Estimating the AIDS incubation period from a prevalent cohort.

Data from a cohort study of 177 homosexual or bisexual men enrolled in Los Angeles in 1982 and 1983 are used to estimate the incubation period distribution of acquired immunodeficiency syndrome (AIDS). The statistical method used is parametric modeling of the joint distribution of the date of human immunodeficiency virus infection and the time from infection to the onset of AIDS. With this method, the unknown dates of infection and the fact that individuals who developed AIDS prior to the enrollment date are excluded from the sample is taken into account. The estimated proportion of individuals who develop AIDS within 4 years of infection is 0.10 (95% confidence interval 0.05-0.21) and that within 8 years of infection is 0.45 (95% confidence interval 0.30-0.60). These proportions are consistent with those obtained from other cohort studies and through other statistical approaches.