Background
The WTC Health Registry was established in 2002 to monitor the long-term physical and mental health of people exposed to the September 11, 2001 terrorist attack on the World Trade Center in New York City. It is the largest post-disaster registry in US history that prospectively follows a diverse population of exposed individuals. In 2003–04, the WTC Health Registry conducted a health survey with 71,434 enrollees. In 2006–07 and 2011–12, the Registry conducted two waves of follow-up health surveys among eligible wave 1 enrollees. All participants gave verbal informed consent to participate in the Registry. The US Centers for Disease Control and Prevention and the New York City Department of Health and Mental Hygiene institutional review boards approved the Registry protocol, including use of the data.
Study population
A detailed description of WTC Health Registry has been reported previously [23,30]. In brief, at the enrollment stage, individuals were identified through lists obtained from lower Manhattan area employers and government agencies (“list-identified”) or using media campaigns that encouraged WTC-exposed individuals to contact the Registry for eligibility screening (“self-identified”). In 2003–04 a total of 71,434 eligible individuals, also called Registry enrollees, completed the wave 1 interview by phone (95%) or in person (5%). With the exception of decedents and those who withdrew from the Registry since wave 1, the entire adult cohort of the Registry was invited to participate in the wave 2 and wave 3 adult surveys in 2006–07 and in 2011–12. In total, 68,959 enrollees and 67,670 enrollees were eligible for inclusion in wave 2 and 3 respectively. This analysis is limited to the 67,670 enrollees eligible for both wave 2 and 3 adult surveys. Figure 1 shows the number of enrollees eligible for wave 3 in relation to their participation in earlier surveys: 36,252 individuals completed all three waves of the Registry surveys (“three-wave participants”); 9,868 are in the “wave 3 drop-out” group (enrollees who completed waves 1 and 2 but not wave 3); 6,682 are in the “wave 3 drop-in” group (enrollees who completed wave 1, did not participate in wave 2, but completed wave 3); and 14,868 individuals participated in the wave 1 survey only.
Follow-up survey methods
Data were obtained from enrollees at both wave 2 and wave 3 using targeted outreach and three data collection modes in the following temporal order: web, a mailed paper survey, and telephone. Wave 2 data collection methods were described elsewhere [23]. For wave 3, starting in June 2011, 36,356 enrollees with valid e-mail addresses received an electronic invitation to access the web survey. After 9 subsequent email reminders, all non-responders to the e-mail invitation were mailed paper questionnaires; the web survey remained available for them. Four additional email reminders, 2 subsequent rounds of mailed paper surveys, and 3 postcard reminders were sent to enrollees who had not responded to the initial web survey invitation. Paper questionnaires were mailed to enrollees who did not have an email address on file beginning in July 2011. Two additional rounds of paper questionnaires and 3 postcard reminders were mailed to non-respondents in this group.
Beginning in September 2011, a six-month effort was made to contact web and paper non-respondents by telephone and to administer the survey by Computer Assisted Telephone Interviewing (CATI) to those successfully reached. Because of the much higher cost of CATI data acquisition relative to web or mail, CATI was directed at selected subgroups of enrollees. During the first five months we focused on enrollees who had participated in both wave 1 and wave 2, on the assumption that these individuals would be more likely to participate in wave 3 than wave 2 drop-outs. This strategy also addressed the desirability of having health assessments at three points in time for analytical studies of the time course of outcomes such as PTSD. During the final month, CATI efforts were expanded to include wave 2 drop-outs. It should be emphasized that throughout the 10-month data gathering period efforts were continuously made to provide every enrollee with an opportunity to respond by web or mail. A total of 6,381 surveys, or 15% of the total, were completed by phone, 19,170 (45%) by web and 17,383 (40%) by mail (total N = 42,934, response rate = 63%).
Study variables
Population characteristics
The wave 1 survey provided data on demographic characteristics (sex, age, race, household income in 2002, and education), eligibility group, and recruitment source (list-identified vs. self-identified). Eligibility group is a mutually exclusive and hierarchical variable with rescue and recovery workers first, followed by lower Manhattan residents, and lower Manhattan area workers and passersby on September 11 [30].
Disaster exposures
Disaster exposure data were obtained in the wave 1 survey. Three measures of exposure are used: witnessing three or more traumatic or horrific events (seeing an airplane hitting the WTC towers, a building collapsing, people running away from a cloud of smoke, anyone injured or killed, or people falling or jumping from WTC towers), sustaining one or more of five listed injury types (cut, sprain or strain, burn, fracture or dislocation, or head injury) due to the WTC attack, and being caught in the dust or debris cloud on 9/11. These three measures were asked of all Registry enrollees.
Physical and mental health status
Enrollees reported their pre- and post-disaster health at wave 1. We chose three wave 1 health indicators to examine whether the baseline health status was associated with follow-up survey participation: 1) new or worsening respiratory symptoms since 9/11, defined as having developed or having worsened persistent cough, shortness of breath, wheezing, sinus problems, or throat irritation since 9/11; 2) probable PTSD, which was assessed as in prior published Registry analyses with a cut-off score of 44 or greater on the PTSD Checklist, a September 11 specific PTSD checklist [31,32]; and 3) a self-reported post-9/11 physician diagnosis of at least one of the following chronic conditions: angina, asthma, hypertension, coronary heart disease, heart attack or other heart problems, stroke, emphysema, diabetes, and cancer, nearly all of which have been reported to be elevated among 9/11 exposed individuals [23,33-36]. The experience of any of this group of illnesses was chosen as the indicator because of the increasing public health importance of chronic diseases.
For assessment of bias in the prevalence of health outcomes measured in wave 2 or wave 3, and bias in the association between exposure and health outcomes, we selected the following health indicators: 1) self-assessed general health, classified as having poor or fair health versus excellent, very good, or good health; 2) recurrent lower respiratory symptoms (LRS), defined as in previous Registry publications as reporting shortness of breath, persistent cough, or wheezing for the first time at wave 1 and for 8 or more days in the last 30 days at the follow-up survey [27,28]; and 3) probable PTSD at wave 2 and wave 3, assessed by the same event-specific PTSD Checklist included in wave 1.
Data analysis
We compared demographic characteristics, study recruitment source, eligibility group, disaster exposure, and wave 1 health status of wave 3 participants (wave 3 drop-ins and three-wave participants) with non-participants (wave 3 drop-outs and those who participated in wave 1 only). The adjusted odds ratios (AOR) and 95% confidence intervals (CI) for the association between wave 3 participation and each of these variables were computed using logistic regression.
We studied associations between wave 3 drop-out status (wave 3 drop-outs versus three-wave participants) and wave 2 health outcomes, using logistic regression models that controlled for demographics, recruitment source, eligibility group, disaster exposure, and survey mode. We used the same methods to study the associations between wave 3 drop-in status (wave 3 drop-ins versus three-wave participants) and wave 3 health outcomes.
We focused on the associations of exposure with two health outcomes, namely probable PTSD and LRS. These outcomes were measured at wave 2 for wave 3 drop-outs or at wave 3 for wave 3 drop-ins. We analyzed the association between these outcomes and exposure using logistic regression models estimated separately for three-wave participants, wave 3 drop-ins, and wave 3 drop-outs. We also estimated pooled logistic regression models that included interaction terms of each exposure variable and either wave 3 drop-in or wave 3 drop-out status as compared to being a three-wave participant to test whether the relationship between disaster exposure and health outcomes differed by survey participation (not shown). In all of these models, we also adjusted for demographics, study recruitment source, eligibility group, and survey mode.
Analyses were conducted in SAS version 9.2 (SAS Institute Inc., Cary, North Carolina).