In early MTCT studies, prior to PMTCT interventions, the
increased relative risk for incident versus chronic maternal
infection was due to high maternal viral load and lower maternal
HIV-specific immune responses in incident infection. With
PMTCT ARVs, rates of transmission among women with chronic
HIV decreased from ,20%–35% to 1%–5%. Since incident
maternal HIV is typically detected weeks after infection, there is a
relative delay in the initiation and effect of ARVs, amplifying the
relative risk in incident versus chronic MTCT rates. Thus, earlier
studies comparing MTCT in incident versus chronic maternal
infection noted 2- to 3-fold increased MTCT, while later studies
note 6- to .15-fold increased MTCT because of differential
PMTCT. A recent modeling study projected that the absolute
number of MTCT events could be reduced by 28% in South
Africa if HIV screening was repeated during late pregnancy or at
6-week infant immunization visits, demonstrating a greater need
for identification and early treatment for women acquiring HIV
during pregnancy [48].