An age-period-cohort analysis of 50 875 AIDS cases among injecting drug users in Europe

Background The long average incubation time from HIV infection to AIDS make s it difficult to estimate recent HIV transmission from AIDS incidence data . Age-period-cohort (APC) analysis can separate out the effects of age, cal endar time and birth cohort to provide a clearer picture of transmission tr ends. Methods AIDS incidence data from 1981 to 1994 drug users (IDU) for 12 Weste rn European countries were used. Yearly incidences per 100 000 population o r 100 000 person-years were calculated by age at diagnosis and 5-year birth cohort (1950-1954, 1955-1959, 1960-1964, 1965-1969 and 1970-1974), and cor rected for reporting delay. Incidence patterns were compared between birth cohorts and countries. Results For most countries the impact was greatest on the cohort born 1960- 1964. Comparing incidence patterns in the 1965-1969 to 1960-1964 cohorts su ggest the epidemic has plateaued at low to intermediate levels in Austria, Greece and the North-Western European countries, and at high levels in Fran ce, Italy and Switzerland. For most countries transmission amongst the 1970 -1974 as compared to the 1965-1969 cohorts could not be assessed due to sma ll numbers and short follow-up time. In Spain the epidemic was uncontrolled with a high incidence among recent birth cohorts. In Portugal the epidemic was still at an early and expanding phase. Conclusions The APC analysis revealed large country differences in the dyna mics of the HIV/ AIDS epidemic among IDU. Full interpretation of these diff erences is dependent on information from other sources about the local publ ic health response and trends in drug injecting behaviours. Earlier introdu ction of the virus and higher prevalence of injecting drug use may explain some of the generally higher incidence in Southern European countries, but the larger part of it is most likely explained by local characteristics of drug users, such as younger age and more frequent sharing of needles and sy ringes, and a less effective public health response.