Background:  A reduction in vertical transmission rates has been demonstrated in European cohorts since the advent of ART and elective cesarean section. In the United Kingdom and Ireland, comprehensive population surveillance of pediatric and obstetric HIV has been ongoing since 1986 and 1990, respectively.

Methods:  Pregnancies in women with diagnosed HIV infection in the United Kingdom and Ireland are notified to the National Study of HIV in Pregnancy and Childhood through a confidential, voluntary, active reporting scheme, and infants are followed to establish infection status. Logistic regression analysis was carried out using Stata.

Results:  From 1990 to 2004, 4229 infants born to diagnosed HIV-infected women were reported, and infection status was confirmed for 87.6% (3703 of 4229). Vertical transmission rates declined from a high of 25.6% (10 of 39, 95%CI, 12.3 to 47.2%) in 1993 to 1.1% (9 of 797, 95%CI 0.5 to 2.1%) in 2004, and were <2% each year since 2000. In multivariable analysis, transmission rates were 4 times lower following elective cesarean section (AOR 0.25, 95%CI 0.14 to 0.45, p <0.001), and 1.7 times lower following emergency cesarean section (AOR 0.58, 95%CI 0.32 to 1.05, p = 0.074), compared with vaginal delivery. Mono/dual therapy was independently associated with a 7-fold reduction (AOR 0.14, 95%CI 0.08 to 0.28, p <0.001) and HAART with a 10-fold reduction in the risk of transmission (AOR 0.09, 95%CI 0.05 to 0.16, p = 0.001), compared with no ART. Among ~2500 women on HAART, elective cesarean section was associated with a 3-fold reduction in transmission (OR 0.34, 95%CI 0.13 to 0.87, p = 0.025), compared with vaginal delivery. The risk of transmission was 1.7 times higher for female than for male infants (AOR 1.7, 95%CI 1.0 to 2.7, p = 0.036), after adjusting for ART and mode of delivery. In 697 mother-child pairs with exposure to ART and vaginal delivery, prematurity (<32 weeks) was significantly associated with vertical transmission (AOR 7.3, 95%CI 2.0 to 27.6, p = 0.003).

Conclusions:  This surveillance study includes all diagnosed HIV-infected pregnant women reported from across the United Kingdom and Ireland, regardless of uptake of interventions or timing of diagnosis. The fact that transmission rates in this diverse and unselected population were <2% from 2000 to 2004 is encouraging. These findings support the suggestion that female gender and prematurity are risk factors for transmission, and that mode of delivery remains important even in the HAART era.