Anal sex may transmit four in ten HIV infections in US women at high risk

A study presented at the 2016 HIV Research for Prevention (HIVR4P) Conference in Chicago, USA suggests that, in women at high risk of HIV infection, 40% or more of the HIV infections in this group might be transmitted via anal intercourse.

Because HIV is transmitted, according to different estimates, from 2 to 18 times more easily via anal than vaginal sex (the Centers for Disease Control's own estimate is 12 times), the contribution of anal intercourse to HIV infections in heterosexual women could be high, or even predominant – even if anal sex is only 5-10% of all sex.

Nearly 10,000 (9304) ‘high risk’ HIV-negative, non-drug-injecting women aged 18-60 who had been in the 2010/2013 National HIV Behavioural Surveillance Survey were contacted and surveyed to find the proportion who had had anal sex during the last year, and the proportion who had anal sex the last time they had sex. Because previous studies have established a link between low income and high HIV risk, the definition of ‘high risk’ was that they had low incomes and lived in one of the 20 highest-prevalence metropolitan areas in the US.

The average age of the women was 36.8 years, and 73% of them were of black ethnicity. Nearly a quarter (22%) had had sex in exchange for money, goods or shelter in the last year.

Thirty-two per cent had had anal sex in the last year, and 27% had had anal sex the last time they had sex. There was some increase in reporting of anal sex with age, with 22% of 18-19 year olds having had anal sex at last sex, and 31% of 40-60 year olds. Women who had transactional sex were twice as likely to have had anal sex during the year than other women, and were 27% more likely to have had it at last sex.

Having anal intercourse was associated with other HIV risks: women who had anal sex reported three times more sexual partners in the last year than women who had only had vaginal sex. They were 50% more likely to report sex with casual partners.

Of particular concern was the fact that they were 60% less likely to have used condoms the last time they had anal, as opposed to vaginal, sex. Overall, 11% of all condomless sex acts among the whole study sample, including women who did not have anal sex at all, were anal.

The prevalence of anal sex varied from city to city. A third of women in San Juan, Puerto Rico (34%) and New York City (32%) reported anal sex on the last occasion they had sex compared with about 15% in New Orleans. The annual rate varied from 43% in New York to 17% in New Orleans – and a very high 60% in San Juan. The prevalence in young women aged 18-19 varied even more widely, from 6% reporting it in the last year in New Orleans to 57% in San Juan.

The researchers calculated the proportion of HIV transmissions that might be due to anal sex, taking into account local HIV prevalence, incidence, and proportion on treatment. Averaged over the 20 cities it came to 38%, ranging from 20% in Denver to 48% in San Juan and 44% in New York. Because these cities had higher rates of anal intercourse than others, the median contribution of anal sex to transmissions was about 33%, roughly what it was for Miami. Thus over a third of HIV infections in women may be due to anal sex.

Among young women under 25, the proportion was slightly lower at 30% and was also lower in non-Hispanic white women, at 25%. In women having transactional sex it was higher, at 43%.

There was considerable statistical uncertainty over these figures, however, given the range of estimates for the transmissibility of HIV via anal sex. This meant that the highest and lowest estimates for the proportion of infections due to anal sex (90% confidence interval) were 15% and 64%. If HIV transmissibility via vaginal sex is roughly the same as it is through being the insertive partner in anal sex – as some studies estimate – then transmissibility via anal sex could be at the upper range of that estimate. In gay men, being receptive in anal sex has 18 times the risk of being insertive.

Another caveat was that women who had injected drugs within the last year were excluded. If they were included then it would mean the proportion of HIV infections due to anal sex in women in general would be lower. Also, this study only looks at a specific population of women already known to be at high risk of HIV. Fewer women at lower risk might have anal sex.

Nonetheless, this study shows that a considerable proportion of HIV infections in US women may be transmitted through anal sex and, especially as condom usage in anal sex appears to be lower, safer-sex advice needs to start incorporating anal sex in messaging for women and consideration should be given to providing PrEP for women more widely.