Background: HIV superinfection (SI) occurs when an infected individual acquires a distinctly new HIV strain, and can occur at rates comparable to primary HIV incidence. This study examined the risk of post-partum SI in HIV-infected women from the Post-Exposure Prophylaxis of Infants (PEPI) trial in Malawi, and evaluated if SI is associated with an increased risk of postnatal mother-to-child transmission (MTCT). PEPI demonstrated that infants who received 14 weeks of extended antiretroviral prophylaxis (ARV; nevirapine or nevirapine plus zidovudine) were at lower risk of acquiring HIV through breastfeeding than infants who received a short course of ARV prophylaxis.
Methods: Samples from women who transmitted HIV to their babies via breast milk (transmitters; infants were HIV- at six weeks of age, but positive later) were screened for HIV-SI (n=91). The 91 cases were matched as a group to women who did not transmit to their babies (non-transmitters) by study arm, follow-up time and sample availability. Samples from delivery and a follow-up visit for each woman were amplified and pyrosequenced in two genomic regions (p24 and gp41). HIV SI was established if the follow-up sample contained a new, phylogentically distinct viral population that was a greater genetic distance from the original viral strain than what would be expected based on viral evolution. HIV SI and MTCT risk were examined by logistic regression, and adjusted for study arm, viral load and CD4 count at delivery, time to resumption of sex, and breastfeeding duration.
Results: Postpartum MTCT was associated with lower initial CD4 counts (p=0.001) and higher viral loads (p<0.0001) at delivery. There were eight SIs in the transmitter group (8/91) with a rate of SI of 7.5/100 person years (pys). This compares to six SIs in the non-transmitter group (6/91) for a rate of SI of 5.3/100pys (p=0.78). HIV SI was not associated with an increased risk of postnatal MTCT after adjusting for factors that influence transmission (adjusted odds ratio=2.73, 95% confidence interval=0.67-11.17; p=0.16).
Conclusions: In this study, there was a relatively high rate of HIV SI in postnatal HIV-infected women. HIV SI did not increase the risk of postnatal MTCT in this study, which may be due to the limited sample size and the extended ARV prophylaxis in a portion of the infants. Further research in different populations will be needed to determine the relationship between HIV SI and transmission.