Since the beginning of the epidemic, surveillance efforts have emphasized determining the number and characteristics of individuals diagnosed with AIDS. The current national AIDS case surveillance system, which was implemented prior to both the identification of HIV as the etiologic agent of AIDS and the development of an antibody test to determine HIV infection, was originally based on epidemiological investigations of an end-stage syndrome (Gostin et al., 1997). Each state requires that all patients diagnosed with AIDS be reported by name to the local, state, and/or territorial health departments. These reports are then forwarded (without names but with unique identifiers) to the Centers for Disease Control and Prevention (CDC), where a national surveillance database is created and analyzed. This surveillance system provides uniform data on trends and distribution of individuals diagnosed with AIDS. Standard records for each case include information on sex, race and ethnicity, state of residence (and metropolitan area, if relevant), mode of exposure to HIV, age at diagnosis, month of AIDS diagnosis, date reported, and other data. Although there are some reporting delays in the system, the data are relatively complete (more than 85 percent of AIDS cases) (Schwarcz et al., 1999; Buehler et al., 1992; Rosenblum et al., 1992) and statistical methods are available to adjust for both reporting delays and incompleteness (Green, 1998). Data from this surveillance system have been used as the basis for allocating federal resources for HIV treatment and care, and as the basis for planning local HIV prevention services (CDC, 1999a).

Until the era of potent antiretroviral therapies, AIDS case reporting, although imperfect, provided a relatively accurate picture of trends in HIV infection, especially relative HIV prevalence in groups defined by geography, race and ethnicity, and primary mode of infection. Estimates of HIV incidence and prevalence were made by statistical techniques, such as calculating backward from reported AIDS cases according to well-established patterns of disease progression (Brookmeyer and Gail, 1994). Recent developments in therapy for HIV and AIDS have at least temporally decoupled HIV infection and its progression to AIDS (Hammer et al., 1996; Collier et al., 1996). As a result, the timing of the progression from HIV infection to AIDS and from AIDS to death is increasingly difficult to predict, making HIV incidence and prevalence estimates based on AIDS cases much less accurate (CDC, 1999a). Consequently, AIDS case reporting is no longer adequate to monitor trends in HIV infection. The United States now faces the challenge of developing an effective HIV surveillance system that can predict where the epidemic is headed.

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