The Use of Backcalculation to Estimate the Prevalence of Severe Immunodeficiency Induced by Human Immunodeficiency Virus in England and Wales

Abstract

As patients infected with the human immunodeficiency virus (HIV) become more severely immunosuppressed, they are at increased risk of developing one of a number of opportunistic infections or lymphomas, which define them as suffering from the acquired immune deficiency syndrome (AIDS). Estimates of the number of individuals who could benefit from antiretroviral treatment and prophylactic therapies against the main opportunistic infections are not readily available, as many individuals infected with HIV are asymptomatic and unaware of their infection. Using the method of backcalculation, we suggest that there are currently at least 2161 individuals with severe immunodeficiency but not AIDS in England and Wales. We demonstrate that this figure is sensitive to the choice of model for the severe immunodeficiency period and to the assumed effect of pre‐AIDS therapy. Using a definition of severe immunodeficiency based on a single CD4 count rather than two consecutive counts leads to substantially increased estimates of the prevalence of severe immunodeficiency.