Age, period and cohort model to estimate prevalence and incidence of HIV infection.
Int Conf AIDS 1992 Jul 19-24; 8:We62 (abstract no. WeC 1091) Verdecchia A, Mariotto A, Capocaccia R, Mariotti S; Istituto Superiore di Sanita, Rome, Italy.
OBJECTIVE. To present an age, period, and cohort (APC) approach both to estimating and analyzing HIV epidemics and to a reconstruction of the HIV epidemics in Italy. METHODS. In a back-calculation framework, a system of integral equations is defined, relating AIDS counts to the probability of new infections among the population, to the AIDS incubation time distribution and to the differential force of mortality from other causes for HIV infected subjects. The equations provide a regression model to fit age and period specific AIDS counts assuming HIV infection incidence as a function of age at the time of infection, period, and/or birth cohort in the logistic scale. Maximum likelihood estimates of the HIV infection incidence are then calculated. A two-stage AIDS incubation time distribution, estimated from data on an Italian cohort of 420 subjects who seroconverted between 1982 and 1990, was found to be dependent on age at infection and on period of infection due to the introduction of AZT therapy in mid 1987; this two-stage distribution also allowed for the estimation of the number of infected individuals in the ARC stage. AIDS counts were corrected for reporting delay. RESULTS. The estimated number of new infections in Italy decreased after 1987, though the number of new AIDS cases has continued to rise, albeit less sharply in recent years. When delay in the progression to AIDS due to therapy is taken into account, the estimated number of persons infected in mid-1990 increases from 52,000 to 67,000, with approximatively 12% of subjects already in the ARC stage. Approximatively 68% of HIV infections in Italy are among intravenous drug users, and 15% are among homosexual males. The age at maximum risk of infection is 25 years in males and 23 years in females. Based on a hypothesis that the rate of new infections will level-off at the 1989 level, it is predicted that ARC counts will continue to rise until 1993, and that AIDS counts will continue to rise beyond 1995. CONCLUSIONS. Although the rate of new infections has been rapidly decreasing in recent years, it is still unacceptable (10-20 new infections every day), especially considering the very young age at which they occur. Efforts discouraging drug use and promoting safe sex are still needed to prevent HIV infection among young people. Special attention should be paid to monitoring trends of new infections in the heterosexual population, in order to prevent a possible spread of the HIV epidemic among the general population.
Keywords: AEGIS, Incidence, Prevalence, HIV Infections, Acquired Immunodeficiency Syndrome, AIDS-Related Complex, Substance Abuse, Intravenous, Disease Outbreaks, Homosexuality, Italy, Human, Female, Male, epidemiology, ICA8 920719
WeC1091
