AN INVESTIGATION ASSESSING THE FRACTION OF HUMAN-IMMUNODEFICIENCY-VIRUS INFECTION ATTRIBUTABLE TO ETHNIC-GROUP VARIATIONS IN RISK

Considerable speculation and a recent publication has suggested that much of the human immunodeficiency virus (HIV-1) infection in the subset of the UK population represented by women having babies in Metropolitan London is substantially imported from Africa. Other data are supportive of this hypothesis. Ades et al. (1992) concluded that the fraction of HIV seroprevalence, identified in a large London-based study using neonatal blood from the dried blood spots on Guthrie cards, attributable to mothers born in Africa was 92.4% with 95% Cl : 82-100%. This is an important observation which requires confirmation. This paper reports on an attempt to do this with closely similar methodology using the Unlinked Anonymous Survey (UAS) of HIV-1 seroprevalence in pregnant women attending antenatal centres, together with data from the Office of Population Censuses and Surveys (OPCS) on the country of origins of mothers delivering in 1991 at the 15 London centres included in the UAS. As in Ades' analysis there appeared to be a strong association between centre prevalences and the proportions of women of African origin delivering at those centres. This was consistent with a high fraction of seroprevalence attributable to such women, but it was also clear that the assumptions required for the estimation of the attributable fractions were not fully met. Analyses modified to justify the assumptions produced attributable fraction estimates ranging from 44% to 96% with tower 95% confidence limits 20% or less. There is clearly an association and it may result from a proportion of HIV infections being imported to the UK from elsewhere, but the nature of the variation makes estimation of the magnitude of such effects somewhat speculative. Furthermore, associations observed in such ecological analyses are not generally adequate for inferring a causal relationship. The correct explanation of these associations can only be identified by studies when information on all the necessary variables is available at the individual level.