Does bacterial vaginosis make HIV easier to pass to men?
Men whose HIV-positive female partners had bacterial vaginosis were about three times more likely to catch HIV from them than men whose partners had normal vaginal bacteria.
Source
Bacterial vaginosis associated with increased risk of female-to-male HIV-1 transmission: a prospective cohort analysis among African couples
Study at a glance
- Design
- Cohort — Secondary analysis of a prospective cohort nested in the Partners in Prevention HSV/HIV Transmission trial; vaginal flora graded by Gram stain every 3 months, with Cox models for time to genetically linked male HIV-1 infection.
- N
- N=2236 · 2,236 couples in which the woman had HIV-1; 50 genetically linked male infections with vaginal flora data were analysed as events.
- Population
- HIV-1 serodiscordant heterosexual couples in southern and East Africa where the female partner was HIV-positive (and HSV-2 positive, CD4 at least 250, not on ART) and the male partner HIV-negative.
- Outcome
- Female-to-male HIV-1 transmission confirmed as genetically linked by viral sequencing; secondary outcomes were genital and plasma HIV-1 RNA levels.
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What they did
The researchers followed 2,236 African couples in which the woman had HIV and the man did not, for up to two years, as part of a trial of herpes-suppressing therapy. Every three months the women's vaginal swabs were graded by Gram stain as normal flora, intermediate flora or bacterial vaginosis, and men were tested for HIV. New infections in men were counted only if viral sequencing showed the virus came from their partner, and the analysis adjusted for sexual behaviour, circumcision, other sexually transmitted infections, pregnancy and the woman's blood viral load.
What they found
There were 57 genetically linked transmissions, of which 50 had vaginal flora results available. Transmission incidence was 2.91 per 100 person-years when the woman had bacterial vaginosis, versus 0.76 with normal flora, and after adjustment bacterial vaginosis carried a 3.17-fold higher risk. Intermediate flora was not linked to higher risk, and sensitivity analyses using different timing of the swab gave similar results. Bacterial vaginosis was associated with only a small rise (about 0.2 log) in genital HIV levels, which does not seem to fully explain the higher risk.
The limits
What it doesn't show
Vaginal flora was checked only every three months, but it can change weekly, so the flora at the moment of infection is unknown. The number of transmission events was small, making the estimate imprecise, and unmeasured confounding cannot be excluded in an observational analysis. Participants were a selected group (couples tested and counselled together, enrolled in a trial, women with relatively healthy immune systems and HSV-2 infection), so results may not generalise. The study does not show that treating bacterial vaginosis would reduce transmission; that requires a trial.
Key terms
- Bacterial vaginosis (BV)
- A common change in vaginal flora in which protective Lactobacillus bacteria are replaced by other bacteria such as Gardnerella vaginalis and anaerobes.
- Nugent score
- A 0–10 Gram-stain score of vaginal bacteria; 0–3 is normal flora, 4–6 intermediate, and 7–10 bacterial vaginosis.
- Serodiscordant couple
- A couple in which one partner has HIV and the other does not.
- Genetically linked transmission
- A new infection confirmed by comparing viral gene sequences to have come from the study partner rather than someone outside the relationship.
- Person-years
- The total follow-up time contributed by all participants, used as the denominator for incidence rates.
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Common questions
Why did the researchers use viral sequencing?
Some men caught HIV from outside partners. Sequencing ensured only infections that came from the female study partner were counted, which avoids misattributing risk to her vaginal flora.
If BV only slightly raises genital HIV levels, why would it raise transmission so much?
The authors suggest BV-associated bacteria may be shared with the male partner and cause inflammation in the penis or urethra, recruiting more HIV target cells and making the man more susceptible.
Should BV treatment be used as HIV prevention?
Possibly, but this study cannot show it. A previous trial of presumptive STI treatment including metronidazole did not reduce HIV incidence, and BV often recurs, so new approaches such as probiotics are being studied.
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