Table compares the baseline characteristics of the two cohorts. On average, the NY cohort was five years older than the PR cohort (mean ages 38.6 vs. 33.2, p < 0.001). The NY cohort had a higher proportion of females than the PR cohort (22.0 vs. 14.1%, p = 0.003), and NY participants were more likely to be married than PR participants (26.2 vs. 18.5%, p = 0.008). History of victimization, sexual abuse, and social isolation were all more likely to be reported by the NY cohort than by the PR cohort (victimization: 81.2 vs. 70.8%, p < 0.001; sexual abuse: 13.0 vs. 7.2%, p = 0.007; social isolation: 30.6 vs. 21.3%, p = 0.002). Compared to the NY cohort, the PR cohort was more likely to have injected drugs during the previous 30 days (87.0 vs. 96.9%, p < 0.001) and among those currently injecting, to inject much more frequently (mean number of injections in the last 30 days 77.5 vs. 182.0, p < 0.001). NY participants were more likely than PR participants to have used alcohol to intoxication (6.4 vs. 2.2%, p = 0.005) and to have smoked crack (50.7 vs. 33.9%, p < 0.001) but were less likely to have injected speedballs (49.3 vs. 87.8%, p < 0.001). NY participants were also more likely to report prior episodes of drug overdoses than PR participants (41.4 vs. 29.5%, p < 0.001). NY participants were more likely than their counterparts in PR to have received syringes from a needle exchange program (46.6 vs. 33.9%, p < 0.001), to be in drug treatment (56.8 vs. 9.7%, p < 0.001), and to be covered by health insurance (67.2 vs. 38.6%, p < 0.001). HIV seroprevalence was slightly higher in the NY cohort than in the PR cohort (28.9 vs. 22.9%, p = 0.048), and among HIV-positives, NY participants were more likely to be taking antiretroviral therapy (37.0 vs. 9.6%, p < 0.001).
Cohort profiles as measured upon recruitment
During the course of the study the NY cohort recorded 2,615 person years and 33 deaths, an annual mortality rate of 1.3 per 100 person years (95% CI 0.8–1.7). The PR cohort recorded 1,196 person years and 56 deaths, an annual mortality rate of 4.8 (95% CI 3.5–6.0). The risk of mortality of the PR cohort was 312 times higher than that of the NY cohort. The mortality rates were also estimated after stratifying the cohorts by HIV serostatus at recruitment. The estimates are shown in Table . In both cohorts the mortality rates of HIV-positives were about five times the mortality rates of HIV-negatives (NY: 3.2 vs. 0.5; PR: 12.7 vs. 2.9). Relative risks of mortality of the PR cohort compared to the NY cohort did not differ noticeably between HIV-negatives and HIV-positives (incidence rate ratios of 5.4 and 4.0, respectively).
Mortality rates stratified by HIV-serostatus at time of recruitment
Figure shows the plots of the annual cumulative survival probabilities of the two cohorts. The cumulative survival probability of the PR cohort descended at a much faster rate than that of the NY cohort, and the difference was statistically significant (p < 0.001).
Cumulative survival probability of each study cohort. Kaplan Meier Log Rank test, p < 0.001
The mortality rates of the NY and PR cohorts were compared to those of their respective general populations by calculating standardized mortality ratios (SMRs, data not shown). The mortality rate of the general population of Puerto Rico was about 35% higher than that of the general population of Hispanics in New York City (321 vs. 240 per 100,000 population). The mortality rate of the NY cohort was four times higher than the rate expected based on the mortality experience of the Hispanic population of New York City (SMR = 4.4, 95% CI 3.5–5.4). The SMR of the PR cohort was 312 times higher, 16.2 (95% CI 15.7–16.6). In both cohorts, the SMRs of the females were several times higher than that of the males (NY SMRs: females = 10.6, males = 3.6; PR SMRs: females = 85.7, males = 12.9).
Causes of deaths are shown in Table . The causes of death of the two cohorts differed significantly (p = 0.016). The four principal causes of death of the NY cohort were HIV/AIDS (50.0%), drug overdoses (13.3%), cardiovascular conditions (13.3%), and pulmonary conditions (10.0%). The four principal causes of the PR cohort were HIV/AIDS (37.0%), drug overdoses (24.1%), sepsis (13.0%), and homicide (11.1%). Table also compares the causes of death across gender groups. The distributions were not statistically significant (p = 0.762), but there were some notable differences. Compared to males, a greater proportion of females died due to pulmonary conditions (4.7 vs. 15.0%), and none died of sepsis (6.3 vs. 0.0%).
Causes of death by study site and gender
Table shows the results from modeling time to death with Cox proportional hazards regression analysis. The table shows the hazards ratios estimated after entering each covariate separately (without adjusting for the other variables) and the hazards ratios estimated after adjusting for all other covariates. The relative risk of mortality of the PR cohort as compared to the NY cohort increased from 3.7 prior to entering other covariates in the model to 9.2 after adjusting for other covariates. The other covariates found to be significantly associated with time to death were age (35-44 years old—HR = 5.1, p = 0.028; 45 years or older—HR = 6.6, p = 0.014), female gender (HR = 1.7, p = 0.048), education, (HR=0.6, p = 0.049), social isolation (HR = 2.4, p < 0.001), use of alcohol to intoxication (HR = 1.8, p = 0.041), and HIV seropositivity at recruitment (HIV-positive not receiving antiretrovirals—HR = 2.7, p < 0.001; HIV-positive receiving antiretrovirals—HR=5.6, p < 0.001). Interaction effects between study site and all the other covariates were tested, and none were found to have a statistically significant effect over the risk of mortality (tests not shown).
Results of regressing time to death with Cox’s proportional hazards regression