Summary: Critical to understanding the spread of the human immunodeficiency virus via needle sharing among drug users is the infectivity, i.e., the conditional probability of infection given injection with a shared, contaminated syringe. A simple mathematical model was constructed that relates infectivity to the prevalence of infection in needles used by drug users, the mean shared injection frequency among drug users, the probability that a needle is disinfected prior to use, and the mean AIDS incubation time. Three of these parameters have been estimated using data from the New Haven, Connecticut legal needle exchange program. Using the polymerase chain reaction to test for the presence of HIV proviral DNA in a sample of returned needles, we determined that 67.5% were HIV positive. We were able to estimate shared injection rates and disinfection rates from surveys of drug users enrolled in the needle exchange and our syringe tracking system. Current estimates of the mean AIDS incubation time are available in the literature. Our model implies that the probability of infection per injection with a contaminated syringe equals 0.0067, which is slightly higher than the transmission probabilities of 1/300 to 1/200 estimated from needlestick studies, and a factor of 3 higher than estimates of the probability of HIV transmission per vaginal sex act from an infected man to an uninfected woman.
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