The "next wave"?
Recent predictions of a catastrophic "next wave" of the HIV pandemic in the populous countries of Asia and Eastern Europe (1, 2) are a cause of controversy within international development agencies and have been condemned by some of the national governments concerned (3). The predictions suggest that prevalent HIV infections in China, India and the Russian Federation alone could reach between 35 million and 68 million in 2010, compared with the current global total of 38 million estimated by the Joint United Nations Programme on HIV/AIDS (UNAIDS) and WHO (4). Undoubtedly the drug-using and sexual behaviours that transmit HIV are increasing in these countries (5), and numbers of newly diagnosed HIV infections have surged in recent years (4). However, next-wave scenarios are contingent on a generalized heterosexual spread occurring on a scale similar to that seen in sub-Saharan Africa. Is this possible?
While many studies have shown the correlation between an individual's sexual behaviour and infection with HIV, there is little evidence for population-level measures that can explain differences in adult HIV prevalence (6–8). This lies at the heart of the difficulty in forecasting the scale of heterosexual HIV epidemics, because no measures of sexual behaviour in the population have been identified as accurate predictors of the size of an epidemic. Current forecasts therefore have to rely on scenarios proposed by experts using Delphi or similar techniques or on mathematical models. Models that attempt to forecast the size of a heterosexual HIV epidemic tend to be based either on fitting epidemic curves to available prevalence data and extrapolating (9) or on mathematical descriptions of sexual behaviour and the associated transmission of HIV (10). There is a massive level of uncertainty in such forecasts (5). Thus, while next-wave scenarios can be deemed unlikely or derived through applying a worst-case scenario systematically (11), they can only ever be disproved after the event. It is the role of UNAIDS, WHO and the scientific community to make clear what can and cannot be said about the future of the HIV pandemic. If accurate forecasts are so problematic, is there an alternative approach that can be adopted?