Maternal viral genotypic zidovudine (ZDV) resistance and infrequent failure of ZDV therapy to prevent perinatal transmission

To assess the importance of ZDV resistance on mother-to-infant transmission we evaluated maternal plasma specimens from 96 women enrolled in ACTG protocol 076 to determine the prevalence of HIV-1 genotypic ZDV resistance at entry, if ZDV resistance developed on study, and the role of ZDV resistance in vertical transmission of HIV-1 despite ZDV therapy. Low and high levels of genotypic resistance were assessed by differential hybridization, oligoligation or direct sequencing of plasma HIV-1 RNA for codons K70R and T215Y/F, respectively. None of the women had high-level genotypic resistance to ZDV at study entry or at delivery. The upper 97.5% confidence limits for baseline prevalence and delivery incidence of low-level ZDV resistance were 6.8% and 14.0%, respectively. After adjusting for the plasma HIV-1 RNA level at delivery, low-level ZDV resistance was not strongly associated with an increase in vertical transmission risk (odds ration 4.8; 95% CI 0.2-131; P = . 35) . Infrequent failure of ZDV to prevent vertical transmission to some infants was not explained by the presence of maternal virus with genotypic ZDV resistance.