The impact of each study on the summary estimate was evaluated by successively omitting each of the studies and obtaining a summary for all the other studies. For HIV incidence studies, the RR varied little, ranging from 1.58 [after excluding the study by Taha et al. (1998) ] to 1.93 [after excluding the study by Martin et al. (1999) ]. The POR estimate from the study by Greenblatt et al. (1999 ), by far the most precise of the estimates from the prevalence studies, was highly influential. Although the homogeneity test P value remained less than 0.0005 upon removing this result, it substantially influenced the symmetry of the funnel plot. When this result was removed, the symmetry tests produced P = 0.2 (Begg) and P = 0.6 (Egger); the ‘trim and fill’ analysis imputed just one hypothetically missing result, regardless of whether a fixed effect or random-effect model was used.
Sensitivity analyses were conducted to determine the impact of the decision to systematically choose Nugent's score results over clinical criteria for bacterial vaginosis diagnosis for studies with both results available. These sensitivity analyses were conducted by repeating the analyses after replacing, for studies that used two diagnostic methods, estimates based on Nugent's scores with those based on clinical criteria. The sensitivity of the effect estimate to this choice was found to be robust with an overall RR in HIV incidence studies of 1.47 (95% CI: 1.10, 1.95). Heterogeneity in prevalence studies remained unchanged with the POR estimate in low HIV-risk groups of 2.30 (95% CI: 1.68, 3.15) still being higher compared with the POR in high HIV-risk groups of 1.53 (95% CI: 1.29, 1.82).
This is an updated systematic review and meta-analysis of the association between bacterial vaginosis and HIV infection. Overall bacterial vaginosis prevalence was high in several populations of women studied, with prevalence rates as high as 70%. Our analyses of HIV incidence studies indicate that bacterial vaginosis increases the risk of HIV acquisition by approximately 60%(95% CI 21–113%). This was slightly higher than that reported (40%) in a previous review of two studies . Studies of HIV prevalence tended to find higher HIV prevalence in women with bacterial vaginosis. However, these prevalence study estimates were heterogeneous and had evidence of funnel plot asymmetry.
The bacterial vaginosis-HIV association tended to be weaker in high HIV-risk groups, though the small number of prospective studies limited the confirmation of this trend in HIV incidence studies. A weaker association in high-risk women may possibly be due to a depletion of susceptibility to HIV resulting from women in high-risk groups having a greater risk of acquiring HIV from causes other than bacterial vaginosis. Once HIV infected, they are no longer at risk of acquiring HIV attributable to bacterial vaginosis, thus reducing the effect of bacterial vaginosis in this group. More data from prospective cohorts are needed to better examine the heterogeneity, by HIV-risk group, in the effects of bacterial vaginosis on the risk of acquiring HIV. This information could be helpful in identifying specific sub-populations, with a stronger association between bacterial vaginosis and HIV, in whom to target bacterial vaginosis control measures.
Bacterial vaginosis results in several changes in the vaginal flora that provide biological plausibility for an increased risk of HIV acquisition in bacterial vaginosis-positive women. Bacterial vaginosis is associated with a depletion of hydrogen peroxide-producing lactobacilli that may reduce vaginal defense against microorganisms including HIV [42,43]. Higher vaginal pH (>4.5) that occurs with bacterial vaginosis may also increase the availability of vaginal HIV target cells by increasing CD4 lymphocyte activation and multiplication . High vaginal pH may also increase the adherence and survival of HIV . Bacterial vaginosis has also been associated with a reduction in vaginal fluid levels of secretory leukocyte protease inhibitor (SLPI) , which has been shown to block HIV infection in vitro. Finally, by increasing intravaginal levels of interleukin-10, bacterial vaginosis may increase the susceptibility of macrophages to HIV . These changes, combined with the difficulties of successfully eradicating bacterial vaginosis , may explain the increased risk observed in most epidemiology studies.
Some methodological limitations to this review need to be considered. The first are concerns on whether a meta-analysis of observational studies can effectively control for confounding and bias . An attempt at reducing these was made by the preferential use of adjusted estimates in the estimation of summary measures. Meta-regression also revealed little difference between the adjusted and unadjusted estimates used in the final analysis. The second limitation has to do with the relatively few prospective studies included in this analysis. The restricted number of HIV incidence studies prohibited any sub-group analysis. However, this did not appear to be necessary as there was no heterogeneity among the estimates from these studies. The limited number of studies also prohibited any reliable analysis of other potential sources of heterogeneity, such as pregnancy or age. More prospective studies are needed to accurately evaluate the causal association between bacterial vaginosis and HIV. Third, this review was limited to that of published studies. This had little impact on the estimate from HIV incidence studies as there was no evidence of funnel plot asymmetry. The impact of publication bias on HIV prevalence studies was, however, unclear because of the heterogeneity in the estimates and discrepancies in the results of the various methods of assessing publication bias (Begg's versus Egger's methods; Duval and Tweedie's ‘trim and fill’ random versus fixed models). The funnel plot of the POR estimates was unusual in that the one estimate that was by far the most precise (Greenblatt et al. 1999 ) fell well outside the range of the other estimates. Given the pronounced heterogeneity among all POR results, this result was not given a very high weight when the trim and fill analysis was conducted using a random-effect model. With a fixed-effect model, however, the exceedingly high inverse-variance weight assigned to this estimate caused the trim and fill analysis to suggest publication bias so profound that one-third of all prevalence results are unreported, all of them on the reduced-prevalence side of the null. Although some publication bias in that direction might have occurred, we are not inclined to believe that it could have been great. In any event, the prevalence results were much too heterogeneous to warrant aggregating them to produce a single, summary estimate. We found nothing obvious about the Greenblatt et al. (1999 ) study that should have caused it to produce an estimate so unlike the remainder of the literature. It was one of five studies conducted in the United States and one of four US studies designed to include sizable proportions of HIV-positive and HIV-negative women. Yet it was the only one to produce an inverse association. We are inclined to consider its departure from the main thrust of the literature an unexplained anomaly.
Limitations in the original studies included in this meta-analysis could also impact our estimates. With bacterial vaginosis being a time-dependent condition, prospective studies of HIV incidence are susceptible to misclassification in the definition of bacterial vaginosis status resulting from the use of either bacterial vaginosis status at enrollment or bacterial vaginosis status at prior visit as indicators of bacterial vaginosis status immediately preceding HIV acquisition. Misclassification could also result from false positive or false negative diagnosis of bacterial vaginosis using either clinical or bacteriologic criteria. Both of these mechanisms of misclassification are expected to be nondifferential and thus lead to more conservative estimates of the effect of bacterial vaginosis on HIV acquisition. Original studies of HIV prevalence by bacterial vaginosis status (using case–control or cross-sectional designs), in addition to the aforementioned susceptibility to misclassifying bacterial vaginosis status, could also be subject to selection bias when HIV cases are enrolled from high sexual-risk populations in whom bacterial vaginosis is more frequent. If not controlled, such a bias would result in an overestimate of the association between bacterial vaginosis and HIV. However, we do not think this may have been substantial as most studies did control for at least one indicator of sexual risk, thus attenuating the impact of selection bias. Finally, all the observational studies included in this analysis are subject to residual confounding, which could result in an underestimate or overestimate of the magnitude of the association between bacterial vaginosis and HIV.
Despite these limitations, this review was strengthened by an extensive search of published literature using multiple databases and references of identified publications. Furthermore, by separating HIV incidence studies from HIV prevalence studies, the effect of bacterial vaginosis on incident HIV was separately analyzed. This distinction is important as studies of incident HIV are not liable to reverse causation bias that would result from HIV-infected women being more likely to acquire bacterial vaginosis. The analysis of sources of heterogeneity also allowed us to identify HIV-risk group, and not method of bacterial vaginosis diagnosis, as an important source of heterogeneity in prevalence study results. Finally, we refrained from using summary estimates in the presence of heterogeneity. It has been argued that even when a random-effect model is used to obtain a summary estimate, the latter is not always conservative and is potentially misleading, if interpreted as an average effect .
The high prevalence of bacterial vaginosis in certain populations (particularly, those most impacted by the HIV pandemic) implies that notwithstanding the relatively modest effect of bacterial vaginosis on HIV infection, a high proportion of HIV infection could be attributable to bacterial vaginosis. In a population of women having a bacterial vaginosis prevalence of 30%, with a RR of 1.6, the population attributable risk proportion (PARP), the proportion of HIV in a population that is attributable to bacterial vaginosis, is estimated at 15%. Although other STIs have been shown to increase the risk of HIV infection with a higher RR in the order of 2–5 , the relatively lower prevalence of these STIs as seen in some of the studies included in this analysis [26–29] may result in similar proportions of HIV infection being attributable to these STIs as to bacterial vaginosis.
The potential impact of bacterial vaginosis could also be expressed in the number of women who need to have bacterial vaginosis for each additional case of HIV. This depends on the baseline risk of HIV among women without bacterial vaginosis. For instance, with a 2.0% baseline risk of HIV seroconversion among bacterial vaginosis-negative women , a RR of 1.6 would correspond to an absolute risk increase of approximately 1.2%, or about one additional case of HIV for every 80–90 women with bacterial vaginosis. These data suggest that greater attention needs to be given to bacterial vaginosis in the global fight against HIV infection. Randomized clinical trials (RCT) to determine the effect of bacterial vaginosis control measures on HIV acquisition may be worth considering. A previous RCT of the effects of mass treatment of STIs on HIV conducted in Rakai (Uganda) used a single dose of oral metronidazole 2 g and found no effect on HIV acquisition . However, although 2 g of metronidazole can cause short-term remission, it is not the recommended treatment [51,52], thus limiting the inference that can be made on the effect of bacterial vaginosis treatment from the Rakai study. Future RCTs assessing this effect will need to use the recommended treatment regimen with a longer duration associated with lower recurrence rates. In addition to the need to evaluate the potential of bacterial vaginosis treatment to prevent HIV acquisition and transmission, a better understanding of its risk factors and determinants of bacterial vaginosis recurrence is required.
We are grateful to Jean Blackwell for assistance during the literature search. We also thank the authors of the original studies included in this analysis.
J.A. is a Fogarty AITRP fellow supported by NIH Fogarty grant DHHS/NIH/FIC 5 D43 TW01039-08 AIDS International Training and Research Program to the University of North Carolina, Chapel Hill. C.P. was supported in part by a grant from the National Institute of Environmental Health Sciences (P30ES10126). J.S.S. was supported by the UNC Center for AIDS Research (CFAR).
J.A., P.N., A.A.A. and J.S.S. defined the question and designed the study. J.A. and J.S.S. abstracted and reviewed the data. J.A., C.P. and J.S.S. conducted the analysis. All authors wrote or reviewed the article.
An earlier version of this study was presented at the International Society for Sexually Transmitted Disease Research (ISSTDR) 2005 conference in Amsterdam.
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