EPIDEMIOLOGY AND SOCIAL SCIENCE Estimating Vertically Acquired HIV Infections and the Impact of the Prevention of Mother-to-Child Transmission Program in Zimbabwe Insights From Decision Analysis Models Sabada Dube, MPH,* Marie-Claude Boily, PhD,* Owen Mugurungi, MD,† Agnes Mahomva, MD,† Frank Chikhata,† and Simon Gregson, PhD*‡ Background: The World Health Organization recommends a single-dose nevirapine (NVP) regimen for prevention of mother-to- child transmission (PMTCT) of HIV in settings without the capacity to deliver more complex regimens, but the population-level impact of this intervention has rarely been assessed. Methods: A decision analysis model was developed, parameterized, and applied using local epidemiologic and demographic data to estimate vertical transmission of HIV and the impact of the PMTCT program in Zimbabwe up to 2005. Results: Between 1980 and 2005, of approximately 10 million children born in Zimbabwe, a cumulative 504,000 (range: 362,000 to 665,000) were vertically infected with HIV; 59% of these infections occurred in nonurban areas. Mother-to-child transmission (MTCT) of HIV decreased from 8.2% (range: 6.0% to 10.7%) in 2000 to 6.2% (range: 4.9% to 8.9%) in 2005, predominantly attributable to declining maternal HIV prevalence rather than to the PMTCT program. Between 2002 and 2005, the single-dose NVP PMTCT program may have averted 4600 (range: 3900 to 7800) infections. In 2005, 32% (range: 26% to 44%) and 4.0% (range: 2.7% to 6.2%) of infections were attributable to breast-feeding and maternal sero- conversion, respectively, and the PMTCT program reduced infant infections by 8.8% (range: 5.5% to 12.1%). Twice as many infections could have been averted had a more efficacious but logistically more complex NVP + zidovudine regimen been implemented with similar coverage (50%) and acceptance (42%). Discussion: The decline in MTCT from 2000 to 2005 is attributable more to the concurrent decrease in HIV prevalence in pregnant women than to PMTCT at the current level of rollout. To improve the impact of PMTCT, program coverage and acceptance must be increased, especially in rural areas, and local infrastructure must then be strengthened so that single-dose NVP can be replaced with a more efficacious regimen. Key Words: decision analysis models, HIV, mother-to-child trans- mission, prevention of mother-to-child transmission, vertical trans- mission (J Acquir Immune Defic Syndr 2008;48:72–81) I n 2001, the United Nations General Assembly Special Session on HIV/AIDS (UNGASS) passed a declaration of commitment to reduce global mother-to-child transmission (MTCT) of HIV by 20% by 2005 and by 50% by 2010. 1 The commitment stressed that the goal should be achieved by implementing a range of public health interventions guided by nationally coordinated strategies. These include ensuring that 80% of pregnant women with access to antenatal care are provided with HIV prevention services, including voluntary counseling and testing (VCT) and antiretroviral therapy (ART). Prolonged efficacy of ART for prevention of mother-to- child transmission (PMTCT) of HIV has been confirmed in clinical studies in non–breast-feeding populations in middle- and high-income countries. 2–5 Efficacy is typically somewhat lower in breast-feeding populations in Africa. 6–12 In devel- oping countries, the effectiveness of these ART regimens at the population level remains unknown, 13 because empiric studies to evaluate their effectiveness are costly. In addition, where attempts have been made to measure the effectiveness of these interventions in real-life settings, sample sizes have been too small for the findings to be generalized. 14,15 Mathematic models, parameterized using local data, can be a useful tool to help explore the likely effectiveness of these interventions and to provide information to improve program impact. 16,17 With HIV prevalence in pregnant women attending antenatal clinics (ANCs) recorded at 32.1% in 2000 and at 23.9% in 2004, 18 the risk of MTCT of HIV in Zimbabwe, as in Received for publication August 28, 2007; accepted February 4, 2008. From the *Department of Infectious Disease Epidemiology, Imperial College London, London, United Kingdom; †Ministry of Health and Child Welfare, Harare, Zimbabwe; and the ‡Biomedical Research and Training Institute, Harare, Zimbabwe. Funded by the US Centers for Disease Control and Prevention through Cooperative Agreement U62/CCU022406-01. Disclaimer views expressed in this paper are those of the authors and do not reflect the views of the United States Department of Health and Human Services/Centers for Disease Control and Prevention. Correspondence to: Sabada Dube, MPH, Department of Infectious Disease Epidemiology, Imperial College London, St. Mary’s Campus, Norfolk Place, London, W2 1PG, United Kingdom (e-mail: dube70@yahoo.co.uk). Copyright Ó 2008 by Lippincott Williams & Wilkins 72 J Acquir Immune Defic Syndr  Volume 48, Number 1, May 1, 2008 Copyright © 2008 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.