|Less transmission of HIV means fewer new cases. However, it is very difficult for regular monitoring systems to measure new cases--incidence data generally come only from sophisticated and expensive longitudinal cohorts. National M&E systems therefore tend to use cross-sectional prevalence data to monitor the spread of infection. But with chronic diseases such as HIV, prevalence data are problematic as a proxy indicator for recent infections. This is especially so when the data come from sentinel surveillance systems built around selected populations such as women in antenatal clinics. ANC data for HIV are biased by mortality, by a reduction in fertility in HIV-positive women and by other factors.
Second generation surveillance aims to make better use of data generated by sentinel surveillance, partly by changing sampling and analysis strategies so that data better reflect more recent infections (see Panel 4).
One of the constraints of sentinel HIV surveillance in generalised epidemics is that few sentinel systems provide any data on men. Other proxy measures of impact in men can be used, for example the incidence of self-reported or clinical STIs. Since interventions aimed at reducing the spread of HIV ought also to have an impact on STIs--and a much more rapid one at that--STI measures can be useful as indicators of recent changes in risk behaviour for HIV.
In theory at least, pregnant women presenting for antenatal care are regularly screened for syphilis and treated where necessary. This regular screening is potentially an important source of impact data for AIDS programmes, since it is at least somewhat more responsive to recent trends in risk behaviour than is HIV prevalence data. However even where testing is systematic, these data have rarely been systematically reported through the AIDS programme. This is a prime example of where existing data could be better used in M&E systems.
Measures of HIV and STI incidence and prevalence give an idea of the health impact of the HIV epidemic and of programmes designed to limit it. Mortality data also provide powerful impact indicators. It is recognised, however, that the impact of HIV and AIDS goes far beyond health or even mortality. Indicators of illness or long-term incapacity and orphanhood give a crude idea of the potential social and economic impact of the epidemic at a household level; they will grow in importance as the epidemic matures. More refined indicators are needed to measure the social and economic impact of HIV and AIDS--and of the success of national AIDS programmes in mitigating that impact. It is hoped that the existing toolkit will be expanded to include more measures of socio-economic impact as new methodologies are developed.