Two cross-sectional studies were designed to assess the prevalence of HIV and risk behavior among IDUs in Estonia. Detailed descriptions of the studies are provided elsewhere.13,14
In both the studies, current IDUs were recruited for an interviewer-administered risk behavior survey covering demographics, drug use history, and HIV risk behavior, and biological sample collection for HIV testing.
was conducted in 2004 and recruited a convenience sample of 162 IDUs from two syringe exchange projects in Tallinn. Eligibility criteria included reporting injecting drugs within past 90 days, and age 18 years or older. A questionnaire was administered by a trained interviewer, with questions on injecting and sexual behavior risk behaviors within last 90 days. Venous blood was collected from participants and tested for the presence of HIV antibodies using commercially available test kits (HIV-1/HIV-2 III Plus from Abbott Laboratories (Abbott Park, IL, USA) at the State HIV/AIDS reference laboratory.
was conducted in Tallinn in 2005, and recruited 350 IDUs using respondent driven sampling (RDS).15,16
Eligible participants reported injecting drugs within past 28 days and were aged 18 or older. A structured questionnaire was administered by a trained interviewer, with questions on injecting and sexual behavior risk behaviors with in the last 28 days. Dried blood spot specimens were collected and tested for HIV antibodies using GACELISA, reactive specimens were confirmed using anti-HIV GACPAT immunoassay, with confirmatory testing conducted on discordant results using the HIV Blot 2.2 Western Blot assay (AbbotMurex). The testing was undertaken at the UK Health Protection Agency. Of note, assays used in both studies for detecting antbodies to HIV had high sensitivity and specificity.17–19
The wording in questionnaires was exactly the same for key variables (gender, ethnicity, age at IDU initiation) in both studies, though the time frame of measurement for recent drug use and risk behavior differed (last 90 days in Study 1 and last 28 days in Study 2). Subtraction of age at first injection from current age gave a measure of the number of years injecting.
We defined ‘new injectors’ in each survey wave as persons who reported their first injection as occurring within 3 years of the study interview. For calculating time since first injection, we assigned persons who had first injected at their current age to have been injecting for 6 months, persons who first injected in the previous year to have been injecting for 1 year, persons who had first injected 2 years prior to their current age to have been injecting for 2 years, and persons who had first injected 3 years prior to their current age to have been injecting for 3 years.
We then estimated HIV incidence among new injectors using the following assumptions: (i) all of them were HIV seronegative when they began injecting; (ii) the HIV seropositives became infected at the midpoint between beginning to inject and the time of blood sample collection and (iii) no HIV seropositives are lost to AIDS or other causes among the new injectors.
The time at risk for HIV seronegative new injectors is the total time from first injection to the time of the interview. The estimated HIV incidence rate was the number of HIV seropositive new injectors divided by the sum of the time at risk for the HIV seropositive new injectors (one-half total time from beginning to inject to time of interview) and the time at risk for the HIV seronegative new injectors (total time from beginning to inject to time of interview).20
Risk behaviors and characteristics were compared between the two groups of IDU (new and long-term injectors). Pearson's χ2
-tests were used for categorical variables and t
-tests with equal variance for continuous variables. RDS analysis Tool v.
was used to weight the sample to control for differences in network size and homophily (the principle that contact between similar people occurs at a higher rate than among dissimilar people22
) to provide population-based estimates of the characteristics of IDU (Study 2 only).
In order to facilitate comparisons with Study 1, the Study 2 data presented are the observed values for the subjects.
Ethical approval was obtained from the Ethics Review Board of the University of Tartu (Studies 1 and 2), and from the Riverside Research Ethics Committee, UK (Study 2).