Back-calculation by birth cohort, incorporating age-specific disease progression, pre-AIDS mortality and change in European AIDS case definition

Objectives: To adapt and improve methodology for back-calculation of AIDS i n Europe and to examine the feasibility of estimating past HIV incidence by birth cohort. Methods: Empirical Bayesian back-calculation (EBBC) used Markov disease pro gression models, modified to allow for three diseases added to the AIDS cas e definition in 1993 and for pre-AIDS mortality,and estimation by penalized maximum likelihood with a neighbour prior. EBBC by 5-year birth cohort ass umed a minimum age at infection and age-dependent progression rates; three versions, with varying age effects, were investigated using AIDS cases diag nosed prior to the introduction of highly active antiretroviral therapies ( HAART). Results: Compared with the no age-effect version, EBBC by birth cohort tend ed to produce flattened HIV incidence curves in country-exposure groups wit h < 1000 AIDS cases, reflecting effects of the neighbour prior when data be come sparse. Otherwise, birth cohort analysis, with moderate effects of age on progression, gave initially increasing incidence curves and consistent patterns across countries, with the 1960-1964 cohort most affected. In the European Union, incidence is estimated to have peaked in 1983 among homosex ual men and in 1988 among injecting drug users; 460 000 persons were estima ted to be living with HIV/AIDS at the end of 1995. Conclusions: Our improved methodology deals effectively with the change in AIDS case definition and has allowed quantitative assessments of the HIV ep idemic by birth cohort using all AIDS cases diagnosed before 1996, thus pro viding a sound basis for public health policy at a time when estimation of more recent prevalence is compromised by the effects of HAART. (C) 2000 Lip pincott Williams & Wilkins.