On April 16, 2014, the Sewol ferry sank off the southwest coast of South Korea. Only 181 of the 476 passengers survived. Among the survivors was a group of students from the same high school who had been on a school trip and were known at the time of the accident as the ‘Danwon High School survivors.’ The group consisted of only 75 students, as 250 Danwon High School students were missing or dead.

Individuals in post-disaster settings may experience disaster-related psychiatric disorders, exacerbation of pre-existing psychopathological conditions, and/or psychological distress [1]. A high proportion of New York City public school children had probable mental disorders, including agoraphobia (14.8%), separation anxiety (12.3%), and posttraumatic stress disorder (10.6%), 6 months after September 11, 2001 [2]. Compared with untraumatized individuals, higher psychopathological symptom scores and suicide rates have been observed in adolescent survivors of earthquakes even 6 years after the event, and long-term psychological support may be required [3].

A study using Twitter data to explore how the public mood in Korea changed following the Sewol ferry disaster demonstrated that the disaster generated a significant short-term negative response, which gradually declined. However, public anger was easily provoked by various events that occurred in the aftermath of the tragedy [4]. This underlined the importance of monitoring mood changes in the general population following a disaster.

However, very little is known about the long-term mental health of the Danwon High School students who survived the Sewol ferry disaster. Analysis of follow-up data obtained from these individuals to understand how relevant psychological variables changed is important for the development of more accurate predictions of the mental health outcomes of adolescents who survive tragedies. The students who survived the Sewol ferry disaster experienced not only life-threatening trauma but also loss of close friends. Additionally, failures in the national disaster response process left them feeling let down and resentful, and the intrusive broadcasting that followed the tragedy increased their antipathy and mistrust. Moreover, distorted news reports led to cyberbullying and malicious comments, which left the students feeling hurt and confused [5].

The aim of this study was to examine relevant changes in psychological parameters, including anxiety, depression, posttraumatic stress symptoms (PTSS), and complicated grief. To this end, we performed retrospective chart and prospective follow-up reviews of the Danwon High School survivors from 9 months after the disaster until the first summer vacation after they graduated from high school, which was 27 months after the tragedy.

The retrospective chart review yielded complete responses from 68, 66, and 40 students at T1, T2, and T3, respectively (Table 1). The prospective follow-up review at T4 yielded data from 44 students (Table 1). Only data from the 32 subjects who completed all four assessments were analyzed to examine changes in the relevant psychological variables.

Changes in the number of subjects with clinically relevant symptoms over time are shown in Table 2. STAIC-State scores indicated that 15.6-37.5% of subjects had clinically relevant symptoms and that the scores peaked at T2. Additionally, 18.8-34.3% of subjects had clinically relevant symptoms according to their CES-D scores. In terms of the CROPS assessment, 21.9-31.3% of subjects had scores that above the cutoff point. The ICG indicated that 12.5-28.1% of subjects had clinically relevant symptoms and that the scores peaked at T2.

All the psychological variables followed similar trends during the study period (Figure 1). Scores tended to increase until T2 (12 months) and then slowly decreased until the follow-up assessment at T4 (27 months). Analyses using one-way repeated-measures ANOVA revealed that the severity of anxiety and complicated grief symptoms, measured using the STAIC-State and ICG assessment tools, changed significantly over time. However, the severity of depression and PTSS, measured using the CES-D and CROPS assessment tools, did not change significantly during the study period. The mean STAIC-State score increased from 35.43±6.9 at T1 to 37.81±7.3 at T2; it then decreased to 33.31±7.8 at T4 [F (3, 93)=5.35, p<0.01]. The mean ICG score increased from 15.87±11.9 at T1 to 17.93±13.0 at T2; it then decreased to 11.12±10.0 at T4 [F (3, 93)=4.91, p<0.01].

We described changes in the psychological profiles of a sample of students who survived the Sewol ferry disaster. The Danwon High School survivors showed aggravated symptoms of anxiety and complicated grief at the first anniversary of the tragedy but significant improvements in these symptoms 27 months after the disaster, following their graduation from high school. However, their symptoms of depression and PTSS did not change significantly throughout the study period.

Several empirical and longitudinal studies have reported that an anniversary of a traumatic event can trigger an increase in distressing memories of this event. Morgan et al. [14] examined anniversary responses to Operation Desert Storm and Operation Desert Shield in veterans 2 and 6 years after the end of each conflict. When the veterans’ worst months were compared with the dates of the traumatic events that occurred during the Operations, we found that 38% of veterans had reported that their worst month coincided with the month in which their trauma had actually occurred. Our study noted that Danwon High School erected a makeshift memorial wall of flowers with photographs of the dead and missing students on the anniversary of the disaster. Many of the surviving students spent considerable time at the memorial, and this could have had an effect on their response to the anniversary of the tragedy [15].

Our findings are broadly consistent with those of previous long-term follow-up studies of children who witnessed a fatal incident at school. As time passed, mean anxiety symptom scores decreased, but symptoms of depression remained largely unchanged [16]. In our study, the school itself might have triggered traumatic memories, and graduation might have reduced students’ symptoms of anxiety and complicated grief. The surviving Danwon students also had to cope with distorted news reports, malicious comments, and cyberbullying, some of which resulted from the government’s decision to give them special status for admission to college on the assumption that they were unable to study following the deaths of nearly 300 of their schoolmates. The students might have felt freed of this stigmatization after they graduated from school and went to university or became integrated into society.

In contrast, we found that the severity of PTSS remained relatively unchanged throughout the study period. This differs from the results of many studies that found a general decline in the prevalence of post-traumatic stress disorder (PTSD) over time. For example, data from survivors of the 1991 mass shooting in Killeen, Texas suggest that the prevalence of PTSD decreased from 27.2% at 6-8 weeks to 17.7% at 13-14 months after the episode [17]. The prevalence of PTSD in tsunami victims was 57.3% at 6 weeks, but it declined sharply 2 years after the event [18]. Other studies reported increases in PTSD prevalence over time. For example, the prevalence of PTSD in a cohort of Dade County, Florida residents exposed to Hurricane Andrew in 1992 increased from 26% to 29% between 6 and 30 months following the disaster. Clearly, the course of PTSD needs to be assessed over a significant period of time to accurately assess the prognoses of survivors of traumatic events [19].

In our study, the prevalence of PTSD was 25.0% at 27 months after the disaster. The development of PTSD in juveniles following a tragedy can vary significantly, and its prevalence can range from 0% to 95%. The prevalence of PTSD observed in this study was comparable to that following man-made disasters (20.3-29%) [20] and higher than that reported in studies on flood survivors (15.74%) [21]. A number of factors could account for differences in the observed prevalence of PTSD in different studies. These include differences in the type of trauma, the degree of exposure to the trauma, the characteristics of the traumatized population, the specific measurements used to define PTSD, and the time between the traumatic event and the assessment of PTSD and anxiety.

This 27-month follow-up study found that the prevalence of depression among subjects was 18.8%. This is higher than the prevalence of depression in an indirectly affected sample from Ansan, where most of the students on the ferry had lived [22]. However, this indirectly affected population had a higher prevalence of psychiatric problems compared with control communities. The rates of depression, moderate-to-severe stress, somatic symptoms, anxiety, and suicidal ideation in the Ansan population sample were 11.4, 8.8, 11.9, 5.9, and 17.7%, respectively.

This study had several limitations. First, some sampling bias may have occurred. Although 59-91% of the sample population participated in the study between T1 and T4, only 43% of the surviving students completed all four assessments and provided data that were used in the statistical analyses. Therefore, the results should be interpreted with caution. Second, the prevalence of PTSD was measured using a self-report assessment tool rather than a structured clinical interview. Selfreporting does not always provide an accurate representation of PTSD symptomatology. However, the CROPS assessment tool used in this study was well validated for Korean children and adolescents. Finally, we did not include comparison groups and could not compare the study subjects with a control group.

In conclusion, despite these limitations, our study provides significant insight, as we demonstrated that the Danwon High School survivors clearly responded to the first anniversary of the Sewol ferry disaster. Additionally, we examined longitudinal changes in key psychological variables and found that the severity of anxiety and complicated grief symptoms decreased significantly but that symptoms of depression and PTSS persisted even after graduation from high school. These results have unique implications in terms of the need for regular screening to improve the mental health of disaster survivors. They also underscore the importance of providing longterm follow-up services for mental health problems in general and for PTSD in particular. A combination of quantitative and qualitative data could provide further insight into why some psychological variables changed more than others in survivors of the Sewol ferry disaster.