In this prospective study of more than 2,200 southern and East African HIV-1–seropositive women and their HIV-1–seronegative male partners with genetic linkage of HIV-1 transmission pairs, we found that BV was independently associated with a 3-fold increased risk of female-to-male HIV-1 transmission. The potential significance of this finding is substantial, given that 35%, 15%, and 52% of women in this study had BV at enrollment, throughout follow-up, and at least one interval of BV during the 2 y of follow-up, respectively. This proportion of HIV-1–seropositive women with BV is consistent with prior studies demonstrating a prevalence of BV ranging from 30%–55% among women in sub-Saharan Africa 
. Thus, assuming that the association we report is causal, BV may account for a substantial population attributable risk percent for new HIV-1 infections in men in Africa.
Although genital HIV-1 RNA predicts female-to-male HIV-1 transmission independent of the HIV-1 RNA concentration in blood 
, we found only a modest (0.2 log10
) increase in HIV-1 RNA in women with BV in comparison to those with normal vaginal flora. Thus, it is likely that increased genital HIV-1 RNA caused by BV only partially explains our results. Most cross-sectional and longitudinal studies have found that women with BV have higher concentrations of HIV-1 RNA in genital secretions in comparison to women with normal vaginal flora 
. However, two prospective studies did not find differences in genital HIV-1 RNA concentration associated with BV 
. Differences with previous studies that found an association between BV and genital HIV-1 shedding could be due to the proportion of women with BV who were symptomatic, with potentially higher levels of inflammation associated with symptomatic BV, and the short duration (i.e., 14 d) after BV treatment in which vaginal samples were collected in the longitudinal studies to measure genital HIV-1 RNA, which may have been too soon for decreased T-cell activation 
, proinflammatory cytokines 
, or reestablishment of lactobacilli predominant flora 
An additional hypothesis to explain our findings is that BV in a female partner may indirectly increase HIV-1 susceptibility in men. A growing body of evidence suggests that the female and male genital microbiota is shared between sexual partners 
. Recent data from Uganda have demonstrated that male circumcision reduces the risk of BV in female partners and that bacterial flora associated with BV commonly colonize the penis including the distal urethra 
. Anaerobic and other bacteria increased in male partners of women with BV may cause inflammation by activating Langerhans cells and CD4+ T-cells, thereby increasing target cells for HIV-1 and susceptibility to HIV-1 infection 
. Interestingly, male circumcision did not affect or modify the relationship between BV and female-to-male HIV-1 transmission in our study. In comparison to pre-circumcision abundances of bacterial phylotypes, post-circumcision abundances of anaerobic bacteria decreased, while abundances of facultative anaerobic bacteria increased significantly in the Rakai study 
. Potential mechanisms need investigation, including whether the male genital microbiota, in particular anaerobes, are associated with urethral and penile inflammation and activation of Langerhans cells and CD4+ T-cells, which could increase risk of HIV-1 infection in men.
Lower socioeconomic status has been associated with higher BV prevalence 
. Previous studies have also implicated race, multiple sex partners, trichomoniasis, HIV-1 infection, intrauterine device use, vaginal douching, recent antibiotic use, and the absence of vaginal colonization by H2
-producing lactobacilli as risk factors for BV 
. Following treatment, BV clinically recurs in 20%–30% of women within 3 mo 
, and recurs in approximately 75% of women with symptomatic as well as asymptomatic BV within 2 mo of treatment 
. One reason for the high prevalence of BV and its frequent recurrence may result from the transfer of potentially pathogenic bacteria between heterosexual partners through genital, and potentially orogenital contact 
The high prevalence and frequent recurrence of BV has led to the development of several new strategies including frequent presumptive antibiotic treatment 
, and use of probiotic lactobacilli as an adjuvant or an alternative to antimicrobial therapy 
. Recent advances in understanding the microbiota associated with BV 
, including the ability of G. vaginalis
and to a lesser degree Atopobium vaginae
to form biofilms recalcitrant to antibiotic treatment 
, may eventually lead to therapies that maintain a lactobacilli-predominant flora in the vagina.
Our study has several strengths starting with its large and diverse population of HIV-1–serodiscordant couples recruited from across multiple sites in southern and East Africa. Furthermore, genetic linkage of female-to-male transmitted HIV-1 minimized misclassification in our analysis 
. One limitation of our analysis is that we do not know the specific vaginal flora present at the time of HIV-1 infection since vaginal microbiota can fluctuate weekly 
. To address this, we conducted two sensitivity analyses, the first evaluating vaginal Gram stain results at the same visit when HIV-1 seroconversion was first noted, and the second evaluating the severity of vaginal flora between that and the prior visit. Both evaluations confirmed the results of our primary analysis. The cohort was a highly selective population (e.g., all participants underwent couples HIV counseling and testing, enrolled in an HIV-1 prevention randomized clinical trial, and index participants had a CD4 count ≥250 cells/mm3
at enrollment), which could impact on the generalizability of our results. Furthermore, all HIV-1–infected partners were co-infected with HSV-2; however, HSV-2 seroprevalence is >80% among HIV-1–infected persons in sub-Saharan Africa 
and thus is unlikely to limit the generalizability of our findings. In addition, the relatively small number of female-to-male HIV-1 transmissions (nine among women with normal vaginal flora versus 31 among women with BV) requires mention. Finally, residual or unmeasured confounding, which cannot be completely excluded, could affect the significance of our findings.
This study clearly demonstrates that BV is associated with an increased risk of female-to-male HIV-1 transmission. BV is a highly prevalent condition among HIV-1–infected women. The association of BV with increased infectiousness of HIV-1–infected women requires additional research to understand potential pathogenic mechanisms as well as the etiology, treatment, and prevention of BV. While a large community randomized controlled trial that provided presumptive treatment of STIs including metronidazole for BV failed to reduce HIV-1 incidence 
, ongoing studies are evaluating more frequent presumptive BV therapy 
, while others are studying naturally occurring and genetically enhanced probiotics to reduce recurrent BV 
. A lactobacillus-predominant vaginal flora might not only reduce the risk of HIV-1 acquisition in women 
, but also HIV-1 transmission to male partners, and points to the potential benefits of using the human microbiota to prevent disease.