In most countries of Asia and Eastern Europe, in contrast to sub-Saharan Africa, HIV currently remains concentrated within identifiable vulnerable populations such as injecting drug users and their sex partners, men who have sex with men, and sex workers and their clients. In many of these countries it has been possible to estimate the size of these populations, using data from surveys and censuses or indirect estimates based on multiplier or capture–recapture techniques (12). HIV prevalence estimates from sentinel surveillance sites among these populations can then be multiplied by population size to estimate the number of people infected with HIV. UNAIDS and WHO have developed software and held training workshops worldwide to assist national AIDS programme staff and others in the implementation of this approach (13) (see Table 1 for an example from Indonesia).
Once HIV reaches vulnerable populations where condom use is inconsistent or needle sharing is common, epidemics are usually inevitable (14). Ignoring the possibility of heterosexual HIV transmission in the wider adult population, it is possible to explore the effect of different assumptions about the peak prevalence and timing of HIV epidemics among vulnerable populations on future epidemic size. Assumed peak prevalence levels can be informed by comparison with long exposed populations from the same or neighbouring countries. Using this approach, it has been estimated that the epidemics in Eastern Europe and Asia are likely to add about 21 million infections to the HIV pandemic by 2010 if there is not a rapid and expanded response (15). Thus, even without a generalized heterosexual epidemic, the need for action is clear.