Purpose: To assess the effectiveness of a behavioral HIV intervention in reducing the risk of transmission from HIV-infected persons to their sexual partners, using a Monte Carlo simulation implemented in a commercially available spreadsheet Method: We simulated sexual partnership formation as a Poisson process, and varied partnership duration according to an exponential distribution. During each partnership, each sexual contact and the probability of HIV transmission to an uninfected partner was modeled as a Bernoulli trial. Sexual partnerships, number of protected and unprotected sexual contacts and the risk of HIV transmission were simulated repeatedly over time. Parameters such as the number of sexual contacts per partner and the per-contact probability of transmission can change with HIV disease progression, and be randomly distributed to represent individual variability. We applied data from a Prevention Case Management (PCM) program to changes in transmission risk behaviors (proportion of unprotected sexual contacts and number of sex partners), and estimated the reduction of HIV transmission to partners. In the sensitivity analysis, we explored the impact of partnership duration and of behavioral changes on the transmission reduction estimates.  Results: A hypothetical cohort of 10,000 HIV-infected individuals was simulated over 5 years. Without PCM, each HIV-infected individual would infect, on average, 0.243, 0.212 and 0.2 uninfected partners, assuming the average partnership duration of 6, 12 and  24 months accordingly. The table below shows the number of transmissions to partners (% reduction) when all PCM clients reduced their sexual risk behavior (39.8% and 30.2% reduction in unprotected sexual contacts and numbers of sex partners) for 3, 6 and 12 months.                                                                                                                                                                                                                                                                

Average partnership           Duration that behavioral changes sustained

duration                        3 months               6 months               12 months

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6 months                       0.234 (3.33%)      0.231 (5.17%)       0.218 (11.15%)

12 months                     0.212 (0.08%)      0.2 (5.61%)           0.194 (9.96%)

24 months                     0.199 (0.68%)      0.191 (5.14%)       0.175 (11.68%)                                                                   

Conclusion: A behavioral intervention such as PCM could potentially reduce the risk of HIV transmission, particularly if the behavioral changes are sustained over time. Information regarding partnership duration is important to better estimate transmission risk and intervention effectiveness. Spreadsheet models have the benefit of being accessible to many users while still incorporating complex parameters such as random variations in individual characteristics, including partnership formation and duration.