An Age and Period Reconstruction of the HIV Epidemic in Italy

Abstract

The majority of AIDS cases in Italy are among intravenous drug users (68%) and homosexual men. An age, period and cohort (ARC) model is presented and used to reconstruct the HIV epidemics in Italy. Projections of AIDS-related conditions (ARC) and AIDS cases are attempted based on a hypothesis of minima and an endemic hypothesis. The model is a generalization of the usual back-calculation method which considers age, competitive mortality, susceptible population and therapy effects. Estimates of the HIV epidemic in Italy are obtained using Italian AIDS counts (corrected for delay in reporting), and an incubation time distribution (estimated from data of an Italian cohort), which was found to be dependent on the age at infection. The impact of AZT therapy, introduced in Italy in mid1987, is evaluated using a modification of the incubation time distribution dependent on period of infection. The estimated number of new infections in Italy declined after 1987, although the number of new AIDS cases has continued to rise, albeit less steeply in recent years. When delay in the progression to AIDS due to therapy is taken into account, the estimated number of people infected in mid 1990 increases from 52 000 to 67 000, with approximately 12% of subjects already in the ARC stage. The age at maximum risk of infection is 25 years in males and 23 years in females. Using a hypothesis of no more HIV infections after 1990, AIDS counts would be still rising in Italy up to 1993 as a result of past infections and of the long incubation period.