Setting and study population
This study began in 1989, at the same time as a specific programme for heterosexual HIV serodiscordant couples was launched in a clinic for HIV and sexually transmitted infections in Madrid, Spain. Each patient with a diagnosis of HIV infection was advised that his or her sexual partner should visit the clinic. The programme includes comprehensive medical consultation for both members of the couple, with check ups scheduled every six months. At each visit, the index partner undergoes clinical follow-up, the partner is recommended to undergo an HIV test, genitourinary infections (including sexually transmitted infections, bacterial vaginosis, and vaginal candidiasis) are ruled out, and women are offered an annual gynaecological examination. Access to free antiretroviral treatment is provided when the index partner meets current international guidelines.24
Couples are systematically advised against having unprotected sex. The staff responsible for this programme did not change throughout the study period.
Design and variables
With the informed consent of both partners, stable heterosexual couples treated in this programme were prospectively included in an observational study to quantify the risk of heterosexual HIV transmission according to sexual risk behaviours.
In a cross sectional analysis of HIV seroprevalence, we included couples recruited in 1989-2008, when the non-index partner came to the clinic for his or her first HIV test, who met the following criteria: ongoing sexual relationship during the past six months, the index partner had received a diagnosis of HIV with a well identified probable route of infection, and the non-index partner had no previous HIV diagnosis and no known risk exposure other than the heterosexual relationship with the index partner. HIV prevalence in non-index partners was evaluated according to treatment of the index partner.
We included all heterosexual couples who were serodiscordant for HIV who returned for at least one follow-up visit in a prospective cohort analysis of HIV seroconversion. Follow-up began on the date of the first negative HIV test result within the programme, and the end point was the first HIV-1 positive test result. For those participants who failed to return for check ups for more than 24 months, whose relationship ended, or whose non-index partner reported any risk exposure outside the relationship, data were censored at the last follow-up visit. For the remaining couples, follow-up was censored at the last check up before 31 December 2008.
We used a series of forms designed at the beginning of the study to collect epidemiological, clinical, and sexual behaviour information; these forms were subsequently modified to incorporate new laboratory tests and treatments. At all visits trained medical practitioners collected data. They asked non-index partners about the number of acts of sexual intercourse (vaginal and anal) during the previous six months (at the first visit) or since the previous visit (follow-up). For each type of practice, respondents reported the number of protected (with condom) and unprotected contacts using a semi-quantitative scale (never, less than half of the times, more than half of the times, and always); to estimate frequencies we assigned the coefficients 0, 0.33, 0.67, and 1, respectively. Vaginal or anal intercourse without a condom was considered to be a “sexual risk practice.” “Risky sexual exposures” included sexual risk practices and condom breakage or slippage during intercourse.
For each visit we categorised couples according to antiretroviral therapy of the index partner: not taking antiretroviral treatment, taking monotherapy/dual therapy, and taking combined therapy with at least three active drugs. Only treatments taken for at least three months were considered.
At baseline and every visit thereafter each partner provided a venous blood sample. Non-index partners were tested to determine serum antibodies to HIV-1/2, and reactive samples were confirmed by western blotting. Syphilis was routinely evaluated with a reaginic test and by Treponema pallidum passive particle agglutination assay or the fluorescent treponema antibody absorption test. Men were evaluated if they had symptoms of a sexually transmitted infection or if one had been diagnosed in their partner. Gynaecological examinations included screening for infections in cervical and vaginal exudates. CD4 count in the index partner was determined by flow cytometry and plasma HIV RNA by a branched DNA assay. The lower limit of detection was 500 copies per ml until 1999, and 50 copies per ml thereafter.
We estimated HIV seroprevalence at enrolment, the incidence rate of seroconversion by couple years of follow-up, and the probability of transmission per sexual risk exposure. We compared estimates for couples in which the index partner was taking combined antiretroviral therapy with those for couples in which the index partner was not receiving any antiretroviral treatment. In the follow-up analysis, we evaluated HIV seroconversions per couple years and per sexual risk exposures between two successive visits with respect to sexual behaviours, antiretroviral treatment, and characteristics of the couple reported for that specific period. Index partners who started or changed antiretroviral treatment between two successive visits were classified in the lower treatment category. Genitourinary infections and other circumstances detected at a visit were considered as covariates present during the whole time period since the previous visit.
We compared continuous variables with the Wilcoxon test, proportions with Fisher’s exact test, and rates with exact methods. Confidence intervals for rates were calculated by assuming a Poisson distribution, and confidence intervals for risks by assuming a binomial distribution.