UC San Diego

Referred to as an "Epidemic Without Borders," the escalating HIV/AIDS epidemic in the U.S./Mexico border region is in urgent need of attention. Although the epidemic is currently concentrated there is potential for it to advance to a generalized level. Recent studies have estimated that 1 in 125 adults aged 15-49 in Tijuana is HIV-infected, with sub-groups most affected being men who have sex with men (MSM), injection drug users (IDUs), and female sex workers (FSWs). Given this there is urgent need to targeted HIV prevention initiatives for these populations. As a result we proposed the following: 1) estimate the total number of men and women aged 15-49 years infected with HIV in 2006; 2) project the new number of HIV infected adults ages 15-49 among non-IDU/FSWs over the next 30 years; 3) evaluate the potential impact of increasing condom use between non-IDU/FSWs and their client on the HIV epidemic curve over a period of 30 years. We calculated HIV prevalence estimates among men and women aged 15-49 in Tijuana, Mexico for 2006, overall and by risk groups (MSM, IDUs, FSWs, pregnant women, low risk men and women). General demographic data such as total population size, gender ratio, and age distribution were obtained from the 2005 Mexican census. To obtain the most updated estimates of HIV prevalence, a review of published literature and available data from community- based studies was conducted. Results of this modeling project allowed us to develop a range of possible estimates for the number of HIV infected individuals, both overall and for each of the at-risk sub-groups. A mathematical model to predict the course of the HIV epidemic over a 30-year period (2006-2036) among FSWs who are not injection drug users (non-IDU/FSWs) was developed. More specifically a deterministic compartmental mathematical model was constructed based on biological (i.e., probability of HIV transmission per sexual act, prevalence of sexually transmitted infections), behavioral (i.e., number of unprotected sexual encounters), and demographic (i.e. population age distribution and population sizes) parameters. Data for the model were derived from community-based studies, literature searches, population census, and communication with Mexican public health officials. A second deterministic compartmentalized model was developed to assess the impact of decreasing unprotected sex among non-IDU/FSWs on the prevalence and incidence of HIV/AIDS. More specifically decreases of 5%, 10%, 20%, 50% and 75% in unprotected sex were modeled. Data on baseline levels of unprotected sex among non-IDU/ FSWs was taken from a community-based study. Sensitivity analysis for both deterministic models consisted of developing low- and high-growth scenarios for each of the modeled estimates. The estimated overall prevalence of HIV in Tijuana Mexico among adults ages 15-49, is 0.54% (N = 4, 347) (Range: 0.22% (N = 1,750) - 0.86% ( N = 6,944). This means that as many as 1 in every 116 male and female adults between the ages of 15-49 could potentially be infected with HIV. However, HIV prevalence is most concentrated among sub-groups of the population such as men who have sex with men (MSM), female sex workers who are injection drug users (FSW-IDUs), female-IDUs, non-IDU/ FSWs and male-IDUs. Results of the deterministic mathematical model show that accumulative HIV incidence will continue to rise over the 30-year modeled period (2006-2036). According to the low- and high-growth scenarios the HIV epidemic will become sustained at approximately 19% by the year 2036. A greater reduction in future HIV prevalence was observed when unprotected sex was decreased by greater than 50% among non-IDU/FSWs. The model also predicted that the greater the reductions in unprotected sex, the lower the level at which the epidemic would become a sustained epidemic