HIV incidence was very high among young women in this population and we identified several potentially preventable factors strongly associated with incident HIV including consumption of alcohol. A relationship with alcohol has been observed in several recent African studies7,11
. In our study, women consuming ten or more drinks per week were at a more than three-fold increased risk of acquiring HIV than non-drinkers. Drinking establishments in sub-Saharan Africa are known HIV high transmission sites12
. Alcohol can lead to behavioural disinhibition with resultant risky sexual behaviour, partner violence and coercive sex, as documented in South Africa and Uganda13,14,15
. Since alcohol may be purchased for women by their clients, it may also be a proxy marker for sex with high risk partners. Alcohol may also influence susceptibility to HIV at a biological level16,17
, possibly due to a suppressive effect on the degradation of the HIV envelope protein, gp12018
We found an increased risk of HIV associated with current gonococcal infection as has been seen in sex workers in Nairobi19
but no association with trichomonasis or C. trachomatis
, unlike recent studies19,20
. This may have been partly because women were diagnosed in the field and offered prompt treatment and because STI were diagnosed aetiologically at selected visits, with syndromic management being offered at other visits, so reducing study power to detect an association with HIV acquisition.
Condom use was not associated with a reduction in HIV incidence. As in other studies in East Africa21
, reported condom use during the trial remained disappointingly low, despite repeated education and counselling sessions on HIV prevention. Our results indicate the need for more intensive HIV prevention programmes in this population.
Genital ulcers were not significantly associated with HIV but may have been underestimated since participants were only examined every six months unless they had symptoms. There was no significant association between HIV acquisition and cervical ectopy in contrast to a South African study where ectopy of more than 20% was associated with HIV incidence22
Hormonal contraception was associated with HIV incidence. This has been found in HSV-2 seronegative women in Uganda and Zimbabwe23
and sex workers in Kenya24
although no association between hormonal contraception and HIV incidence was observed in Cape Town25
. Our data suggest that hormonal contraception can increase the risk of HIV in high risk women although it is possible that this association may in part be attributable to residual confounding by sexual behaviour.
VCT has been associated with self-reported behaviour change in Kenya26
. In common with studies in Uganda and Zimbabwe, we found no difference in HIV incidence between those who accepted or did not accept VCT27,28
. Our selected trial population, however, received repeated risk reduction counselling irrespective of uptake of VCT which may have diluted any impact that VCT could have had as an HIV prevention tool in this setting.
Injections given outside our research clinic were also associated with HIV acquisition. Estimates of the proportion of HIV infections attributable to injections in sub-Saharan Africa range from 1-30%29,30
. However the causal direction of this association is not straightforward. In our trial the main reason for injections was malaria treatment. Over-diagnosis of malaria is common in sub-Saharan Africa31,32
and our clinical data suggest that, in at least seven cases, injections were given to treat symptoms possibly related to a seroconversion illness which may have been misclassified as malaria or another infection. A similar temporal association between a possible seroconversion illness and injections has been seen in Uganda33
. If injections are given for symptoms related to seroconversion, this emphasises the necessity to maintain safe injection practices since risks of onward transmission will be high at this stage of HIV infection due to high viral load.
We could not examine the independent effect of HSV-2 on HIV incidence. However, 80% of women in this population were found to be HSV-2 seropositive at screening5
. Our results will therefore be generalizable to many women in similar occupational settings. A further limitation is reliance on self-reported sexual behaviour since such information may be subject to social desirability bias34,35
. Finally, with 63 seroconversions, the power of our study to detect significant associations with uncommon exposures will have been limited.
In conclusion, this prospective study has demonstrated a strong association between HIV incidence and young age, alcohol consumption, injections, payment for sex, gonococcal infection and hormonal contraception. Interventions are needed to address the risk associated with alcohol use and to sustain control of other STIs. In addition prevention messages should be targeted to young women in this population. Further work is needed to examine the effect of injections and hormonal contraception on HIV incidence in high risk settings.