Maternal viral load and vertical transmission of HIV-1: an important factor but not the only one.

Objectives: To investigate the association between maternal RNA load, risk of vertical transmission of HIV-1, and other variables. Methods: Plasma or serum samples from mothers of 373 children, enrolled in the prospective European Collaborative Study, were collected around time of delivery, and HIV-RNA quantified using two types of commercial assay. Women and children were followed according to a standard protocol. Adjusted odds ratios (AOR) were calculated to estimate the effect of RNA load and other maternal Factors on vertical transmission. Results: Maternal RNA levels, mode of delivery and gestational age were independently associated with transmission. Vertical transmission increased with increasing RNA levels, but there was no threshold below which transmission did not occur. The risk was more than double for women with RNA above the sample specific median [AOR 2.36 (1.23-4.52)]. Elective caesarean section was associated with a substantial and significant decrease in transmission [AOR 0.19 (0.06-0.55)], and delivery before 37 weeks gestation with an increased risk [AOR 2.67 (1.33-5.38)]. Elective caesarean section was effective in both subgroups defined by median RNA level [AORs 0.37 (0.08-1.71) and 0.15 (0.03-0.64) below and above median respectively]. The predicted rate of transmission in a woman with a low RNA load delivering by elective caesarean section or vaginally after 37 weeks is around 2%, and 11%, respectively. interpretation: Mother-to-child transmission of HIV-1 is multi-factorial; high RNA load is an important determinant but clearly not the only one. Interventions that target risk factors other than maternal RNA load remain important.