Use of AIDS surveillance data to describe subepidemic dynamics

Official reports on AIDS surveillance mainly consist of absolute numbers of AIDS cases or cumulative incidence rates. More detailed analyses focusing on the clusters of subepidemics within Italy seemed necessary for a better understanding and more accurate description of the epidemic.

Age-specific AIDS incidence rates were calculated with reference to resident population by sex, calendar time and geographical area. Age-standardized incidence rates, with the Italian resident population in 1990 as standard, were used to present time trends and geographical distributions. All analyses were repeated for injecting drug users, homosexual/bisexual men, heterosexual contacts and individuals with other or undetermined risk factors.

Annual incidence rates for AIDS in Italy increased over the study period. The highest rates were observed in the North and in Sardinia, while Southern regions showed generally lower rates. This heterogeneity was more evident when examining small geographical areas (i.e. provinces). Epidemics in some of the smaller provinces, such as Imperia and Livorno (Northwestern port towns), were shown to be important in that they greatly affect AIDS incidence rates in the regions in which they are located.

According to our analysis, the crude presentation of data from the Italian AIDS Registry is not adequate for understanding the national spread of the AIDS epidemic in terms of several local subepidemics, which may differ by size, temporal trend, and risk group composition. Classifying cases according to their place of residence, which we considered as a good proxy of the place of life, was fundamental for correctly locating these subepidemics. Furthermore, the use of age-standardized rates allowed for unbiased comparisons between regions whose population may have a different age structure and dynamics.