`Incredibly high risk` of male-to-female HIV transmission reported from South Africa

In sub-Saharan Africa, young women are at high risk of HIV infection, with a prevalence of 21% among 21 year-olds recently reported from South Africa. Now, the same team of US and South African scientists report that this prevalence occurs in young women with few lifetime partners, suggesting a much higher rate of male-to-female transmission than previously reported. The researchers report their findings in the April 23^rd edition of AIDS.

Similar prevalence rates have been reported from elsewhere in sub-Saharan Africa among young women with few sexual contacts. “Unfortunately”, write investigators from the University of North Carolina, United States, and the University of Witwatersrand, South Africa, “previous studies of transmission probabilities were limited by a focus on serodiscordant couples or study designs that could not capture the risk properly.” So the team used a cross-sectional national household survey of South Africans aged 15-24 years to estimate the per-partnership probability of transmission of HIV from men to women.

The study was conducted in 2003 among 11,904 young South Africans, using oral collection of samples for HIV testing. All sexually experienced people were asked about lifetime partners, their first sexual partner, and the last three sexual contacts in the past twelve months. The overall number of sexual contacts was estimated by multiplying the number of months of each relationship with the number of sexual contacts reported in the past month. If the relationship was reported as ongoing and the number of sexual contacts in the past month was reported as zero, a rate of 0.5 contacts per month was used. The authors then used mathematical modelling to estimate per-partnership HIV transmission.

Of 3993 women who had available data, the mean reported number of partners was 2.3 [95% CI 2.1-2.4], with an overall HIV prevalence among sexually active women of 21.2% [95%CI 17.9-24.5%]. A significant increase in prevalence was seen with increasing numbers of sexual partners, from 15.2% of women reporting one partner to 28.5% among those with more than three partners.

Notably, the team found evidence of significant reporting error of number of sexual partners, as seen in previous studies. Part of the reason for under-reporting might be due to participants failure to report forced sex as a sexual contact, which may be as high as 10% of women, the authors suggest. However, all models done to account for reporting error and also the possibility that the sexual partners of female participants had higher HIV prevalence than the general population suggested male-to-female transmission rates in excess of 70%.

“For a range of plausible values of the partner prevalence, the estimated per-partnership transmission probability varied from 0.74 to 1.00” [95% CI 56-1.00], report the authors. These probability rates are described by the team as “very high”, since the majority of previous studies have estimated male-to-female HIV transmission as lower than 50% per partnership (AIDS 21: 861-865, 2007).

Conservative estimates of the transmission rates per sexual encounter ranged from one in fifty to one in 16. The authors note that factors related to higher levels of HIV sexual transmission including increased infectiousness due to sexually transmitted infections and acute HIV infection. Susceptibility in young women may be increased by abnormal vaginal flora, hormonal contraception, and pregnancy.

“Our data indicate an incredibly high risk of HIV transmission per-partnership in young women in South Africa. These findings are supported by data from another large population-based study of young people in South Africa, which reported a per-partner HIV transmission probability estimate close to 1.0 for young women”, the authors note. These results, say the team, “demonstrate extreme risk that demands a rethinking of HIV risk and prevention interventions. Prevention programmes targeting young women and their partners that accurately and effectively communicate the extreme risk young women face and that provide improved strategies for HIV prevention cannot be delayed further”, they urge.