Short-course antiretroviral regimens have been shown to be effective in reducing peripartum HIV transmission but their beneficial effect is partially nullified by the subsequent transmission associated with breastfeeding [1–3]. In the industrialized countries, the avoidance of breastfeeding has virtually abolished postnatal transmission. However, in resource-limited countries, breastfeeding has nutritional, immunological, economical, logistical and social advantages that strongly influence maternal choice. It has been hypothesized that the type of breastfeeding could play an important role in transmission. Exclusive breastfeeding (breastmilk only with no other feed or fluid) was reported to be associated with postnatal transmission rates similar to those obtained with formula feeding and lower than those in mixed-fed infants (infants who receive breastmilk with other fluids) [4,5]. Although these data derive from a single observational study, World Health Organization guidelines for infant feeding for HIV-positive women recommend exclusive breastfeeding for the first months of life, when replacement feeding is not feasible .
The aim of this study was to evaluate the relation between mode of infant feeding and HIV transmission in a cohort of children born to HIV-positive mothers enrolled in a prevention of mother-to-child transmission (PMTCT) program at St Francis Hospital Nsambya, Kampala, Uganda.
HIV-positive pregnant women enrolled in the PMTCT program from 30 September 2000 to 30 October 2002 were asked to participate in the study. The purpose of the study was explained and consent forms reporting aims and methods were signed by participating women. Short-course regimens of zidovudine  or nevirapine  were administered according to Ugandan PMTCT guidelines. Compliance with drugs was considered adequate when women reported to have taken the drugs as prescribed and, for zidovudine, for at least 1 week before delivery. Women planning to breastfeed were counselled to adopt exclusive breastfeeding for not more than 6 months, to wean rapidly and to avoid mixed feeding. Formula feeding was provided in powder, free of charge, to mothers willing to refrain from breastfeeding; specific counselling was provided and the home environment was evaluated for feasibility of safe use of formula feeding. Size of the house, number of the people living in the house, presence of a kitchen as a separate space and presence of clean kitchen utensils were assessed in order to support mothers in their feeding choice. Women–children pairs were seen after delivery at different time points (weeks 1, 6, 10 and 14, then months 4, 5 and 6). Information about feeding practices and about mother–infant health was collected at each scheduled visit during follow-up. Counsellors conducted unscheduled home visits to verify feeding practices. At 6 weeks and 6 months after delivery, HIV RNA or DNA was measured by the polymerase chain reaction (Amplicor v 1.5, Roche Molecular Systems, Branchburg, New Jersey, USA) on samples collected from enrolled children. Pediatric HIV infection was defined by one positive HIV RNA or DNA result.
For the analyses, women–children pairs were initially classified according to the choice of feeding at delivery into two groups: breastfeeding (BF) and formula feeding (FF). In a secondary analysis, women–children pairs were reclassified into three groups according to the information about infant feeding collected during follow-up: exclusive breastfeeding (EBF), exclusive formula feeding (EFF) and mixed feeding (MF). The EBF group comprised children who received only breastmilk with no other concomitant fluid or feed. Children who were weaned early and children who stopped breastfeeding before 4 months and were switched to formula feeding were included in the EBF group. The EFF group included infants who were formula-fed only and who were never breastfed. The MF group included children who received breastmilk plus other food or fluids or formula feeding plus breastfeeding. Statistical analyses were performed using STATA 8.0 software (Stata Corp., College Station, Texas, USA). HIV transmission rates were calculated as the number of HIV-infected infants divided by the number of mother–infant pairs. Univariate analyses compared rates of transmission by Fisher's exact test. The relative risks of transmission in the secondary analyses were calculated by Kaplan–Meier life tables and the multivariate Cox's proportional hazards model. All statistical tests were two sided, and P values of < 0.05 were considered significant.
Characteristics of the mother–child pairs enrolled in the study are reported in Table 1.
The analyses included 306 children who attended the week 1 visit, 181 in the BF group and 125 in the FF group. The HIV status was determined for 304 infants at week 6: 27 children were diagnosed with HIV infection (transmission rate, 8.9%). Of the children who were HIV negative at week 6, 28 were lost to follow-up before month 6 (17 in the BF group and 11 in the FF group). Six new infections were newly diagnosed at the month 6 visit: 33 out of 276 children evaluated (12.0%) were HIV infected. Analysing these data according to the mode of infant feeding (as indicated at delivery), transmission rates at week 6 were 12.8% [95% confidence interval (CI), 8.3–18.6)] and 3.3% (95% CI, 0.9–8.2) in the BF and FF groups, respectively; at month 6, transmission rates were 16.7% (95% CI, 11–23) and 5.3% (95% CI, 2–11), respectively (Table 1).
Child growth and the cumulative rate of adverse events in children during follow-up were similar in the BF group and the FF group; fewer episodes of respiratory tract infections were reported in the FF group (12.6%) than in the BF group (16.5%) (P = 0.04).
Breastfeeding was still occurring in the BF group in 83% at week 6, 37% at month 3 and 9% at month 6; 45 (25%) of the women in this group reported to have given mixed feeding to children before weaning. Among the FF group, 13 women (11%) reported to have breastfed their children at least once, and eight of these practiced mixed feeding regularly. There were 52 home visits (29 in the BF group, 23 in the FF group) and only two minor discrepancies with respect to the information collected at follow-up visits were found, thus confirming the validity of women's reports.
In a secondary analysis, children were reclassified in three groups, according to mothers’ reports on feeding modalities: EFF, EBF and MF. The cumulative probability of HIV infection in the three groups is shown in Fig. 1. The differences among the groups were already highly significant at week 6, when only 4 of 117 children (3.4%) in the EFF group were infected compared with 17 of the 152 (11.2%) in the EBF group and 6 of the 35 children (17.1%) in the MF group. At month 6, transmission rates were 3.7% (4/108) in the EFF group, 16.0% (19/119) in the EBF group and 20.4% (10/49) in the MF group. Among the 306 enrolled children, there were five deaths (three were HIV positive) before 6 months: one in the EFF group, three in the EBF group and one in the MF group. In a multivariate Cox's model, EBF and MF groups were associated with a significantly higher risk of HIV infection (hazard ratio for the EBF group, 4.4; 95% CI, 1.5–13.1; P = 0.007; hazard ratio for the MF group, 6.6; 95% CI, 2.0–22.1; P = 0.002) in comparison with EFF group. Poor compliance with antiretroviral drugs (hazard ratio, 2.3; 95% CI, 1.1–4.7; P = 0.03) and symptomatic HIV infection (WHO stage II–IV) of the mothers at enrolment (hazard ratio, 2.3; 95% CI, 1.0–5.4; P = 0.05) were also associated with HIV transmission; elective caesarean section and status of perineum were not associated. A similar multivariate model, not including EFF group, showed no statistically significant risk difference between the EBF group and the MF group (hazard ratio for the MF group, 1.4; 95% CI, 0.6–3.3; P = 0.4).
Programs for the prevention of mother-to-child HIV transmission are being implemented on an increasing scale in developing countries. Introduction of short-course antiretroviral regimens can strongly decrease peripartum transmission, but it does not provide protection from breastfeeding-associated transmission. The contribution of transmission associated with breastfeeding to overall transmission has been estimated to be between 23 and 42% ; therefore, prevention of postnatal HIV transmission is a public health priority [10–12]. It had been hypothesized [4,5] that exclusive breastfeeding could represent a safe option (as safe as formula feeding) in comparison with mixed feeding (which is usually the norm in Africa). The explanation of this difference was believed to be the capacity that mixed feeding had to increase gut permeability and to induce local inflammation, thus facilitating the passage of HIV.
In our study, we were able to verify the compliance with feeding modalities. Indeed, we found that, on the one hand, 25% of the women who had chosen exclusive breastfeeding actually gave other food to their infants, demonstrating that, as expected, strict exclusive breastfeeding was difficult to practice because of cultural or other reasons. On the other hand, a significant proportion (11%) of the women who had chosen replacement feeding reported to have breastfed at least once, probably because of social pressure. We were, therefore, able to identify three groups of infants according to the infant feeding modalities and to measure the associated rates of transmission.
Our findings clearly showed that exclusive breastfeeding was associated with an increased risk of HIV transmission in comparison with formula feeding and that there was no significant difference between the risk associated with exclusive breastfeeding and that associated with mixed feeding.
Previous studies  have suggested that the first month of life may be a particularly high-risk time for transmission of HIV through breastfeeding. In our study, the difference in transmission rates between breastfed and formula-fed infants was already highly significant at 6 weeks of age, and only few transmissions occurred after. The fact that the breastfeeding period was relatively short in our cohort may have contributed to the small size of the increase in the transmission rates observed during follow-up. However, our data strongly support the hypothesis that transmission through breastfeeding occurs mainly early after delivery.
Our study has several limitations. First, it was not a randomized trial and the comparison between the feeding groups could have been biased by differences in the populations. Second, our study groups represented a subsample of the women attending the hospital who agreed to be closely monitored and, therefore, the results may not be valid for the entire population. In this regard, it is important to underline that our population included middle- and high-class women who had been trained by health personnel in the use of formula feeding. Our data, although limited, suggest no increase in morbidity associated with formula feeding. Third, loss to follow-up was high between week 6 and month 6. However, since the loss was relatively low in the FF group, the difference among groups would have persisted even in the conservative hypothesis that all the children not tested in the EBF and MF groups were HIV negative.
In conclusion, our study showed that formula feeding was the safest way to feed children when compared with both exclusive breastfeeding and mixed feeding and that most of the transmissions occurred in the first 6 weeks. However, since formula feeding is feasible only for a limited number of women in less-developed countries, new preventive approaches such as the administration of antiretroviral prophylaxis to children or of antiretroviral therapy to mothers during breastfeeding need to be evaluated.
Sponsorship: This study was supported by grants from The Istituto Superiore di Sanità National AIDS Clinical Research Program, in 2000–2003.
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