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The increasing marginal benefit of condom usage.

Annals of Epidemiology 2000;10(3):154-159.

Chesson HW, Gift TL.

Abstract
PURPOSE: Condom use is promoted as a primary strategy for preventing sexual transmission of human immunodeficiency virus (HIV). This paper analyzes how incremental changes in condom compliance rates can affect an individual's risk of acquiring HIV. METHODS: We developed a simple mathematical model of HIV transmission in which the cumulative probability of HIV infection depended in part upon the percentage of acts in which a condom was used. We applied basic methods of calculus to differentiate the mathematical model with respect to the probability of condom usage. We applied values from published studies to the model to illustrate how the marginal benefits of condom usage vary across different populations. RESULTS: In general, the marginal benefit of condom usage increases as condom compliance increases. CONCLUSIONS: The marginal benefits of increased condom usage vary across different risk groups and across different levels of condom compliance. These results offer insight into the motivation behind the decision of whether or not to use condoms, and indicate possible ways to optimize the use of resources devoted to increasing condom usage by at-risk populations.