(introduction...)

For the first decade or so of the HIV epidemic, many countries concentrated resources on tracking the spread of the virus itself. Industrialised countries focused on AIDS case reporting, while many developing nations, particularly those of sub-Saharan Africa, set up sentinel surveillance systems to detect the spread of HIV. After stripping personal identifiers from blood samples taken for other purposes - most commonly antenatal syphilis testing of pregnant women - sentinel surveillance systems test blood for HIV. This data is thought to give some indication of the levels of HIV infection in the general population.

However, because a person may be infected with HIV for a decade or more without showing any symptoms, HIV prevalence rates can reflect a combination of recent infections and infections that are many years old. Consequently, the prevalence rate is very slow to reflect changes in new infections. Prevalence that is stable or falling may mean that people are behaving more safely and fewer are becoming infected than in previous years. It may, however, simply reflect the fact that HIV-infected people are dropping out of the tested population because they have died, moved away, or are too sick to go to the health facility where they might be tested. It may mean that nearly everyone with risk behaviour is already infected, or that the group of people tested has changed over time. Indeed, the relationship between HIV incidence and prevalence is so complex that in some cases falling prevalence may mask a still rising incidence of HIV infections, especially among young people.

Clearly, then, HIV prevalence rates do not serve as a good indicator of changes in new infections or as a measure of the success of programmes designed to reduce new infections. What are the alternatives? HIV incidence is costly and problematic to measure, since it involves testing the same group of individuals repeatedly over time or using costly testing methods on large numbers of people to detect a small number of new infections. Other physical markers that track sexual risk behaviour more closely than HIV are curable sexually transmitted diseases (STDs). Bacterial STD prevalence rates more closely reflect incidence rates because they are usually treated with antibiotics upon detection. However, surveillance of STDs in most countries is of lower quality than HIV surveillance. It is also extremely incomplete in the many countries where most surveillance data are collected in the public sector, while most treatment occurs in the private sector.

Although measuring changes in new HIV and STD infections is difficult, it is possible to track changes in the behaviours that lead to those infections. There are several reasons to do this, and they vary in importance according to how widespread HIV is in a country and which communities are affected.