The long incubation period of AIDS, with variation in infectiousness over its course, has emphasized the need to model progression of the disease process. The models used for progression of HIV infection to AIDS have generally been staged Markov models that imply a one-way progression from infection to AIDS to death and so do not allow for temporary remissions in the progression of the disease. Such models have negative exponential distributions for the transit times in a stage and independence of transit times in successive stages (Longini et al. 1992, Longini et al. 1991). In our studies to estimate transmission probabilities from data on the Chicago MACS cohort, by stage of infection, we found it necessary to examine progression in the cohort.

The Multicenter AIDS Cohort Study (MACS) involves 4 cohorts of male homosexuals recruited in 1984 in 4 cities: Baltimore, Chicago, Los Angeles and Pittsburgh (Kaslow et al. 1987). Approximately every 6 months, the participants had physical examinations, had blood drawn and filled out a questionnaire on sexual practices. We examine progression in the Chicago MACS cohort which consisted of 1020 individuals at the start of the study. We present data on the first to twelfth waves of examinations, covering the period 1984–90.

Cumulative plots of seropositivity for HIV-1 show that approximately 40% of the Chicago cohort was HIV(+) by the first wave and that about 70 more seroconversions occurred from wave 1 to wave 12. The experience of the other cohorts was similar. Thus roughly 85% of the infections occurred before the first wave of examinations.