ROLE OF THE PRIMARY INFECTION IN EPIDEMICS OF HIV-INFECTION IN GAY COHORTS

A review of the data on infectivity per contact for transmission of th e HIV suggests that the infectivity may be on the order of 0.1-0.3 per anal intercourse in the period of the initial infection, 10(-4) to 10 (-3) in the long asymptomatic period, and 10(-3) to 10(-2) in the peri od leading into AIDS. The pattern of high contagiousness during the pr imary infection followed by a large drop in infectiousness may explain the pattern of epidemic spread seen in male homosexual cohorts in the early years of the epidemic. Simulations of cohorts of homosexual mal es, using that range of parameter values, indicate the following: (a) The initial fast rise and then more or less rapid flattening of the in cidence curve of seropositives is primarily due to rapid initial sprea d, yielding a group of infecteds all of whom pass into the low infecti vity asymptomatic period at close to the same time. All this occurs on ly if the basic reproduction number for the primary infection is >1. ( b) The behavioral changes that have been reported all started after th e incidence of new infections began to fall, too late to have a major effect on the initial rise. The behavioral changes had a major effect in slowing down the subsequent rise in the number of seropositives. (c ) High activity groups play an important role in the early rapid rise of the epidemic. However, it is not likely that the rapid decrease in rate of growth of seropositives is solely due to saturation of these v ery high activity groups. Although the evidence for this interpretatio n of the role of the primary infection is not conclusive, its implicat ions for prevention and for vaccine trials are so markedly different f rom those of other interpretations that we consider it to be an import ant hypothesis for further testing.