Introduction

Human immunodeficiency virus (HIV) infection is spreading rapidly in women of childbearing age worldwide. Women of childbearing age constitute nearly half of adults currently living with HIV globally [1]. The epidemic of HIV in women of this age group signifies a serious threat to children. The United Nations has estimated that there were approximately 370,000 newly infected children in the year 2007, and majority of which acquired the infection vertically from their mothers [1]. The developing countries, those with the least available resources, have been the most severely affected by the burden of the HIV pandemic. The increasing number of HIV-infected adults, particularly women, makes the prevention of mother-to-child transmission of HIV a public health priority in many developing countries.

Risk Factors for Mother-to-Child HIV Transmission (MTCT)

HIV transmission from a pregnant woman to her infant mostly takes place at around the time of birth [2, 3]. Interventions to interrupt transmission at the time of delivery, such as antiretroviral prophylaxis given in late gestation or peripartum and elective cesarean section, have been shown to be effective in reducing MTCT.

There are multiple risk factors that may affect MTCT. Maternal plasma HIV RNA copy number, also known as HIV viral load, appears to be among the important risk factors for MTCT [4–9]. However, although higher plasma HIV RNA levels have been noted among women who transmitted the virus to their infants, overlap in HIV RNA levels has been observed in women who transmitted and those who did not transmitted the virus [5, 9]. Despite the fact that the risk for MTCT in women with plasma HIV RNA below the detectable level appears to be extremely low, transmission has been reported across the entire range of HIV RNA levels. The HIV RNA threshold below which there is no risk for transmission has not been identified [5, 9]. Nevertheless, antiretroviral prophylaxis has been shown to be effective in reducing MTCT regardless of maternal HIV RNA levels. Since transmission can occur even at low or undetectable HIV RNA copy numbers, HIV RNA levels should not be used as a determining factor when deciding whether to use antiretroviral prophylaxis.

Levels of HIV in maternal genital tract secretion may affect MTCT [6, 10]. Previous studies, demonstrating that risk of MTCT is increased in vaginal as compared with elective cesarean deliveries, following prolonged rupture of amniotic membranes, and among first-born twins, suggest that exposure to infected secretions in the birth canal influences transmission [11–13]. Although there is a general correlation between viral load in plasma and in genital secretion, discordance has also been observed [14, 15]. In the short-course zidovudine (ZDV) trial in Thailand, plasma and cervicovaginal HIV RNA levels were reduced by ZDV treatment, and each independently correlated with the risk of MTCT [10]. Decreased CD4+ T-lymphocyte counts are an indicator of worsening immune deficiency and have been associated with increased MTCT [7]. However, a low CD4+ T-lymphocyte count may be a result of a high plasma HIV RNA level and, therefore, may not be an independent risk factor for MTCT.

Maternal use of illicit drugs such as cocaine and heroin has been associated with a risk up to three-fold higher of transmitting HIV to the infants [16]. Cigarette smoking during pregnancy may also increase the risk of transmission [17]. Discontinuing these practices during pregnancy might reduce the risk of MTCT.

Genital ulcer diseases, including herpes simplex virus (HSV) and syphilis, may increase the risk of MTCT [18, 19]. Higher levels of cervical HSV DNA has been associated with higher levels of expressed HIV and with the more frequent detection of HIV-infected cells in cervical secretions [20]. The administration of valaciclovir to HIV-infected women reduced their genital and plasma HIV viral loads [21]. It is unclear whether treatment of HSV infection in HIV-infected pregnant women will result in further reduction of MTCT.

Vitamin A deficiency and malnutrition can worsen immune deficiency and can cause disruption of mucosal integrity and are associated with increased MTCT. However, an analysis of a randomized trial in South Africa concluded that vitamin A supplementation lacked its efficacy in preventing MTCT [22]. Interestingly, the women receiving vitamin A in this trial were less likely to have a premature delivery, but this was not observed in the Tanzania trial [23]. Among the preterm infants in the South Africa trial, those whose mothers received vitamin A were less likely to be infected than those in the placebo group.

Rapid HIV Testing and the Prevention of MTCT

New HIV infection in infants continue to occur among HIV-infected pregnant women who either did not obtain prenatal care or were not offered HIV testing during pregnancy. These are the most common missed opportunities for prevention of MTCT reported by the US Centers for Disease Control and Prevention [24]. Use of rapid HIV testing for women with unknown HIV status at labor and delivery, because of either lack of prenatal care or lack of testing during pregnancy, would allow initiation of prophylactic strategies to reduce MTCT. In addition, rapid testing will allow identification of an HIV-infected woman for subsequent medical care for her own as well as for her partner. A study of 4849 pregnant women already in labor when they presented to health providers demonstrated that the sensitivity, specificity and positive predictive value of the rapid HIV test were 100%, 99.9% and 90%, respectively [25]. The median turnaround time for the rapid HIV test result was 25 min in one study; over 95% of rapid test results were available in 1 h [26]. The rapid HIV testing has significantly increased the proportion of pregnant women accepting and receiving HIV results in the resource-limited setting [27]. Most facilities in less developed countries have employed rapid HIV testing as part of routine prenatal/intrapartum care.

Modes of Delivery and MTCT

Since majority of MTCT occurs during peripartum period when fetal exposure to maternal body fluid is most likely, efforts have been focused to interrupt transmission during such period. Early studies suggested that cesarean delivery before labor began or before the membranes ruptured (elective cesarean delivery) was effective in reducing the rate of transmission [28, 29]. Subsequently, two large studies have confirmed that MTCT can be reduced by cesarean delivery. In the European Mode of Delivery Collaboration, HIV-infected pregnant women were randomly assigned to elective cesarean section at 38 weeks of pregnancy or to vaginal delivery [30]. Seven (3.5%) of 203 infants who were born by cesarean section were infected, compared with 17 (10.2%) of 167 born by vaginal delivery. Within the cesarean section group, four (2.4%) of 169 infants born by elective cesarean section were infected, compared with 3 (8.8%) of 34 born by emergency cesarean section (after labor began or after the membranes ruptured). Since intrauterine transmission cannot be prevented by cesarean delivery and ZDV cannot completely prevent intrauterine transmission, a small number of infants will still be infected despite all the efforts. Intrauterine transmission however is rare. Of interest, the transmission rate was lower in infants born by vaginal delivery whose mothers received ZDV (3.3%) than in infants born by cesarean section whose mothers did not receive ZDV during pregnancy (6.8%).

In the meta-analysis of more than 7,800 mother-infant pairs, the transmission rate among women undergoing elective cesarean section (8.2%) was significantly lower than that among women having either non-elective cesarean section or vaginal delivery (16.7%) [31]. The rates of HIV transmission were 19% with other modes of delivery but without antiretroviral prophylaxis; 10.4% with elective cesarean section but without antiretroviral prophylaxis; 7.3% with other modes of delivery and antiretroviral prophylaxis during prenatal, intrapartum, and neonatal periods; and 2% with both cesarean section and antiretroviral prophylaxis during all three periods. Interestingly, both studies were prior to the use of highly active antiretroviral therapy (HAART) and the measurement of maternal HIV RNA levels in pregnancy.

The use of HAART, which could significantly reduce the risk for transmission, may diminish the potential impact of elective cesarean delivery upon MTCT. One study demonstrated that women with HIV RNA levels <1,000 copies/mL on combination therapy had transmission rates of 0.8% with elective cesarean delivery and 0.5% with all other delivery modes [32]. In a report from 4,525 women in the European Collaborative Study, the overall transmission rate among women on HAART was 1.2% [33]. Among women with undetectable HIV RNA levels, elective cesarean delivery was associated with a significant reduction in MTCT in univariate analysis. However, after adjustment for antiretroviral therapy, the effect was no longer significant. These data support the benefit of elective cesarean section if HIV RNA is more than 1,000 copies/mL but do not confirm or rule out its benefit among women with HIV RNA <1,000 copies/mL who are receiving HAART.

A cesarean section delivery can have potential adverse effects for the mothers. Several studies reported slightly greater risk of complications among HIV-infected versus uninfected women undergoing cesarean delivery, usually on an urgent rather than elective basis. Complication rates were inversely related to immunologic status and clinical stage of HIV disease [34]. In a large cohort of HIV-infected women, one or more serious complications occurred in 12% of emergency cesarean deliveries, in 6.4% of elective cesarean deliveries, and in 4% of vaginal deliveries. The adjusted relative risk of any postpartum complication was increased by 1.85 after elective cesarean delivery and 4.17 after emergency cesarean delivery, compared with vaginal deliveries [35]. Thus it appears that cesarean delivery carried out electively before labor and with intact membranes have a low risk of complications, whereas cesarean procedures performed on an emergent basis have a higher risk of complications. Women with more advanced HIV disease carry a higher risk of complications. In developing countries, the risk-benefit ratio and the cost-effectiveness of cesarean section are different. The post-operative complications are more likely to occur, so the routine use of cesarean section to prevent MTCT may not be of overall benefit.

The use of antiseptic vaginal and cervical cleansing has been suggested as an inexpensive way to reduce potential viral exposure to infants during delivery. In a trial in Malawi, vaginal disinfection with chlorhexidine 0.25% solution did not reduce MTCT [36]. Results from a study in Kenya using different concentrations (0.2–0.4%) of chlorhexidine also yielded a similar outcome [37]. However, vaginal disinfection before the membranes are ruptured was associated with a reduction of transmission, especially with higher concentrations of chlorhexidine. The trial in West Africa evaluated daily vaginal suppository with benzalkonium chloride during the last month of pregnancy and labor plus bathing of the infants. The results did not demonstrate any benefit of benzalkonium chloride disinfection on MTCT or perinatal and infant mortality [38]. At present there is insufficient and inconclusive evidence on the effect of vaginal disinfection on the risk of MTCT.

Antiretroviral Prophylaxis to Prevent MTCT

Numerous randomized clinical trials have assessed the efficacy of a number of antiretroviral interventions. This article will discuss the results of the published clinical trials in non-breastfeeding and breastfeeding populations. When possible, observational comparisons between the trials will be made to provide ways to better understand prevention strategies.

Clinical Trials of Antiretroviral Prophylaxis in Non-Breastfeeding Populations

In 1994, the Pediatric AIDS Clinical Trials Group (PACTG) Protocol 076, conducted in the US and France, demonstrated the effectiveness of antiretroviral therapy for the prevention of MTCT [39]. In this trial, HIV-infected pregnant women were given oral ZDV, a nucleoside reverse-transcriptase inhibitor, from 14–34 weeks gestation till the onset of labor and intravenous ZDV during labor. The newborns received oral ZDV for 6 weeks. Of interest, most women in this trial had asymptomatic or mildly symptomatic HIV disease. The results showed that the transmission rate for infants in the placebo group was 22.6%, compared with 7.6% for those in the ZDV group, a 66% reduction in risk for transmission [9]. The efficacy of ZDV prophylaxis in pregnant women with more advanced HIV disease and low CD4+ lymphocyte counts were evaluated in the PACTG 185 trial [40]. The objective of this trial was to evaluate the efficacy of the combination of HIV hyperimmune globulin and ZDV versus the combination of immune globulin and ZDV. The ZDV regimen was similar to the PACTG 076 regimen and approximately 25% of women had received ZDV prior to their current pregnancy. The overall transmission rate was only 4.8% and did not differ by whether the women received HIV hyperimmune globulin or immune globulin. The results confirm the efficacy of ZDV and extend its efficacy to women with advanced disease, low CD4+ lymphocyte counts, and prior ZDV use.

The results of PACTG 076 study quickly lead to a widely use of ZDV prophylaxis in HIV-infected pregnant women, particularly in developed countries, to prevent MTCT. However, the high cost of the full PACTG 076 regimen and its complexity restrict the use of such regimen in the resource-limited countries where the intervention is seriously needed. A trial evaluating short-course antenatal/intrapartum ZDV prophylaxis among non-breastfeeding HIV-infected pregnant women was subsequently conducted in Thailand [41]. The regimen included administration of ZDV 300 mg twice daily to pregnant women starting at 36 weeks gestation and 300 mg every 3 h orally during labor until delivery. No ZDV was given to the newborn infants. The transmission rate was reduced by 50% from 18.9% in the placebo group to 9.4% in the ZDV group.

A second trial in Thailand compared four different prophylactic ZDV regimens: antenatal ZDV starting at 28 weeks gestation plus neonatal ZDV for 6 weeks (long-long regimen); antenatal ZDV starting at 28 weeks gestation plus neonatal ZDV for 3 days (long-short regimen); antenatal ZDV starting at 36 weeks gestation plus neonatal ZDV for 3 days (short-short regimen); and antenatal ZDV starting at 36 weeks gestation plus neonatal ZDV for 6 weeks (short-long regimen) [42]. All women received an identical ZDV regimen orally during labor. At an interim analysis, the transmission rate in the short-short regimen was 10.5%, which was significantly higher than the rate in the long-long regimen. At this point, the short-short regimen was stopped. At the end of the study, the transmission rates were 6.5% for the long-long regimen, 4.7% for the long-short regimen, and 8.6% for the short-long regimen. The efficacy of the three regimens was not statistically different. There was a higher rate of in utero transmission, defined by infant’s HIV PCR positivity within 7 days after birth, with the short antenatal arms compared with the long antenatal arms. This suggests that longer treatment of the infant cannot substitute for longer treatment of the mother.

An epidemiologic study in non-breastfeeding population from New York demonstrated that the transmission rate in the absence of antiretroviral intervention was 26.6% but was reduced to 10% in women who did not receive antepartum ZDV but received ZDV intravenously during labor and whose infants were given ZDV for 6 weeks [43]. Even when the antepartum and intrapartum components of prophylaxis could not be given, some benefits could be gained by giving ZDV to infants within 48 h after birth (transmission rate, 9.3%). No benefits were observed if neonatal treatment was initiated on day 3 of life or later. Therefore, antiretroviral prophylaxis should be considered in HIV-infected pregnant women and their infants even if the identification of HIV seropositivity is delayed. The rapid HIV testing should help identify HIV infection in pregnant women, who have no prenatal care or prenatal records, so that antiretroviral prophylaxis can be quickly initiated during this critical period.

The benefit of adding nevirapine (NVP), a nonnucleoside reverse-transcriptase inhibitor, to ZDV in non-breastfeeding population was evaluated in a randomized trial of three ZDV-NVP regimens among pregnant Thai women who were receiving ZDV during the third trimester [44]. In one group mothers and infants received a single dose of NVP (NVP-NVP regimen); in another, mothers and infants received NVP and placebo, respectively (NVP-placebo regimen); and in the last, mothers and infants received placebo (placebo-placebo regimen). All infants also received 1 week of ZDV after birth. At the interim analysis, the transmission rate in the NVP-NVP group (1.1%) was significantly lower than that in the placebo-placebo group (6.3%). At the final analysis, the transmission rate in the NVP-NVP group (1.9%) was not inferior to the rate in the NVP-placebo group (2.8%). No serious adverse effects were associated with NVP therapy. Therefore a single dose of NVP to the mother, with or without a dose of NVP to the infant, added to oral ZDV prophylaxis starting at 28 weeks’ gestation, is effective in reducing MTCT.

In an international multicenter study in the US, Europe, Brazil and Bahamas, mother-infant pairs were randomized to receive either single-dose NVP at labor and single-dose NVP to the infant or a similar placebo regimen [45]. All mother-infant pairs were to receive the PACTG 076 ZDV regimen as a minimum therapy. In this study, more than 75% of women were on combination antiretroviral treatment and one-third had elective cesarean delivery. The transmission rates in the NVP group and the placebo group were 1.4% and 1.6%, respectively. Therefore, there is no benefit from additional intrapartum/newborn NVP when the mothers already receive prenatal care and antenatal antiretroviral therapy in the setting where elective cesarean section, the PACTG 076 regimen and safe formula feeding are available. This is because in such setting the risk of MTCT is already very low.

In developed countries where there is no or minimal resource constraint, HAART has become the standard of care for HIV-infected persons. It is recommended that HIV-infected pregnant women should be offered such therapy for their own health, if indicated, according to current guidelines [46]. A prospective study in USA and Puerto Rico showed the HIV transmission rates of 10.4% in women receiving ZDV monotherapy, 3.8% in those receiving dual antiretroviral therapy, and only 1.2% in those receiving protease inhibitor-containing HAART [47]. The protective effect increased with the complexity of the regimen, and HAART was associated with the lowest rate of MTCT.

Breastfeeding and Mother-to-Child HIV Transmission

Postnatal transmission of HIV through breastfeeding remains a significant problem in resource-limited countries. The risk of breastfeeding transmission has been estimated to range from 25% to 48% [48]. Recognition that HIV could be transmitted through breastfeeding has precipitated a major public health dilemma particularly in the developing world. Long promoted as a mean of preventing malnutrition and decreasing infant morbidity and mortality, especially in resource-poor areas, breastfeeding now poses a significant risk of HIV transmission and is responsible for most of postpartum HIV transmission from mothers to infants. Several studies have helped document the risk of HIV transmission associated with breastfeeding and have suggested that type and duration of breastfeeding are the key factors that affect transmission. Nduati et al conducted a trial in Kenya, in which HIV-infected women were randomly assigned to feed their infants either breast milk or infant formula [49]. The cumulative probability of HIV acquisition among breastfed infants was 37% compared with 21% among the formula-fed infants. Compliance with the feeding protocol was poorer among the women assigned to formula arm; a significant number of infants in this study arm received both breast milk and formula. The effect of type of breastfeeding on HIV transmission was evaluated in South Africa [50]. Mixed feeding, consisting of breast milk and other foods, was associated with highest rates of transmission when compared with exclusive breastfeeding or formula feeding. The study suggests that for women who choose to breastfeed, exclusive breastfeeding may reduce the risk of HIV transmission.

Mbori-Ngacha et al reported the morbidity and mortality among formula-fed and breastfed infants of HIV-infected women in Nairobi, Kenya [51]. Both groups had similar mortality rates and incidences of diarrhea and pneumonia during the first 2 years of life. There was an increased incidence of diarrhea and dehydration in the formula arm during the first 3 months of life, a finding that underscores the protection by breast milk against diarrheal disease in young infants. Infants in the breastfeeding arm had better nutritional status, particularly during the first 6 months of life. However, HIV-free survival (remaining alive and uninfected) at 2 years of age was significantly higher in the formula arm.

Exclusive breastfeeding from 6 weeks to 6 months was found to carry a 4% risk of HIV transmission through breast milk in South Africa [52]. In Zimbabwe, the HIV transmission rate between 6 weeks and 6 months among infants exclusively breastfed for at least 3 months was about 1.3% [53]. The difference in rates could be the results of the methodological differences and the heterogeneity in risk factors for transmission and survival. The Zambian study assessed whether a shortened period of breastfeeding would be of benefit in terms of reduced transmission of HIV and mortality [54]. The rates of HIV-free survival at 24 months were not significantly different between the group that stopped breastfeeding at 4 months (68.4%) versus the group that continued on breastfeeding for as long as the women wanted (64%, with median duration of breastfeeding of 16 months). Infants who became infected by 4 months and had weaned early had a higher mortality by 24 months than the HIV-infected infants who continued on breastfeeding (73.6% versus 54.8%). Abrupt weaning of breastfeeding by HIV-infected women at 4 months in the Zambian setting does not improve the rate of HIV-free survival by 24 months and is harmful to HIV-infected infants.

The most appropriate infant feeding option depends on individual circumstances, including maternal health status and the local situation such as availability of health counseling and support for infant feeding. Only when formula or other replacement feeding is acceptable, feasible, sustainable and safe should such feeding be recommended to HIV-infected women in the resource-poor setting. This is particularly important to lessen the risks of formula feeding. If formula feeding is not a suitable option, means of making breastfeeding safer for the infants of HIV-infected women should be sought. Such means include exclusive breastfeeding for the first 6 months of life, wet-nursing by an HIV-negative woman or family member, and avoidance of nursing when there is breast inflammation, which may increase the risk of HIV transmission [55]. Infants who are known to be infected should continue breastfeeding up to 2 years or beyond. Implementation of the above practices requires significant modification of mixed and prolonged breastfeeding commonly practiced in much of developing world [56]. Specific counseling should be provided to HIV-infected expectant women so that each woman can select the feeding method that maximizes benefits and minimizes risks given her individual situation. There is increasing evidence that the use of antiretrovirals by women or their infants during breastfeeding will likely lower the risk of HIV transmission to infants via breastfeeding, as described later in this article.

Clinical Trials of Antiretroviral Prophylaxis in Breastfeeding Populations

Numerous clinical efficacy trials have been reported in breastfeeding population and all were conducted in Africa. The RETRO-CI study and the DITRAME study in Côte d’Ivoire used a treatment regimen identical to the first Thailand ZDV regimen. The women started oral ZDV 300 mg twice daily at 36–38 weeks gestation and repeated doses of 300 mg every 3 h from beginning of labor until delivery. Data were pooled from both studies and the efficacy of the short peripartum ZDV regimen at 24 months of age was assessed [57]. The cumulative risks of transmission were 30.2% in the placebo group and 22.5% in the ZDV group, a 26% reduction in transmission. The HIVNET012 study was conducted in Ugandan breastfeeding women [58, 59]. The regimen consisting of a single 200 mg dose of oral NVP given to women at onset of labor and a single 2 mg/kg dose to infants within 72 h after birth was compared with oral ZDV given to women every 3 h during labor and to the infants for 7 days. The estimated risks of transmission in infants by age 18 months were 25.8% in ZDV arm and 15.7% in NVP arm. The efficacy of NVP to reduce transmission compared with ZDV was 41%. Because there was no placebo group in this trial, no conclusions can be drawn regarding the efficacy of NVP or of the short ZDV regimen versus no treatment. The simplicity, the efficacy and the low cost of NVP regimen now make a large-scale implementation to prevent MTCT even in the most resource-poor settings possible.

The PETRA trial in Uganda, Tanzania and South Africa comprised four study arms to evaluate the effectiveness of three short regimens of a combination of ZDV plus lamivudine (3TC) [60]. HIV-infected mothers were randomized to one of four regimens: A, ZDV 300 mg plus 3TC 150 mg twice daily starting at 36 weeks gestation, followed by oral intrapartum dosing of ZDV every 3 h and 3TC every 12 h and by 7 days postpartum dosing in mothers and infants; B, as regimen A, but without the antepartum component; C, intrapartum ZDV and 3TC only; or D, placebo. The combined HIV transmission and infant mortality rates at 6 weeks of age were 7.0% for group A, 11.6% for group B, 17.5% for group C, and 18.1% for group D. HIV infection rates at 18 months were 15%, 18%, 20% and 22%, respectively. Even though regimens A and B were effective at week 6 after birth, their benefits diminished considerable after 18 months, most likely due to breastfeeding transmission.

The SAINT study assessed the efficacy of NVP versus ZDV + 3TC in South Africa [61]. For the NVP arm, mothers received a 200 mg dose of NVP at labor and again at 24–48 h postpartum, and infants received a 6 mg dose of NVP at 24–48 h after birth. For the ZDV/3TC arm, mothers during labor received ZDV 600 mg for one dose then 300 mg every 3 h and 3TC 150 mg every 12 h followed by 7 days postpartum dosing in mothers and infants. The overall estimated HIV infection rates in infants by 8 weeks were 12.3% for NVP group and 9.3% for ZDV/3TC group. Excluding infections detected within 72 h (intrauterine infection), new HIV infections were detected in 5.7% and 3.6% of infants in the NVP and ZDV/3TC groups, respectively, at 8 weeks after birth. This is consistent with the HIVNET012 and PETRA data and suggests that the short intrapartum and postpartum NVP and ZDV-3TC regimens have similar efficacy.

The HIVNET012, PETRA, and SAINT trials provide good evidence that intrapartum and early postpartum antiretroviral prophylaxis reduce MTCT substantially among breastfeeding populations. These interventions target the critical period around labor and delivery when the highest rate of MTCT is known to occur.

Antiretroviral Prophylaxis for Infants to Reduce Breastfeeding Transmission

The utility of prophylactic antiretrovirals given to infants to minimize breastfeeding transmission was assessed in MASHI, SWEN, PEPI and MITRA studies. MASHI study in Botswana is a 2x2 factorial randomized trial with peripartum (single-dose NVP versus placebo, with maternal ZDV prophylaxis stating at 34 weeks gestation) and postpartum infant feeding (formula versus breastfeeding plus 6-month infant ZDV prophylaxis) interventions [62]. The 7-month infection rates were higher but mortality was lower in the breastfed + ZDV group but this difference diminished beyond month 7. Cumulative mortality or HIV infection rates in infants at 18 months were indifferent between the 2 feeding groups (13.9% in formula group versus 15.1% in breastfed + ZDV group). ZDV did not decrease breastfeeding transmission as effectively as formula feeding, but was associated with a lower mortality.

SWEN study is combined analysis of studies in Ethiopia, Uganda and India comparing single-dose maternal/infant NVP alone to single-dose NVP plus extended daily infant NVP through age 6 weeks [63]. There was a 46% decrease in postnatal HIV infection at age 6 weeks in infants uninfected at birth with extended NVP compared with the control group. However the rates of infection at age of 6 months were not different, suggesting that a longer course of daily infant NVP to prevent HIV transmission via breast milk might be more effective where affordable and safe formula feeding is not available.

A randomized study in Malami, PEPI, evaluated a longer regimen of infant prophylaxis, comparing a control regimen of single-dose maternal/infant NVP plus 1 week of infant ZDV to the control regimen plus 14 weeks of daily infant NVP or 14 weeks of infant ZDV/NVP [64]. Among infants who were not infected at birth, the control group had consistently higher rates of HIV infection from the age of 6 weeks through 18 months. The estimated protective efficacy of the extended regimens when compared with the control group was 66–67% at 14 weeks, 49–60% at 6 months, and 40–51% at 9 months. There were no differences in efficacy between the two extended-prophylaxis groups except more neutropenia in the extended ZDV/NVP group. Of note, there remained a continued risk of transmission to infants who continued to be breastfed after discontinuation of prophylaxis.

The Use of HAART to Prevent MTCT in Resource-Poor Settings

The use of HAART is increasingly feasible in a dynamic world of changing prices as well as expanding experiences, international resources and commitment to increasing access to quality health services in developing countries. In addition to the benefit of HAART on maternal health, it is possible that the substantial decrease in maternal HIV viral load in breast milk from the postpartum regimens may reduce the risk of breastfeeding transmission to infants. In the observational Mitra-Plus study, HAART consisting of ZDV + 3TC + NVP was provided to HIV-infected pregnant women starting at 34 weeks gestation and continued through up to 6 months of breastfeeding [65]. The cumulative risk of HIV infection among infants was 5% at 6 months and 6% at 18 months of age. The risk of postnatal infection through breastfeeding between 6 weeks and 6 months was only 1%. This risk was substantially lower than the risks reported in the above-mentioned studies in which there was no extended postpartum prophylaxis.

KIBS study in Kenya used ZDV + 3TC + NVP in HIV-infected pregnant women from 34 weeks gestation to 6 months postpartum [66]. NVP was later changed to nelfinavir in women with CD4 count >250 cells/mm3 due to fatal hepatotoxicity related to NVP reported in women with higher CD4 count [67]. Infants received single-dose NVP at birth. Women were advised to exclusively breastfeed and wean rapidly at 6 months. Cumulative HIV transmission rates were 3.9% at 6 weeks, 5% at 6 months and 5.9% at 12 months. Low 12-month infant HIV transmission rates were achieved using maternal HAART from late pregnancy through 6 months of breastfeeding. There was no difference in transmission based on maternal CD4 or regimen used.

The Kesho Bora study randomized women to receive HAART (ZDV + 3TC + lopinavir/ritonavir) starting at 28–36 weeks gestation through 6-month postpartum compared with antepartum ZDV starting at 28–36 weeks, with single-dose NVP a labor and 1 week of postnatal ZDV/3TC [68]. All infants received single-dose NVP and 1 week of ZDV. The rates of in utero infection were similar between the two groups. However cumulative infection rates at 6 months were lower (4.9% versus 8.2%) in the HAART group. The rate of infection between 6 and 12 months when prophylaxis was discontinued was similar in the two groups, 0.7% in the HAART group and 1% in the short-course ZDV/3TC group.

The BAN study is a 3x2 factorial design in which mother-infant pairs were randomized within 1 week of birth to one of 3 interventions: 1) maternal ZDV + 3TC + lopinavir/ritonavir twice daily up to 28 weeks during breastfeeding (MHAART), 2) infant NVP daily up to 28 weeks during breastfeeding (INVP), or 3) no additional drugs (control), and 2 nutritional arms (with or without maternal nutritional supplement) [69]. During intrapartum, all mothers and infants received single dose NVP and 1 week of ZDV + 3TC. Mothers breastfed exclusively for 24 weeks. Among 2637 mother-infant pairs, in utero transmission as reflected by HIV infection at 1 week was 4.9%. The estimated risk of HIV transmission by 28 weeks in those uninfected at 1 week was higher in the control arm (6.4%) compared to either of the intervention arms (3.0% in MHAART and 1.8% in INVP arm). The estimated risk of HIV transmission or death by 28 weeks was higher (7.6%) in the control arm compared to 4.7% in the MHAART arm and 2.9% in the INVP arm. Therefore the use of HAART in mothers postpartum or the use of extended NVP prophylaxis in infants is effective in reducing HIV transmission during breastfeeding.

The Mma Bana trial in Botswana compared two HAART regimens, a triple nucleoside regimen versus a protease inhibitor-based regimen, in HIV-infected pregnant women starting at 26–34 weeks gestation and through 6 months of breastfeeding [70]. Plasma HIV RNA was < 400 copies/ml in more than 90% of women at delivery or throughout breastfeeding and did not differ by treatment arms. The cumulative rates of infant infection were low, 2% in triple nucleoside and < 1% in protease inhibitor-based regimen (p = 0.53). Majority of infants (71%) were breastfed for at least 5 months. Infant 6-month mortality was 2–3% in this study. Provision of HAART to women from pregnancy through 6 months postpartum allowed for safe breastfeeding, with an overall HIV transmission rate to infants of ≤ 2%.

A nonrandomized interventional cohort study in Rwanda allowed HIV-infect pregnant women to choose the mode of feeding for their infants: breastfeeding with maternal HAART for 6 months or formula feeding [71]. All received HAART from 28 weeks of gestation. Of the 532 infants, 43% were breastfeeding and 57% were formula feeding. Overall, seven (1.3%) children were HIV-infected of whom six were infected in utero. Only one child in the breastfeeding group became infected between months 3 and 7, corresponding to a 9-month cumulative risk of postnatal infection of 0.5% with breastfeeding. HIV-free survival by 9 months was 95% in the breastfeeding group and 94% for the formula feeding group. In the DREAM program in Mozambique, HIV-positive pregnant women received HAART antenatally and up to 6 months after delivery, nutritional supplementation, counseling and lifelong care [72]. The cumulative infection rate at 6 months of age was 2.7% for formula-fed infants and 2.2% for breastfed infants. Overall the transmission rates were extremely low and comparable to those seen in developed countries. The mortality rate at 6 months of age was 27 per 1000 person-years among formula-fed infants and 28.5 per 1000 person-years in breastfed infants—both considerably lower than the rates of 101 per 1000 person-years observed in Mozambique. In conclusion, maternal HAART while breastfeeding significantly reduces the risk of breast milk HIV transmission to the infant and this strategy could be a promising alternative in resource-limited countries.

Safety of Antiretroviral Therapy in Pregnancy

Nucleoside reverse-transcriptase inhibitors such as ZDV, 3TC, stavudine and didanosine are generally well tolerated and cross the placenta. Teratogenic effects have not been shown in animals in concentrations similar to those used in humans. Nucleoside reverse-transcriptase inhibitors bind to mitochondrial DNA polymerase gamma and can cause mitochondrial dysfunction, which may be manifested as myopathy, cardiomyopathy, neuropathy, lactic acidosis, fatty liver or hepatic failure [73]. Three cases of fatal lactic acidosis were reported in women who were either pregnant or postpartum and whose antiretroviral therapy during pregnancy included stavudine and didanosine in combination with other antiretroviral agents. Although hepatic failure and lactic acidosis have been reported most commonly with long-term use of stavudine and didanosine, the potential exists with all nucleoside reverse-transcriptase inhibitors. Physicians and pregnant women should be aware of the nonspecific clinical presentations of liver dysfunction and lactic acidosis. Mitochondrial dysfunction might develop in infants who are exposed to nucleoside reverse-transcriptase inhibitors. Possible mitochondrial dysfunction with severe outcome were reported in a large cohort of infants exposed to ZDV or ZDV/3TC in utero and during neonatal period [74, 75]. In a review of more than 16,000 children exposed to antiretroviral drugs in USA, no increase in death rates was found among children exposure to nucleoside reverse-transcriptase inhibitors as compared with children with no such exposure [76, 77]. No deaths were found to be definitely related to mitochondrial dysfunction. No increases in the incidence of tumors were found in a similar study using lower doses [78]. The data from PACTG 076 and the Women and Infants Transmission Study demonstrated no tumors of any nature among ZDV-exposed, HIV-uninfected children during the follow-up period of up to 6 years [79]. Thus, the potential risk of antiretroviral drugs in mothers and infants appears to be very small and should be compared against the clear benefit of antiretroviral prophylaxis in reducing perinatal transmission of a yet-fatal HIV infection.

Nonnucleoside reverse-transcriptase inhibitors such as NVP and efavirenz readily cross the placenta in primates. Use of efavirenz in early pregnancy is not recommended because of birth defects, including anencephaly and anophthalmia observed in cynomolgus monkeys after exposure in utero [46]. Fatal hepatotoxicity related to NVP has been reported in women with CD4 count above 250 cells/mm3 [67]. Therefore NVP should be avoided in women with CD4 count above 250 cells/mm3. The common toxic effect of nonnucleoside reverse-transcriptase inhibitors is rash and it could progress into severe reactions with fatal outcome. In HIVNET 012 trial in which NVP versus ZDV was used, the rates of serious adverse events and clinical and laboratory abnormalities were similar between the two study groups [58].

There appears to be minimal transplacental passage of protease inhibitors in humans [46]. Pre-clinical reproductive toxicity studies, carcinogenicity and mutagenicity studies have been reviewed extensively. According to the US Food and Drug Administration (FDA) classification, indinavir, fosamprenavir, darunavir, tipranavir, and lopinavir are in FDA pregnancy category C. Nelfinavir, atazanavir, ritonavir, and saquinavir are in category B. Category C means that safety in human pregnancy has not been determined and animal studies are either positive for fetal risk or have not been conducted, whereas Category B means that animal studies fail to demonstrate a risk to fetus and well-controlled studies in pregnant women have not been conducted. The toxicity of protease inhibitors among pregnant women appears to be similar to that among non-pregnant women. Hyperglycemia, new-onset diabetes mellitus, exacerbation of existing diabetes, and lipodystrophy have been reported. Pregnancy is itself a risk factor for hyperglycemia; it is unknown if protease inhibitors will increase the risk for pregnancy-associated hyperglycemia. An appropriate diagnostic and monitoring approach to hyperglycemia is essential when the use of protease inhibitors is planned during pregnancy.

The combined data of 3920 mother-child pairs from the European Collaborative Study and the Swiss Mother + Child HIV Cohort Study were evaluated for the relationship between type and timing of initiation of antiretroviral therapy and the outcome of pregnancy [80]. Exposure to ZDV monotherapy was not associated with prematurity, but severe immunosuppression and illicit drug use were. Combination antiretroviral therapy was associated with an increased risk for prematurity, with the multivariate odds ratios of 2.6 and 1.8 for infants exposed to combination therapy with and without protease inhibitors, respectively, compared to no therapy. In contrast, in a large cohort of HIV-infected US women who received antiretroviral therapy during pregnancy, the risk of prematurity or low birth weight was not associated with the use of either monotherapy or combination therapy (with or without protease inhibitors) [81]. The reason for the differences between the two studies is unclear. It is possible that there are other factors not being identified in the studies, such as the status of maternal disease or HIV RNA load, that could explain the differences. A recent study from two large cohorts indicated that pregnancy itself was significantly associated with increased hepatotoxocity in HIV-infected women receiving antiretroviral therapy [82]. These findings however highlight the need for health care providers to discuss risk/benefit and therapy options with HIV-infected pregnant women.

Viral Resistance and MTCT

Viral resistance to antiretroviral drugs has become a concern particularly with the increasing global use of antiretroviral therapy to prevent MTCT. The prevalence of resistance mutations in pregnant women varies. The resistance to ZDV in women and infants after ZDV prophylaxis was rare in the PACTG 076 study and the resistance mutations was not associated with an increased risk of MTCT [83, 84].

As single-dose NVP is the most widely used intervention to prevent MTCT in resource-poor settings, NVP resistance is an important issue that deserves an attention. NVP resistance mutations was evaluated in a large population in HIVNET 012 trial [85]. The mutations were detected in 21 (19%) of 111 women tested at 6–8 weeks after delivery, and the most common mutation found was K103N. A meta-analysis study reported the pooled estimate of postpartum NVP resistance prevalence of 35.7% in women exposed to single-dose NVP. The prevalence was lower (4.5–18%) when women also received additional postpartum antiretrovirals including HAART [86, 87]. The NVP resistance mutations typically fades to undetectable levels in plasma within several months after delivery [85], although the persistence of resistant virus in plasma for up to 5 years and in latent reservoirs of resting CD4 T cells for up to 3 years has been reported [88, 89]. Nevertheless, the efficacy of single-dose NVP to prevent MTCT was not diminished in subsequent pregnancies among women who had previously exposed to single-dose NVP [90]. This suggests that the single-dose NVP would remain effective in subsequent pregnancies. Implications concerning the transient development of NVP resistance from single-dose NVP for future treatment of the mothers could be problematic. Women in Mashi study in Botswana who began NVP-based HAART for her own health within 6 months after receiving single-dose NVP had higher rates of virologic treatment failure than those who received NVP-based HAART without previous exposure to NVP (42% versus 0%, respectively) [91]. In contrast, the virologic failure rates between women who began NVP-based HAART at 6 months or more after exposing to single-dose NVP and those who received NVP-based HAART without previous exposure to NVP did not differ significantly (12% versus 8%, respectively). A similar finding from Thailand showed that viral suppression to less than 50 HIV RNA copies/ml with NVP-based HAART was achieved in 49% of women who had received intrapartum single-dose NVP, as compared with 68% in those who had not [92]. Therefore NVP-based HAART may not be considered as a treatment of choice for women who previously received single-dose NVP especially if it was within the past 6 months.

NVP resistance mutations were also observed in 46% HIV-infected infants in HIVNET012 study, with Y181C as the most common mutation [85]. The mutation difference in women and infants implies that NVP-resistant virus is selected independently in infants after NVP exposure. Similarly to the women, the mutations faded from detection in infants over time. This is most likely due to the reduced fitness of NVP resistant strains and the replacement by wild-type strain in the absence of the drug. Infants who received extended daily NVP prophylaxis for up to 6 weeks to prevent breastfeeding transmission were more likely to develop NVP resistance than infants who received only a single-dose NVP at birth (84% versus 50%) and their NVP resistance tended to persist longer up to 6 months [93]. Among infected infants in Mashi study who received antiretroviral treatment, virologic treatment failure by the 6-month visit occurred significantly more in infants who had received single-dose NVP than in infants who had not [91].

Given the high likelihood of viral resistance associated with single-dose NVP use alone and the uncertain clinical implications in settings where NVP is included in first-line antiretroviral treatment regimen, a short-course postpartum regimen should be strongly considered to reduce the risk of resistance development. However, the potential for selection of NVP-resistant HIV in women and infants receiving NVP prophylaxis must be weighed against the clear benefits of this simple, practical and effective regimen in preventing MTCT.

Public Health Perspectives on Prevention of MTCT

Any specific intervention package to reduce MTCT should be fully integrated in the overall antenatal, obstetrical, and pediatric care, with the primary goal of reducing overall maternal and infant morbidity and mortality. Such a package should include maternal sexually transmitted disease screening and treatment, routine immunization, iron supplementation, appropriate nutritional education, basic obstetric care, information on HIV prevention and care, voluntary HIV counseling and testing, partner counseling, appropriate infant feeding and family planning options. Appropriate antiretroviral prophylaxis should be discussed and provided. Infant feeding options should be thoroughly communicated with pregnant women. If women opt for breastfeeding, exclusive breastfeeding should be strongly supported. Those who decide not to breastfeed their children must be ensured access to sufficient quantities of nutritionally adequate breast milk substitutes they can prepare safely. Continuous care of women and their infants should be addressed with specific emphasis on the prevention against opportunistic infections, antiretroviral therapy, and determination of infant’s HIV infection status. Serious consideration should be given to strategies to prevent new HIV infection and unplanned pregnancy among childbearing women. Implementation of such strategies will require a renewed commitment to increasing access to quality health services. The public health imperative is clear: large-scale efforts can prevent hundreds of thousands of pediatric HIV infection, provide hope for millions of HIV-affected families and reverse the recent precipitous declines in child survival in developing world.