Short-Term Extrapolation of the AIDS Epidemic

For both scientific and administrative reasons it is of vital importance to forecast the future course of the AIDS epidemic. There are essentially two ways of doing this. On the one hand, a mathematical model of the spread of the disease can be constructed and used to provide forecasts for as far ahead as is desired. At the other end of the spectrum, numbers of recorded cases can be plotted against time and the resulting curve extrapolated forwards. The model-based forecasts rely upon estimates of the model parameters which must to some extent be supplied by reference to the case data; as will appear, the extrapolation forecasts are far from being model-free. The purpose of this paper is to describe the data available on the UK epidemic and to see to what extent extrapolation forecasts can be reliably made.