HIV RNA levels and anti-HIV antibody responses among HIV-exposed infants with and without HIV infection. Schematic depiction of the timing of positive HIV-1 antibody testing and RNA levels among HIV-exposed infants. The horizontal axis shows infant age in months. The left vertical axis shows mean HIV-1 RNA level on a logarithmic scale, and corresponds to the green lines on each graph. The right vertical axis shows the proportion of infants for whom an HIV antibody test would likely return positive, and corresponds to the red lines on each graph. The proportion of infants with a positive antibody test in all panels is approximate, based on a wide range of reported ages at which transmitted maternal HIV antibody fades from detection in the sera of uninfected infants [7, 105]. Similarly, the mean RNA level is also approximate, based on several studies of infected infants with and without receipt of antiretroviral drugs for prevention of mother-to-child HIV transmission (PMTCT) [106–109]. (a) Results for an HIV-exposed infant who is born without HIV infection and remains uninfected throughout breastfeeding. In this case, HIV-RNA level remains zero, and maternal HIV antibody fades with time. (b) Results for infants infected before birth, either during the intrauterine period (IU; resulting in a high RNA level immediately after birth) or during the intrapartum period (IP; resulting in a 1-2 week delay before viremia is detectable). Maternal HIV antibody is present at birth; although maternal antibody fades with time, endogenous infant antibody production begins in response to infant infection. (c) Results for an HIV-exposed infant who is uninfected at birth, but becomes infected at approximately 6 months of age through breastfeeding. HIV RNA is undetectable while the infant is uninfected, but rises rapidly within the first few weeks after infection. Maternal antibody is present at birth and begins to fade with time, but infant antibody production begins after infant infection occurs.