Sexually transmitted infections (STIs) and bacterial vaginosis (BV) are widespread globally. These conditions have important sexual, reproductive, and maternal-child health consequences, including genital symptoms, pregnancy complications, infertility, enhanced HIV transmission, and psychosocial effects. The World Health Organization (WHO) estimated that, in 2012, there were 357 million new episodes of 4 curable STIs (chlamydia, gonorrhea, syphilis, and trichomoniasis) [1] and 417 million people had infection with herpes simplex virus type 2 (HSV-2) [2]. Global estimates of BV occurrence are limited [3].

The WHO Global Health Sector Strategy on Sexually Transmitted Infections 2016–2021 (Global Strategy), provides goals, targets, and priority actions for stemming the STI epidemic [4]. As data on STI burden are critical for efforts to prevent, control, and manage STIs, the first strategic direction of the Global Strategy is to increase information, including STI prevalence estimates, for focused public health action. Estimating the global burden of STIs is hampered by the limited availability of STI diagnostic testing and surveillance and a paucity of high-quality published studies of STI prevalence in low- and middle-income countries (LMICs), for both general and key populations. Given these limitations, a WHO Consultation on Methods for Improved Global STI Estimates highlighted the importance of exploring potential data available through clinical research studies and trial networks to improve estimates [5], because many studies gather baseline data on STIs, even when estimating STI prevalence is not a primary study objective.

In one collection of such studies, investigators used combined data from 18 prospective HIV prevention studies in sub-Saharan Africa in an individual participant data (IPD) meta-analysis to examine the association between hormonal contraceptive (HC) use and HIV acquisition [6]. Although the main objective of these studies was not to estimate STI/BV prevalence, all of them captured information on at least some STIs or BV. The 18 studies in the HC-HIV IPD dataset include more than 37,000 women in 9 countries in sub-Saharan Africa [7–33] and present a unique opportunity to explore prevalence of selected sexually transmitted and reproductive tract infections.

In the current study, we used IPD meta-analysis to estimate the prevalence of selected STIs and BV, and the prevalence of co-infections, among women in sub-Saharan Africa participating in these 18 studies. We selected STIs included in the majority of the studies (chlamydia, gonorrhea, trichomoniasis, syphilis, and HSV-2), and present STI/BV prevalence by age, region, and population type.

We followed a protocol and a prespecified analysis plan (S3 Text). We report our findings in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (S1 Text) and provide a checklist of items specific to IPD meta-analyses (S2 Text). PRISMA checklists for previous IPD analyses of these data are also available [6,34]. The US Centers for Disease Control and Prevention, WHO, and the Protection of Human Subjects Committee of FHI 360 determined that the research was exempt from ethical review. All included studies had relevant country-specific institutional ethical review and regulatory board approvals, and all participants within each study provided written informed consent for study participation. All principal investigators agreed to share their data for the present meta-analysis.

All 18 potentially eligible studies included testing for at least 1 STI and/or BV testing at baseline. Test technologies varied across studies (S2 Table). Five studies included multiple geographic regions and/or population types and were split into separate sub-studies for analysis, resulting in a total of 26 studies/sub-studies (hereafter referred to as “studies”) (Table 1). There were 10 studies in South Africa that were conducted among clinic/community-based populations (N = 9) and HIV-discordant couples (N = 1). Nine studies were conducted in Southern Africa (N = 5) or Eastern Africa (N = 4) among clinic/community-based populations (N = 7) and HIV-discordant couples (N = 2). Seven studies were conducted in Eastern Africa among higher-risk populations. Sample sizes of included studies ranged from 138 to 5,654.

Prevalence estimates from studies of HIV-discordant couples were similar to those from studies of clinic/community-based populations, so we combined these population types (subsequently referred to as “clinic/community-based studies”). In addition, prevalence estimates from clinic/community-based studies in Southern and Eastern Africa were similar, and these groups were combined (subsequently referred to as “Southern/Eastern Africa”). Our main results are presented by age group (15–24 and 25–49 years) for South African clinic/community-based populations, Southern/Eastern African clinic/community-based populations, and Eastern African higher-risk populations. Participant characteristics and STI/BV prevalence estimates according to the initial stratification variables for age, region, and population type are shown in S3 Table and S1–S8, S11–S18 Figs.

In this IPD meta-analysis, we combined data from 18 prospective HIV prevention studies representing over 37,000 women in sub-Saharan Africa to estimate prevalence of chlamydia, gonorrhea, syphilis, trichomoniasis, HSV-2, and BV. We observed age-, region- and population-specific patterns in the prevalence of these infections, including higher prevalence of STIs among younger (15–24-year-old) than older (25–49-year-old) women, generally higher STI prevalence in clinic/community-based populations in South Africa than among similar populations elsewhere in Southern/Eastern Africa, and notably greater prevalence among higher-risk populations for certain STIs (e.g., gonorrhea and syphilis), but not for others (e.g., HSV-2 and BV, which had high prevalence across all population types).

A major strength of this analysis is that it provides important information on STI and BV prevalence that would not otherwise be readily available through literature searches of published data. Few of our included studies had published detailed results about STI or BV. None of the data from the included studies were identified through the literature search process used to generate the 2008 and 2012 WHO global curable STI estimates [1,37], because they included only higher-risk populations, they were published outside the time frames of the literature searches, they were not identified in the searches, or the published papers did not provide enough information (e.g., estimates not stratified by age group, sex, or country/region) to include in the WHO estimates. One study that initially did not have enough STI information in the published paper to include in the WHO 2012 estimates was later identified during this STI IPD meta-analysis and subsequently incorporated [30,31]. Combining STI/BV data from a large number of studies allowed us to estimate prevalence by age and population type more precisely for distinct regions within sub-Saharan Africa.

While this IPD meta-analysis provides a unique opportunity to strengthen understanding of the epidemiology of STIs/BV among sub-Saharan African women, several limitations need to be considered. First, the studies included in the HC-HIV meta-analysis dataset were not specifically selected to assess STI/BV prevalence, and should be considered a convenience sample of studies. There may have been additional studies from the region that did not meet inclusion criteria for the HC-HIV meta-analysis, but contained STI/BV data, including data for men. In addition, the women in these studies were not randomly sampled from the populations they represent. Women enrolled from clinical settings in particular may be more likely to be symptomatic or recently exposed to an STI, which would tend to overestimate STI prevalence. Some studies only enrolled women in clinic/community settings who reported recent sexual activity or more than 1 partner in the recent past [33]. Conversely, because all included studies were prospective evaluations of HIV acquisition, women included in this baseline evaluation were all HIV-negative. Thus, we may have underestimated STI/BV prevalence in communities with high HIV prevalence (such as in South Africa), where HIV-negative women may be at lower STI/HIV risk than others in their communities. Reassuringly, the prevalences of curable STIs we observed in clinic/community-based populations, especially in Southern/Eastern Africa, were similar to WHO 2012 estimates for African women (which were limited to general populations) [1]. Another limitation of this meta-analysis is that it included some studies that are more than 15 years old. Summary estimates could be biased if prevalence has changed over time. Evidence of temporal trends in the prevalence of STIs/BV in LMICs is limited, in part due to lack of standardized surveillance systems and differing methodology for repeated sets of regional estimates [1]. Available data from a few large studies have not shown clear evidence of changing epidemiology over the last two decades, beyond methodological issues in the estimation process [1–3]. Aside from syphilis—for which available rapid diagnostic tests and concerted efforts to screen and treat pregnant women to avert mother-to-child transmission have resulted in likely declines over time [1,38]—the other STIs and BV lack clearly effective population-level interventions in these settings. This provides confidence that even though some studies in this IPD meta-analysis are older, the data provide a general idea of the overall STI epidemic in these regions. Future research could combine these data with additional data points to further the understanding of global trends over time.

We observed high heterogeneity across studies when calculating summary estimates for STI/BV prevalence in some populations. Observed heterogeneity can reflect differences in study design, setting, sample size, and recruitment strategies between studies. We present summary estimates irrespective of the degree of between-study heterogeneity to facilitate the comparison of overall patterns between the population groups for each infection; however, observed heterogeneity should be taken into account when interpreting findings. For example, differences in test technology might explain some of the heterogeneity for some infections. We attempted to minimize this by excluding studies with less accurate tests for a particular STI or BV (e.g., studies that diagnosed BV based only on Amsel clinical criteria [39]). For some STIs, however, results from diagnostic tests with varying levels of sensitivity and specificity were included. Some of the most noticeable heterogeneity was observed among trichomoniasis estimates, for which a large number of studies used only wet mount, which is notably insensitive [40]. For HSV-2, although all included studies used EIAs, some studies used EIAs with lower specificity [41]. For chlamydia and gonorrhea, prevalence estimates were relatively consistent across studies despite some high I² values. For syphilis estimates, heterogeneity was less pronounced. To refine prevalence estimates, future work could consider additional investigation into heterogeneity by diagnostic test type. Finally, studies that were conducted in multiple regions or among multiple population types were split into sub-studies; it is possible that estimates from these sub-studies are not independent.

We observed several patterns that highlight important features of the STI/BV epidemics in sub-Saharan Africa, which address the first strategic direction of the Global Strategy for STIs: “information for focused action: the who and the where.” In terms of the “who,” across all populations and regions, STI prevalence was generally higher among younger than older women, except for the prevalence of antibodies to HSV-2, which increased with age. Higher STI prevalence among young people has been observed worldwide and highlights the need for global efforts to improve sexual and reproductive health in adolescent girls and young women [42–44]. Additionally, we found that women who were in HIV-discordant partnerships had similar STI prevalence as women recruited from clinic/community-based populations, perhaps reflecting that although these participants may be at higher risk for HIV transmission than the general population, their risk for STIs/BV may be similar to that of clinic/community-based populations, given that they are in stable relationships.

As for the “where,” we found that women enrolled from clinic or community-based settings in South Africa had particularly high prevalence of STIs. In many cases, STI prevalence in South African women was similar to or greater than that of populations in Eastern Africa that were recruited from settings thought to be higher risk for HIV, such as women who report transactional sex. These findings are consistent with other studies in South Africa showing high prevalence of infections like chlamydia and gonorrhea [45,46]. This is particularly concerning given the likely increased risk of HIV acquisition posed by these inflammatory STIs [47,48]. Because of variability in how studies measured sexual behavior, we were unable to fully explore reasons for differences in STI prevalence across regions. South African women in the included studies were, however, less likely to be married or living with a partner compared with Southern/Eastern African women, although they were more likely than other groups to report condom use at last sex [49,50]. Additional work, such as research that incorporates anthropological and other qualitative methodologies, could help elucidate the cultural and behavioral drivers of observed differences between these populations.

The estimated prevalence of chlamydia in clinic/community populations in Southern/Eastern Africa was very similar to estimates from high-income countries in Europe and in the United States, including countries with well-established chlamydia control programs [51,52]. HSV-2 and BV prevalence was relatively high across all regions and population types. Gonorrhea and syphilis, on the other hand, were primarily concentrated among women recruited from settings thought to be at higher risk for HIV, which in this study were all in Eastern Africa. This suggests that targeting interventions to both populations and geographic regions may be important in addressing 2 of the 3 main areas of intervention focus for the Global Strategy: first, controlling the spread and impact of gonorrhea, particularly to stem the threat of gonococcal antimicrobial resistance [53], and, second, dramatically reducing mother-to-child transmission of syphilis and HIV [54]. The third main area of intervention focus of the Global Strategy is fully utilizing HPV vaccines; however, too few studies in the HC-HIV dataset collected information on HPV infection for us to evaluate this important STI.