Predictors and impact of losses to follow-up in an HIV-1 perinatal transmission cohort in Malawi

Background Large simple trials which aim to study therapeutic interventions and epidemiological associations of human immunodeficiency virus (HIV) infection, including perinatal transmission, in Africa may have substantial rates of loss to follow-up. A better understanding of the characteristics and the impact of women and children lost to follow-up is needed. Methods We studied predictors and the impact of losses to follow-up of infants born in a large cohort of delivering women in urban Malawi. The cohort was established as part of a trial of vaginal cleansing with chlorhexidine during delivery to prevent mother-to-infant transmission of HIV. Results The HIV infection status could not be determined for 797 (36.9%) of 2156 infants born to HIV-infected mothers; 144 (6.7%) with missing status because of various sample problems and 653 (30.3%) because they never returned to the clinic. Notably, the observed rates of perinatal transmission were significantly lower in infants who returned later for determination of their infection status (odds ratio = 0.94 per month, P = 0.03), even though these infants must have had an additional risk of infection from breastfeeding. In multivariate models, infants of lower birthweight (P = 0.003) and, marginally, singletons (P = 0.09) were less likely to return for follow-up. The parents of infants lost to follow-up tended to be less educated (P < 0.001) and more likely to be in farming occupations, although one educated group, teachers and students, were also significantly less likely to return. Of these variables, infant birthweight, twins versus singletons, and maternal education were also associated with significant variation in the observed risk of perinatal transmission among infants of known HIV status. Conclusions Several predictors of loss to follow-up were identified in this large HIV perinatal cohort. Losses to follow-up can impact the observed transmission rate and the risk associations in different studies.