Short-Term Prediction of HIV Infection and AIDS: A Critique of the Working Group's Report to the Department of Health

The Report of the Working Group on the Short-Term Prediction of AIDS/HIV (the Cox Report) is reviewed mainly to assess its calculations of the numbers of people in England and Wales who are infected with the human immunodeficiency virus (HIV). Two main methods are used in the report to estimate this total—the direct method and the back projection method.

The direct method estimates the number of people infected with HIV by attempting to specify the numbers of people in various at-risk groups, and the percentage infected in those groups. Of particular significance are the estimates given for male homosexuals. The Cox Report suggests that between 4.0% and 4.7% of the male population aged between 16 and 59 are homosexual, and that between 1.9% and 4.5% of these are HIV antibody-positive. The basis on which these estimates are made is not substantiated by the Report, and it is quite possible that the upper limit given for HIV prevalence in male homosexuals represents an understatement of the actual number by a factor of 2.5 or more.

The back projection method estimates HIV prevalence from the numbers of cases of the acquired immunodeficiency syndrome (AIDS) and the incubation function, the relationship between HIV infection and the probabilities of AIDS in each of the years following infection. Using this method the Cox Report fails to produce results that are in accordance with our knowledge of how the epidemic developed during the 1980s. As a consequence of this the various calculations of numbers of HIV antibody-positives to 1987 given in the Cox Report are all almost certainly underestimates.

In both the procedures adopted and the results obtained the Report is found deficient. Its assertion that the number of HIV infections to 1987 is likely to be at the lower end of the 20,000 to 60,000 range has not been established. Even less credible is the later model proposed by Anderson et al. in the subsequent Royal Society report that presents results suggesting only 10,000 to 15,000 people were infected with HIV in England and Wales ‘at the period of peak incidence’. (Presumably from their other work this must be up to the end of 1987.) The available evidence suggests that the actual number is greater than 60,000, could be over 100,000, though tighter estimates must await either accurate surveys of population prevalence or further knowledge about character of the AIDS incubation function, and the degree to which AIDS statistics are understating the numbers of AIDS cases and deaths.