Surveillance provides valuable information for tracking the maturing HIV-1 epidemic, developing prevention programs to mitigate the epidemic's impact, and evaluating the success of HIV-1 prevention programs. Risk-based cohorts have provided excellent information on the risk of HIV-1 infection [1–6]. Population-based cohorts consisting of individuals at varying risk for HIV-1 infection also provide information on HIV-1 in the general population. The active duty component of the US Army is a particularly good cohort with which to monitor HIV-1 infection trends because it is a dynamic cohort into and from which young people are routinely added and removed. HIV-1 testing in the US Army provides unique insights into the dynamics of the epidemic in young, race/ethnically diverse, sexually active men and women throughout the US.
The methodology used in this study has been described in detail elsewhere [7–9]. The cohort consists of US Army personnel who were on active duty at any time between November 1985, the beginning of the HIV-1 testing program in the US Department of Defense, and December 1999.
Laboratory testing and control measures have been described in detail elsewhere [10,12]. Briefly, sera were tested with a commercial ELISA. Reactive samples were retested in duplicate, and those found to be repeatedly reactive were tested by Western blot analysis. A positive test was defined as reactive ELISA followed by diagnostic Western blot (at least two of the following bands: p24, gp41 and gp120 and/or gp160) with the same result from a second serum specimen. A negative test was defined as non-reactive ELISA or reactive ELISA followed by a non-diagnostic Western blot. Indeterminate Western blot results were confirmed using other non-diagnostic tests (e.g., PCR).
Endpoints were either a first documented positive HIV-1 antibody test or a last negative antibody test prior to 31 December 1999. The seroconversion date was defined as the midpoint between the dates of the last HIV-1 negative test and the first HIV-1 positive test. Person-years (PY) accrued from the first negative HIV-1 test date to the seroconversion date (for seroconverters) or up to the negative endpoint test date (for non-seroconverters). A positive endpoint was required to be preceded by a negative HIV-1-antibody test by at least 60 days in order to minimize misclassification. HIV-1 incidence rates were calculated as the number of seroconversions per 1000 PY of follow-up.
Trends in HIV-1 seroconversion were assessed with demographic variables available from personnel databases using EpiInfo  and JMP . These included age, race/ethnicity, gender, marital status, length of service, and rank. Three variables (race, gender, and rank/grade) were static over the period of study. The remaining variables were derived annually for each individual in the study. Unadjusted and adjusted relative risk estimates and 95% confidence intervals (CI) for the association between demographic variables and HIV-1 seroconversion were calculated using Poisson regression. Data were analyzed using SAS  and JMP .
Since the Department of Defense HIV-1 testing program began in October 1985, 2582 HIV-1 infections among active duty Army personnel have been detected. Of these, 1275 (49.4%) were HIV-1 seroconversions identified among 2 004 903 soldiers on active duty at any time between 1985 and 1999 who contributed 7 700 231 PY of follow-up. The remaining 1307 (50.6%) were not included in this analysis as a reference negative HIV-1 test date was unavailable for them. Annual incidence rates (IRs) by gender are presented in Fig. 1. Overall HIV-1 incidence declined from a high of 0.45/1000 PY in 1985/86 to 0.08/1000 PY in 1999. There was a statistically significant decline over the analysis period (S2 for linear trend, 189.7;P < 0.000). The IRs in men mirrored the overall IR as the majority (94%) of the cohort consisted of men. Among women HIV-1 IRs declined from a high of 0.26/1000 PY in 1985/86 to 0.05/1000 PY in 1999. Interestingly, the IR in women increased from 1992 to 1997 converging with the IR in men in 1996.
Table 1 presents the HIV-1 IR and risks associated with demographic characteristics. Seroconverters under the age of 25 years represented 45.3% of all seroconverters, reflecting the youth of the US Army. Older age was associated with a decrease in incidence among married personnel from 0.14 to 0.10/1000 PY (< 20 years old versus > 30 years old). However, among unmarried personnel, older age (< 20 years old versus > 30 years old) was associated with an increase in incidence, from 0.20 to 0.27/1000 PY. Neither of these trends was statistically significant.
A disproportionately large number of seroconversions was seen among Black active duty members. As a result HIV-1 incidence rates were greater among Black soldiers than among White soldiers. Bivariate analysis of race–ethnicity with all other demographic covariates revealed that Black personnel uniformly had higher rates.
The overall incidence of HIV-1 infection was greater for men than for women. Among men, the HIV-1 IR declined over time, leveling at about 0.1/1000 PY in 1992. Among women, HIV-1 IRs declined until 1992 after which a non-significant increase was seen. HIV-1 incidence peaked among women aged < 20 years (0.18/1000 PY), and among 20–24-year-old men (0.22/1000 PY).
Fig. 2 presents HIV-1 IRs by gender, race/ethnicity and age group. Although the peak incidence for women occurred in those < 20 years old, particularly among black women, the peak IR for men occurred in those aged 20–24 years.
A majority (61.2%) of seroconverters were unmarried, and for both men and women, unmarried personnel had an approximately twofold higher IR than married personnel. With increasing age, HIV-1 IRs among unmarried and married women declined and converged. Among males, however, the IR declined only for married men (from 0.16 to 0.11/1000 PY), while becoming larger among unmarried men with increasing age (from 0.21 to 0.33/1000 PY). An overall progression in incidence rates by gender and marital status was observed. Female married personnel had an IR of 0.05/1000 PY, female single personnel an IR of 0.10/1000 PY, followed by male married personnel with an IR of 0.12/1000 PY, and finally unmarried men with an IR of 0.26/1000 PY.
The majority (61.3%) of seroconversions occurred among personnel with 3 or more years of service. HIV-1 incidence decreased with longer length of service for Black and White personnel, but increased slightly among Hispanic personnel.
The majority (94.7%) of seroconversions occurred among enlisted personnel. Age and race/ethnicity influenced seroconversion risk differentially for enlisted personnel versus officers. HIV-1 incidence was stable and low among White enlisted personnel and officers across the four age strata. However, among Black officers HIV-1 incidence increased with older age, but decreased with older age among Black enlisted personnel.
HIV-1 seroconversion risk
Black soldiers were at highest risk as were male and unmarried personnel in both the unadjusted and adjusted models. Upon adjustment some changes were seen in each variable. However, adjusted age variables changed markedly, particularly in older ages. This change is reasonable given that older soldiers are more likely to be officers, married, and to have served more than 3 years in the Army, all factors associated with lower HIV-1 risk.
Military populations pose a unique challenge to epidemiologic research on HIV-1 infection due to the sensitive nature of behaviors known to increase risk for HIV-1 infection. As a result, few controlled studies have been performed in military populations. Renzullo et al.  published the first prevalent case–control study of HIV-1 in the Army. After assurances of anonymity and confidentiality, elevated risks were found to be associated with same-sex behavior, sexual activity resulting in contact with blood, sexual contact with prostitutes, and sexual contact with injecting drug users (IDU). In what is, to our knowledge, the only controlled study of risk factors for incident HIV-1 infection in the military, Levin et al.  found significant risk associated with having had six or more lifetime sex partners, engaging in sex with partners on the first day of meeting, and sex with three or more casual partners.
More recent research has confirmed the high prevalence of sexually transmitted infections (STI) among Army personnel. The lifetime prevalence of sexually transmitted infections was reported to be 19.9% in Army personnel . Women reported a higher lifetime STI prevalence than men, 29.5% versus 18.3%. Among sexually active unmarried personnel, 44.3% reported condom use during their last sexual encounter. Various factors could help explain the higher STI prevalence among military personnel including first time away from home, youth, unmarried status, and the risk-taking atmosphere.
In addition to sexual risk behaviors, injecting drug use contributes to HIV-1 transmission. In the military, pre-accession drug screens serve to eliminate a large proportion of individuals who inject drugs, and in addition, random drug screens during service may keep the occurrence of illicit drug use low in military populations. Not surprisingly, lower drug use in the US military compared with the civilian population has been reported . Overall, only 4.9% of all US Army personnel had used any illicit drug (other than marijuana) in the past 12 months. Correlates of any drug use included male gender, younger age, lower education level, unmarried status, and lower rank/grade.
Two strengths in the current study are noteworthy. This cohort involved over 2 million persons with almost 8 million PY of follow-up over 14 years of the HIV-1 epidemic. In addition, excellent demographic information was available for these individuals. Limitations included the unavailability of behavioral and biologic data, such as STI occurrence. The determination of the seroconversion date based upon the midpoint of the last negative and first positive HIV-1 test date is somewhat arbitrary and may have introduced some bias particularly where the seroconversion window length was long. An additional limitation is the probable underestimation of the most recent annual HIV-1 incidence rate. For an individual to be considered a seroconverter in the last year of analysis, both the last negative and first positive HIV-1 test would need to have occurred close to or in the final year.
Given the large number of soldiers in the US Army, the need for continued force readiness, and the potential for rapid deployment around the world, continued assessment of HIV-1 infection among US Army personnel is vital. The surveillance of incident infections allows for careful monitoring of HIV-1 infection trends by demographic groups, including the impact of prevention programs for decreasing HIV-1 transmission. The analysis of HIV-1 seroconversion rates among US Army personnel assists in targeting prevention efforts not only for military populations, but also for civilians. Careful examination of the rate of new infections allows for responsible monitoring of the epidemic and the projection of future resource needs.
The authors thank M. V. Rubertone and V. Desborough of the US Army Medical Surveillance Activity, US Army Center for Health Promotion and Preventive Medicine for their assistance with data collection.
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