[The probability of transmission of the human immunodeficiency virus].

The probability of human immunodeficiency virus (HIV) transmission is one of the key epidemiological parameters in the spread of the AIDS epidemic. The understanding of its possible determinants is necessary for a thorough interpretation of epidemic dynamics and for the evaluation of alternative strategies to control the epidemic. We carried out a bibliographic review of the main studies reporting data on the probability of HIV transmission through sexual contacts and parenteral routes (by transfusion of blood and blood products, by occupational exposures among health care workers, by sharing needles among injecting drug users). Different values of probability are associated to different routes of transmission. The observed high variability depends on the viral load, on the biological fluid (blood or mucous secretions) through which the virus is transmitted and on several other factors related to source infectiousness and to individual susceptibility, both common to all routes of exposures and specific to each of them. It is currently too early to forecast how the introduction of highly active antiretroviral treatment, which strongly reduce the viral load in patients with HIV infection, will modify the probability of transmission. Therefore, the compliance with recommended preventive measures still represents an indispensable way to contrast the virus spread.