British veterans of the recent Iraq conflict [27]. In these studies, combat severity was associated with higher occurrence of PTSD and persistence of symptoms [23–26]. Other notable factors associated with persistent PTSD included lack of social support [23, 26, 27], preexisting depression [24], and reporting of multiple physical symptoms [13, 27]. Other studies have found that PTSD symptoms may continue to increase over time [4]. Studies that are more comprehensive involving U.S. cohorts and a more comprehensive range of potential risk and protective factors are lacking. Understanding the broad interactions of various risk factors in the development of persistent PTSD among U.S. military personnel is essential to providing specialized multidisciplinary treatment for service members and veterans.

This study is the first, to our knowledge, to prospectively assess persistent PTSD symptoms among U.S. military personnel serving since the start of operations in Iraq and Afghanistan. The primary objective of this study was to estimate rates of persistent PTSD and identify factors associated with persistent symptoms among participants in the Millennium Cohort Study, a large cohort study of service members and veterans from all service branches and components [28, 29].

Launched in 2001, the Millennium Cohort Study was designed to examine the impact and long-term health effects associated with military service. Between 2001 and 2013, four separate groups of service members, referred to as panels, were enrolled. Participants were randomly selected from U.S. military personnel and asked to complete a comprehensive web-based or paper survey approximately every 3 years. A detailed description of the Millennium Cohort Study methodology has been published elsewhere [28, 29].

The present study included Millennium Cohort participants who enrolled in the first or second panel. Participants must have completed at least two questionnaires, the 2004–2006 and 2007–2008 surveys, approximately 3 years apart (N = 63,589). Data from the 2004–2006 survey cycle and 2007–2008 survey cycle are referred to as “baseline” and “first follow-up,” respectively, for the purposes of this paper. To be eligible for this analysis, participants must have screened positive for PTSD at baseline (N = 2992). After excluding those with missing data for the key outcome or predictor variables, the study population between baseline and the first follow-up included 2409 individuals (80.5% of the eligible sample). In addition, participants who screened positive for PTSD at first follow-up had complete data for the covariates of interest, and those who had follow-up data at the subsequent 2011–2013 survey cycle (N = 822) were assessed for persistent PTSD at the “second follow-up.”

Of the 2409 service member participants who met criteria for PTSD at baseline, the mean age was 32.2 years (SD = 10.2), 65% were male, and 71% were non-Hispanic white. 49% of the sample deployed between 2001 and the first follow-up visit; of those, 90% reported experiencing combat.

Of the 1792 participants who screened positive for PTSD at baseline and had available data from the three survey cycles, 592 (33.0%) screened positive for PTSD on both follow-up questionnaires. Among those who screened positive for PTSD at the first follow-up and had available data at the second follow-up (N = 822), 585 (71.2%) continued to screen positive at the second follow-up visit. In addition to many of the aforementioned factors associated with persistent PTSD, separation from the military, sleeping less than 4 h per night, and lack of social support were associated with persistence of PTSD at the second follow-up (Table 2).

PTSD is one of the most common disabling disorders among U.S. service members and veterans, and it often becomes a chronic condition. In this study, we found that 47% of those who screened positive for PTSD at baseline were still positive for PTSD at follow-up approximately 3 years later; of those, over 70% remained positive for PTSD at the second follow-up approximately 6 years later.

Results from this study indicate that the intensity of combat exposure during the recent operations in Iraq and Afghanistan is a strong risk factors for persistent PTSD. This finding is consistent with other research that suggests combat exposure, compared with other types of trauma, confers a uniquely high risk of persistent PTSD, with strong linear associations related to combat intensity [2, 27, 41, 42]. Conversely, individuals who deployed without experiencing combat were more likely to screen negative for PTSD at follow-up compared with those who did not deploy. This protective association among non-combat deployers is likely due to the well-known healthy warrior effect, in which healthier service members are more likely to deploy than service members experiencing medical problems [43].

In addition to combat intensity, a number of other important risk factors for PTSD chronicity were identified, in particular indicators of psychological or physical health comorbidities. Not surprisingly, PTSD severity at baseline was strongly associated with persistence of symptoms. However, even after adjusting for PTSD severity and other variables, comorbid health conditions, which are common among those with PTSD, still emerged as important and independent risk factors of persistent PTSD. These findings indicate that those with comorbid conditions, such as depression or multiple physical symptoms, may be different in some ways than those with the single condition of PTSD. These comorbid conditions a may lead to increased dysregulation of neuroendocrine and autonomic nervous system or interoceptive functions. These comorbid conditions could also be linked to pain perception changes or physical symptoms acting as reminders of the traumatic event, either directly from injury that occurred as part of the trauma or indirectly through physiological mechanisms associated with bodily sensations [44]. For these and other reasons, it may be more difficult to effectively treat and care for individuals with PTSD who have other additional conditions.

One notable finding was that participants who reported sleeping less than 4 h per night at the first follow-up survey were more likely to screen positive for PTSD at the second follow-up, suggesting that sleep is an important predictor of long-term persistent PTSD. Sleep problems are common among service members who have deployed, independent of PTSD, and research has shown that those who develop disruptions in normal sleep patterns after a traumatic event are more likely to develop PTSD [45]. Additionally, sleep problems may result from the PTSD symptoms themselves, such as hypervigilance and recurrent nightmares. Sleep problems are also associated with comorbid conditions. Given these multiple interrelated health associations, treatment of persistent PTSD symptoms may include comprehensive approaches that address comorbid physical health problems, chronic pain, and sleep problems [44, 46].

An important aspect of PTSD treatment is having adequate social support. We found that being bothered a lot by not having someone to turn to was associated with persistent PTSD at the second follow-up. Research shows that social relationships can help with treatment compliance, coping with intrusive symptoms, and sharing feelings and thoughts that are experienced with PTSD. Social support both during and following deployment has been shown to be associated with lower PTSD severity [47]. Additionally, separation or retiring from the military was associated with persistent PTSD between the first and second follow-up visits, which may also relate in part to changes in social relationships after transitioning from the military. It is also likely that persistent PTSD itself led to medical discharge from the military for some study participants [48].

Participants born before 1980 were more likely to have persistent PTSD. Military studies have generally observed that the risk of PTSD is highest in younger age groups; however, when examining persistence or chronicity, studies have more often shown an association with older age [27, 49]. Studies among aging war veterans and Holocaust survivors have reported that PTSD severity increases in older trauma survivors [49].

We did not find an association between reported history of sexual trauma and persistent PTSD in adjusted models, though there was an association in unadjusted models. Sexual assault has been shown to be one of the most significant types of traumatic events in predicting PTSD and other mental health outcomes [50, 51]. In our analysis, the null association with sexual assault may be attributable to the small number of sexual assault cases or high correlation with other statistically significant variables such as physical assault. It is also possible that the measure of sexual assault was less than optimal and thus could have missed some cases of sexual assault or lead to misclassification. Further research to examine the role of sexual trauma in PTSD persistence is needed.

Understanding correlates of PTSD chronicity is critical to the long-term health and well-being of service members and veterans. Almost half of participants who screened positive for PTSD at baseline had persistent PTSD at the first-follow-up; among those, over two thirds had persistent PTSD at the second follow-up. Given the large number of factors associated with PTSD persistence, our study highlights the importance of holistic collaborative care models that address complex interrelated comorbid health conditions, including depression, generalized physical symptoms, and sleep problems [32].