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Epidemiology and Social

Incident HIV among pregnant and breast-feeding women in sub-Saharan Africa: a systematic review and meta-analysis

Graybill, Lauren A.a; Kasaro, Margaretb; Freeborn, Kelliec; Walker, Jennifer S.d; Poole, Charlesa; Powers, Kimberly A.a; Mollan, Katie R.a,e; Rosenberg, Nora E.f; Vermund, Sten H.g; Mutale, Wilbroadh; Chi, Benjamin H.c

Author Information
doi: 10.1097/QAD.0000000000002487



HIV acquisition among pregnant and breast-feeding women increases risk of maternal morbidity and mortality, and accounts for a significant, and growing, proportion of pediatric HIV infections globally [1]. A meta-analysis of 19 studies conducted between 1980 and 2012 estimated an average HIV incidence rate of 3.8/100 person-years [95% confidence interval (CI): 3.0--4.6] among pregnant and breast-feeding women in sub-Saharan Africa (SSA) [2]. Although this estimate is above the World Health Organization's (WHO) threshold for substantial risk of HIV acquisition [3], the rapidly evolving HIV prevention and treatment landscape since publication of this review may have important implications for maternal HIV incidence.

In 2013, the WHO updated HIV treatment guidelines, expanding antiretroviral therapy (ART) eligibility to CD4+ ≤ 500 cells/μl [4], and in 2015, it recommended universal treatment for HIV [5]. These changes, together with increased uptake of HIV testing and counseling and medical male circumcision [6–8], coincided with a 30% decline in the estimated number of new adult HIV infections in SSA between 2010 and 2017 [9]. Similar temporal trends in HIV incidence have been observed in three population-based cohort studies in SSA [10–12], with more gradual declines observed among women than among men [11,12]. Although combination HIV prevention and treatment interventions may not directly target pregnant and breast-feeding women, these populations may experience downstream benefits in HIV prevention. In at least one study [13], maternal HIV incidence was considerably lower in a cohort of pregnant and breast-feeding women participating in a community-based HIV prevention program than estimates of maternal incidence from the previous review [2].

Although the previous review observed evidence of heterogeneity among study-specific estimates of the incidence rate and the association between pregnancy and risk of HIV acquisition, their investigation into the underlying factors contributing to this variability was limited [2]. A better understanding of features contributing to variation in estimates is critical for guiding future research and policy, and for developing efficient strategies to reduce horizontal and vertical HIV transmission during pregnancy and breast-feeding.

In this updated review of literature from SSA between 1980 and 2018, we sought to summarize estimates of HIV incidence among pregnant and breast-feeding women; summarize estimates of the associations between pregnancy and risk of maternal HIV acquisition and between breast-feeding and risk of HIV acquisition; and identify population and methodological characteristics contributing to variation in study-specific estimates of incidence and association.


This review is registered with PROSPERO (CRD42017079577) and follows the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Guidelines [14].

Study selection and data abstraction

We searched PubMed, Embase, PsycInfo, and the Cochrane Library for relevant literature published between 1 January 1980 and 1 December 2018 (Table S1, We also searched online abstract archives from HIV Research for Prevention Conference (2014–2018), Conference of Retroviruses and Opportunistic Infections (2014–2018), and International AIDS Society Conferences (2001–2018) using the terms (‘pregnant’, ‘pregnancy’, or ‘postpartum’) and (‘incident’, ‘incidence’, or ‘seroconvert’).

We screened resulting titles and abstracts to identify publications that referred to HIV incidence among women or to pregnancy/breast-feeding and HIV. We conducted a full text review of included publications to identify primary research reports with estimates of (or sufficient information to derive) the incidence rate of HIV among pregnant and breast-feeding women, the incidence rate ratio (IRR) or hazard ratio contrasting HIV incidence between pregnant and nonpregnant periods, and/or the IRR or hazard ratio contrasting HIV incidence between breast-feeding and non-breast-feeding periods. Included studies were restricted to those published in English and conducted in SSA. We requested additional information from authors when publications contained relevant but insufficient information, and reviewed the bibliographies of included publications for relevant references.

Two investigators reviewed each publication at screening and full-text review; disagreements were resolved by consensus. Data on outcomes and exposures of interest and key population and methodological features of each study were abstracted into standardized tables by one reviewer and checked by two others. When more than one publication reported the same outcome from the same study population over the same period, we included the report considered most complete.

Outcome and exposure definitions

HIV incidence, the primary outcome, was defined as the number of new HIV infections per 100 person-years. Pregnancy and breast-feeding represented periods of interest in studies contributing incidence rate estimates, and represented exposures of interest in studies estimating the IRR or hazard ratio. We accepted all definitions in our primary analyses. In a sensitivity analysis, we excluded studies where the breast-feeding period exceeded 24 months postpartum [15].

Statistical approach

We used inverse-variance-weighted random-effects meta-analysis to estimate natural log-transformed measures of the average HIV incidence rate among pregnant and breast-feeding women, the average association between pregnancy and risk of HIV acquisition, the average association between breast-feeding and risk of HIV acquisition, and 95% prediction intervals around summary estimates. The 95% prediction intervals convey the estimated spread of the random-effects distribution, and can be informally interpreted as 95% CI for the true rate or association to be estimated in a randomly selected study population [16–18]. When zero seroconversions were reported, we applied a half-integer continuity correction to prevent the estimate from being omitted. As IRRs roughly approximate hazard ratios [19], we pooled these estimates for meta-analysis and assumed approximate collapsibility since HIV acquisition is rare [20]. Summary estimates and 95% prediction intervals were exponentiated for interpretability.

Because of the potential for publication bias, we drew funnel plots and analyzed them with the symmetry test of Egger et al. and with Duval and Tweedie's trim-and-fill imputation method [21,22]. We analyzed overall heterogeneity using 95% prediction intervals and the P value for Cochrane's Q statistic. We used stratified analyses and univariate random-effects meta-regression to analyze heterogeneity further by comparing average rates and associations by population characteristics of included studies. Meta-regression was also used to explore associations between estimates and methodological aspects related to study quality [23,24]. When a single study contributed information to more than one stratum of a variable, we used robust variances to account for correlation [25]. Given the large number of studies contributing estimates of the incidence rate, we also constructed separate multivariable models for each potential source of heterogeneity of the incidence rate. Each model adjusted for region, years of study implementation, and calendar time to account for differences in HIV prevalence and ART coverage. All analyses were conducted using the Metafor package in R, version 3.5.1 (R Foundation for Statistical Computing, Vienna, Austria) [26].

Sources of heterogeneity

Characteristics related to underlying HIV risk – region, calendar time, age, membership of a high-risk population, and participant enrollment in an HIV-prevention clinical trial – may be associated with estimates of incidence and association. As studies contributing estimates of the association had limited variability in calendar time, and did not provide age-stratified results, these features were only evaluated as sources of heterogeneity of the incidence rate.

We defined region using the World Bank's classifications, and calendar time based on mid-year of study implementation. We examined calendar time continuously, as a quadratic function, and categorically with three periods: precombination HIV prevention (before 2010), early adoption (2010–2014), and program expansion (after 2014). These periods correspond to important updates to WHO HIV treatment and prevention recommendations [4,5,27,28], and their implementation across SSA [6–8]. We defined age groups based on the most commonly reported categorization in the literature: less than 20, 20–24, 25–29, and at least 30. Other age group categorizations were considered in sensitivity analyses. We used a binary variable to distinguish between studies that enrolled a ‘high-risk’ cohort (e.g. HIV-discordant couples or female sex workers) and those that did not. We stratified by type of ‘high-risk’ group in sensitivity analyses. Studies were also classified according to whether participants were enrolled in a clinical trial evaluating an HIV prevention intervention.

The following features related to the measurement of incident infections and person-time may also be associated with estimates of incidence and association: study design, use of results from repeat HIV testing to identify seroconversions, reproductive periods observed over follow-up, use of HIV DNA/RNA PCR in the HIV-testing algorithm, and method for estimating date of HIV infection. As all studies contributing estimates of the association used repeat HIV testing and observed all reproductive periods over follow-up, these features were only evaluated as sources of heterogeneity of the incidence rate.

Finally, estimates of the IRR or hazard ratio may be related to the inclusion of breast-feeding-exposed periods in the reference group, adjustment for confounders, and adjustment for time-varying measures of condom use and intercourse frequency.


Our search yielded 5186 nonduplicate abstracts (Fig. 1). Screening resulted in 202 publications for full-text review, of which 57 met inclusion criteria. After excluding 20 publications because of overlapping cohorts and outcomes, 37 publications remained (Table 1    ). Thirty-four contributed estimates of the HIV incidence rate [13,29–61], and 10 contributed estimates of either the IRR or hazard ratio [55–64]. Follow-up ranged from 45 person-years to 57 240 person-years. Most studies were conducted in southern Africa (n = 20) [13,29,30,32,34,35,39–44,48,51,52,54,55,60,61,64]. The mid-point of follow-up occurred before 2010 in 26 studies [29,32,34–36,38–44,48,50,52–64], between 2010 and 2014 in eight [30,31,33,37,46,47,49,55], and after 2014 in three [13,45,51]. Two studies reported results stratified by calendar time [55,56]. In seven studies, participants were enrolled in an HIV prevention trial [32,43,54,60–62,64]. Four studies enrolled high-risk study populations [54,57,62,63], and two studies reported results stratified by risk-group [58,59]. Eight studies reported estimates of incidence stratified by age [13,30,38,39,44,48,55,58].

Fig. 1:
Study selection flowchart.
Table 1:
Description of studies meeting inclusion criteria.
Table 1 (Continued):
Description of studies meeting inclusion criteria.
Table 1 (Continued):
Description of studies meeting inclusion criteria.
Table 1 (Continued):
Description of studies meeting inclusion criteria.
Table 1 (Continued):
Description of studies meeting inclusion criteria.

There was limited variability in how studies measured incidence after accounting for study design (Table S2, Prospective cohort studies (n = 24) enrolled HIV-seronegative women and retested them over follow-up to identify changes in HIV serostatus. Twenty-one prospective cohort studies contributed estimates of the incidence rate among pregnant and breast-feeding women [13,29,30,32,37–40,42–44,48–50,52,54,58–61], and eight contributed estimates of the IRR or hazard ratio [57–64]. Eleven cross-sectional studies contributed estimates of the incidence rate among pregnant and breast-feeding women [31,33–36,41,45–47,51,53]. In these studies, HIV status at the time of the first antenatal visit was retrospectively assessed at the time of enrollment, which occurred in the third trimester [31,41,46,47], at delivery [33–35,51], or in the postpartum period [36,45,53]. Women classified as HIV-negative in pregnancy were enrolled and current HIV serostatus was assessed to identify new HIV infections. Finally, two studies nested within large population-based surveillance studies contributed estimates of both the incidence rate and the hazard ratio [55,56]. These studies used prospectively collected data from HIV surveillance assessments to assess changes in serostatus over time.

HIV incidence during pregnancy and breast-feeding

Studies contributing estimates of incidence during pregnancy typically captured the period between the first antenatal visit and delivery, while studies contributing estimates of incidence during breast-feeding captured the period from delivery up to 24 months postpartum depending on length of follow-up (Table 1    ).

Thirty-four studies contributed 100 758 person-years of follow-up and generated 44 estimates of HIV incidence among pregnant and/or breast-feeding women. Ten studies reported stratified estimates of incidence during pregnancy and during breast-feeding [13,37,39,44,54–59]. Using all available estimates, we observed little difference in the average HIV incidence rate during pregnancy only (n = 22, 3.4/100 person-years, 95% prediction interval: 1.1--10.4), breast-feeding only (n = 17, 3.1/100 person-years, 95% prediction interval: 1.0--9.5), and pregnancy and breast-feeding combined (n = 5, 4.6/100 person-years, 95% prediction interval: 1.4--15.4). We, therefore, combined estimates into a single HIV incidence rate during ‘pregnancy and breast-feeding’ for subsequent analyses. The estimated average of the HIV incidence rates during pregnancy and breast-feeding was 3.6 per 100 person-years (95% prediction interval: 1.2--11.1; Figure S1, Our results were unchanged after excluding one study with follow-up exceeding 24 months postpartum [48]. There was no visual or statistical evidence of funnel plot asymmetry (P = 0.3). Cochrane's Q statistic indicated evidence of heterogeneity (P < 0.001), which was consistent with the wide 95% prediction interval.

The average HIV incidence rate among pregnant and breast-feeding women was associated with age, calendar time, study design, and method of estimating the timing of HIV infection (Table 2). Average HIV incidence rates were lower among women at least 30 years old than among women less than 20 years old (ratio of average incidence rates: 0.5, 95% CI: 0.3--0.7), and this inverse relationship was robust to different categorizations of age (Table S3, HIV incidence appeared to have an inverted u-shaped association with calendar time (Figure S2, After adjusting for region and length of study, the average incidence rate for studies conducted after 2014 was 0.4 times the average rate for studies conducted prior to 2010 (95% CI: 0.2--0.7). Incidence was also associated with study design. Average rates were the highest among cross-sectional studies (4.7/100 person-years, 95% prediction interval: 1.6--13.5), followed by prospective cohort studies (3.4/100 person-years, 95% prediction interval: 1.2--9.4) and surveillance studies (2.2/100 person-years, 95% prediction interval: 0.6--7.4). Studies that defined the date of seroconversion as the date of the first positive HIV test observed higher incidence rates than studies that used a date between the last negative and first positive HIV test (ratio of average incidence rates: 4.3, 95% CI: 1.4--13.2).

Table 2:
Stratified analysis and meta-regression of the incidence rate of HIV during pregnancy and breast-feeding.

After stratifying by type of high-risk population, we observed higher estimated incidence rates among pregnant and breast-feeding women with known HIV-positive partners than rates estimated in a more general study population (ratio of average incidence rates: 4.7, 95% CI: 2.2--10.2; Table S4,

Pregnancy and HIV acquisition

Ten studies contributed estimates of the association between pregnancy and HIV acquisition. In four, nonpregnant, non-breast-feeding periods served as the referent [55,56,58,59]; in six, nonpregnant periods (which included breast-feeding) were defined as the referent [57,60–64]. There were variability definitions of ‘nonpregnant’ and ‘nonpregnant/non-breast-feeding’ because of heterogeneous definitions of pregnancy and breast-feeding (Table 1    ). All studies used methods that allowed women to contribute person-time to both exposed and unexposed periods.

The average hazard ratio estimating the association between pregnancy and risk of HIV acquisition was 0.9 (95% prediction interval: 0.2--3.8; Figure S3, Although we observed statistical evidence of funnel plot asymmetry (P = 0.05), results were largely unchanged after using a trim-and-fill analysis to impute one possibly missing result (average hazard ratio: 1.0, 95% prediction interval: 0.3--3.3). We also observed evidence of heterogeneity among study-specific estimates of the association (P < 0.001), which was consistent with the wide 95% prediction interval spanning the null. Stratified analyses and meta-regression revealed limited evidence of associations between the average hazard ratios and the measured characteristics of contributing studies (Table 3). Two estimates were generated by studies with partially overlapping cohorts [61,64]; exclusion of either did not change these results substantially (Tables S5, and S6,

Table 3:
Stratified analysis and meta-regression of the association between pregnancy and risk of HIV acquisition.

Breast-feeding and HIV acquisition

Four studies compared the risk of HIV acquisition during breast-feeding to risk during nonpregnant and non-breast-feeding periods. The average hazard ratio estimating the association between breast-feeding and risk of HIV acquisition was 1.0 (95% prediction interval: 0.6--1.6; Figure S4, We did not observe statistical evidence of funnel plot asymmetry (P = 0.2). Compared with estimates of the association between pregnancy and risk of HIV acquisition, estimates of the association between breast-feeding and risk of HIV acquisition were more tightly clustered around the null. We observed little evidence of heterogeneity between the study-specific hazard ratio estimates (P = 0.6), and our analyses revealed limited evidence of associations between the average hazard ratios and the measured characteristics of contributing studies (Table 4).

Table 4:
Stratified analysis and meta-regression of the association between breast-feeding and risk of HIV acquisition.


In this meta-analysis update -- which included 15 new studies and over 77 000 additional person-years of follow-up -- the estimated average HIV incidence rate among pregnant and breast-feeding women was above the ‘substantial risk’ threshold described by the WHO [3], whereas the estimated average associations between pregnancy and risk of HIV acquisition, and breast-feeding and risk of HIV acquisition, were close to the null. Prediction intervals around each of our summary estimates were wide, highlighting the variability of HIV incidence across populations of pregnant and breast-feeding women in SSA.

Our results were consistent with findings from a previous meta-analysis that reported high average HIV incidence during pregnancy and breast-feeding [2]. Hormonal changes during pregnancy may increase susceptibility to HIV through changes in the vaginal epithelial thickness, microbiome, and CCR5 coreceptor expression [65,66]. Pregnancy activates the innate immune system, increasing inflammation and concentration of dendritic cells in the female genital tract, while suppressing the adaptive immune response [67,68]. Such immunologic changes may increase risk of HIV acquisition [69–71], and can last for several months postpartum [72,73]. Behavioral changes occurring during pregnancy may also influence risk of HIV acquisition. Couples may be more likely to engage in unprotected sex during pregnancy [34,58,74], and male partners may be more likely to seek extra-partnership sexual liaisons during extended periods of pregnancy-related or breast-feeding-related abstinence [34,75–77].

Substantial heterogeneity of the incidence rates, however, cautions us from interpreting the average HIV incidence rate estimated in this study as the incidence rate among pregnant and breast-feeding women in SSA. Our results suggest maternal HIV incidence rates may lower among older compared with younger pregnant and breast-feeding women, and higher among women in HIV serodiscordant relationships. Additionally, we observed changes in average HIV incidence over calendar time that follow temporal trends observed in the region since the 1980s: a steady rise in HIV incidence until the early 2000s [78], largely driven by increasing HIV prevalence without viral suppression [79], followed by a slow decline that may be attributed to expanded HIV testing and counseling, medical male circumcision, and ART services. Inverted u-shaped trends in HIV incidence over time have been observed in large population-based cohorts in SSA [10–12], with reported associations between HIV incidence and community-level coverage of ART and medical male circumcision. Models predict that integrated behavioral and biomedical interventions will reduce HIV incidence generally [80,81], and among pregnant women specifically [82], and two cluster randomized trials of combination HIV prevention with universal ART demonstrated some reductions in community-wide HIV incidence [83,84]. Although we expect that HIV-negative pregnant and breast-feeding women may serve as beneficiaries of expanded combination HIV prevention, impact will likely vary across sub-groups.

Prediction intervals around estimates of the average association between pregnancy and risk of HIV acquisition and between breast-feeding and risk of HIV acquisition, were wide with lower and upper bounds on either side of the null. This variability is not unexpected; pregnancy and breast-feeding are periods marked by significant biological and behavioral changes that may have different effects on risk of HIV. For example, the potential increased risk of HIV arising from the pregnancy-induced physiological changes described earlier may be offset by a reduction in sexual intercourse that frequently occurs during late pregnancy and early breast-feeding [34,54,58,74]. The direction of the observed association between pregnancy or breast-feeding and risk of HIV acquisition may, therefore, depend on both study context and analytical decisions regarding covariate measurement and adjustment [85]. Furthermore, as the physiological and behavioral changes that accompany pregnancy and breast-feeding are dynamic, decisions regarding how to define pregnancy, breast-feeding, and the referent state may influence the direction of the observed association. For example, the inclusion of breast-feeding in the referent group may produce estimates closer to the null as incidence rates during breast-feeding appear similar to those during pregnancy, whereas single categories for pregnancy and breast-feeding may obscure periods during pregnancy or breast-feeding when risk is truly elevated or suppressed. Work by Thomson et al.[54] suggests that physiological changes during pregnancy increase susceptibility to HIV, particularly in late pregnancy and early breast-feeding. However, additional work is needed to better understand the interaction between biological susceptibility and behavioral changes on risk of HIV acquisition among pregnant and breast-feeding women in different SSA contexts.

Our results should be interpreted in light of possible limitations. It is unclear if contributing studies enrolled representative cohorts of women, so the extent to which our estimates generalize to all pregnant and breast-feeding women in SSA is unknown. It is possible that investigators targeted clinics in areas of elevated HIV incidence, which may bias estimates of incidence upwards. Few estimates of the incidence rate captured the first trimester of pregnancy, and given the variability of risk over the course of pregnancy [54,58,62], this may bias estimates of incidence. The directionality of this bias is unclear; two studies report higher incidence during early compared with late pregnancy [58,62], whereas one reports the reverse [54]. For this reason, misclassification of early or late pregnancy-exposed periods as nonpregnant person-time may also bias estimates of the association in unknown directions. Finally, our analyses were restricted by the number of studies and the information provided by each study. The small number of estimates may have limited our power to detect associations between estimates and underlying sources of heterogeneity. Differences in populations and methodological features of contributing studies may not have been adequately captured by variables used in meta-regression models, and several important population features were unmeasured by contributing studies.

Although many countries in SSA have placed considerable focus on identifying and treating HIV-infected pregnant and breast-feeding women, HIV-uninfected women have received considerably less attention in antenatal and postnatal settings. Our results support the expansion of bio-behavioral HIV prevention interventions and repeat testing throughout pregnancy and breast-feeding to women at high risk of HIV acquisition. Further work is needed to identify risk factors for HIV acquisition during pregnancy and breast-feeding to facilitate targeted prevention interventions in antenatal and postnatal settings. Offering female-controlled strategies, such as tenofovir-based oral preexposure prophylaxis, and promoting couple-based prevention approaches in these settings, are important next steps that may reduce the risk of HIV-related maternal morbidity and mortality, and ensure continued progress towards the elimination of mother-to-child transmission of HIV.


This study was supported in part by the National Institutes of Health (R01 AI131060, R00 MH104154, K24 AI120796, P30 AI050410, D43 TW009340).

All authors met the criteria for authorship as established by the International Committee of Medical Journal Editors. Contributions were as follows: study concept and design: L.A.G., M.K., K.A.P., W.M., B.H.C.; literature search: L.A.G., J.S.W.; literature review: L.A.G., M.K., K.F., B.H.C.; data abstraction: L.A.G., K.F., B.H.C.; statistical analysis: L.A.G., C.P., K.R.M., K.A.P.; data interpretation: L.A.G., C.P., K.A.P., K.R.M., N.E.R., S.H.V., W.M., B.H.C.; drafting of manuscript: L.A.G., K.F., B.H.C.; critical revisions of manuscript: all authors.

Sources of funding: This study was supported in part by the National Institutes of Health (R01 AI131060, R00 MH104154, K24 AI120796, P30 AI050410, D43 TW009340).

Disclaimer: The funders of this study had no role in study design, data collection, data analysis, data interpretation, or writing of the report. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication.

Duplicate Publication: Results have not been published previously. Results were presented as a research poster at the 11th International Workshop on HIV Paediatrics (abstract number 109) and the 10th International AIDS Society Conference (abstract number TUPEC475).

Conflicts of interest

There are no conflicts of interest.


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