ESTIMATION DU TAUX DE TRANSMISSION DU VIH DE LA MERE A L'ENFANT: PROBLEMES METHODOLOGIQUES ET ESTIMATIONS ACTUELLES

In the last eight years, numerous cohort studies have been conducted to estimate the rate of mother-to-child (MTC) transmission of HIV. Many of these have faced problems in data collection and analysis. This made it difficult to compare transmission rates (TRs) between studies. Two workshops on methodological aspects of the study of MTC transmission of HIV-1 were held in Ghent (Belgium) in February 1992 and September 1993. Fourteen teams of investigators participated, representing studies fom Central (5) and Eastern Africa (3), Europe (2), Haiti (1) and the USA (3). A critical evaluation of the projects was carried out, under four headings: 1) enrollment and follow-up procedures, 2) diagnostic criteria and case definitions, 3) measurement and comparison of MTC TRs and 4) determinants of transmission. Reported TRs ranged fom 13 to 32% in industrialized countries and from 25 to 48% in developing countries. However, no direct comparisons could be made because methods of calculation differed from study to study. Based on this review, a common methodology was developed during the 1992 workshop. Agreement was reached on definitions of HIV-related signs/symptoms, paediatric Aids and HIV-related deaths. A classification system of children born to HIV-1 infected mothers according to their probable HIV infection status during the first 15 months of life allowed the elaboration of a direct method of computation of the TR and of an indirect method for studies with a comparison group of children born to HIV-seronegative mothers. This standardized approach was subsequently applied to selected data sets to update previous estimates and provide a comparison of the MTC TRs of HIV-1 in 13 different settings. TRs were calculated during the 1993 workshop by the participating teams, using the direct and indirect methods. TRs based on the intermediate estimate obtained with the direct method ranged between 12.7 and 42.1%. Estimates of TRs obtained with the indirect method ranged fom 20.7 to 42.8%. TRs observed in developed countries ranged from 14 to 25% with the direct method. In the developing world, these rates ranged fom 13 to 42% with the direct method, from 21 to 43% with the indirect method and most of the studies reported a TR in the range of 25 to 30%. In general, both methods provide a reasonable estimate of the true TR. The risk of MTC transmission of HIV-1 tends to be higher in children born to HIV-seropositive mothers in Africa than in Europe. It is unlikely that there is a single explanation for this variation, and determinants of transmission are likely to differ between populations. This methodology can now be applied to all studies with sufficient follow-up. This is of importance for the design and implementation of trials evaluating interventions aimed to reduce or prevent mother-to-child transmission of HIV.