Joint analysis of HIV and AIDS surveillance data in back-calculation

AIDS surveillance data are the main source of information to perform back-c alculation of HIV incidence. We propose a method to incorporate additional information gained by linkage with an HIV surveillance system, containing d ata on the time of first positive HIV test. In this paper we generalize an earlier method that was developed to use HIV testing data available only fo r AIDS cases. The new method also makes use of cases with an HIV positive t est who have not yet developed AIDS, typically a substantial proportion of the HIV-infected population. Furthermore, we use a more realistic model for the HIV testing rate, incorporating dependence on both time since infectio n and calendar time. The method makes use of an EM algorithm with generaliz ed additive model smoothing, and is applied to data from Veneto, a region o f northern Italy. Our results show that HIV incidence in Veneto peaked in t he late 1980s, and decreased thereafter. Importantly, the HIV incidence est imates based on joint analysis of HIV and AIDS surveillance data are more e fficient than estimates based on AIDS surveillance data alone. Our estimate s also show a decreasing trend in the HIV testing rate over time, which lea ds to the conclusion that the interval between HIV infection and first posi tive test has lengthened over time. Furthermore, it is found that for infec ted individuals, the probability of seeking on HIV test is highest soon aft er infection. Copyright (C) 2000 John Wiley & Sons, Ltd.