THE IMPACT OF THE 1993 EUROPEAN REVISION OF THE AIDS CASE-DEFINITION ON BACK-CALCULATION ESTIMATES - AN APPLICATION IN ITALY

Objectives: The revision of the case definition for AIDS within a give n observation period causes discontinuity points on the epidemic curve to which back-calculation procedures can be sensitive. The aim of thi s work is to characterize the impact of the 1993 European revision of AIDS case definition and to evaluate the degree of distortion by which back-calculation estimates obtained on Italian data are affected. Met hods: The back-calculation procedure used, is a generalization of stan dard methods for estimating and projecting the AIDS epidemic. Age at o nset of HIV infection, incubation time distribution, susceptible popul ation and competitive non-AIDS mortality rates for infected persons ar e included in the estimation process. Results: Back-calculation estima tes resulted to be sensitive to the discontinuity point that the 1993 revision of AIDS case definition induced on the epidemic curve. The re sulting effect is a tendency to overestimate HIV incidence in recent y ears by 20-40%. The maximum effect was found using data with end of AI DS cases at December 1993. The overestimation is a transient effect, t hat is, it is expected to almost disappear in 1995-1996. Conclusions: The introduction of the 1993 European revision of AIDS case definition caused distortion of back-calculation estimates in Italy, leading to overestimate incidence and prevalence of HIV infection for recent year s. It is believed that this effect has occurred in all countries that adopted the 1993 European revision of AIDS case definition, and its ch aracterization is potentially useful for further changes of AIDS case definition in the future.