Does delaying sex contribute to falls in HIV infection?

This situation may be changing: the 2010 UNAIDS report¹ found that HIV prevalence in young women aged 15 to 19 was ‘only’ twice that of young men the same age, and the disparity was not as great as it was among young women and men five years older.

This may be nothing to do with abstinence, however: it may be because more young people are using condoms when they do have sex. This was certainly one of the reasons South African researchers² cited for the fact that annual HIV incidence appears to have declined in young women aged 15 to 24, from 5.5% in 2003-05 to 2.2% in 2005-08. This was attributed to an increase in condom use from 31.3% in 2002 to 64.8% in 2008 among respondents aged 15 to 49 and, specifically, not to a reduction in the number of partners they had.

Other studies, however, have found that falls in the age at which people start having sex, and the number of partners they have, can change significantly over time and are associated with falls in HIV incidence. One study³ found that the proportion of urban young women aged 15 to 19 in Uganda who said they had ‘never had sex’ increased from 44 to 60% between 1990 and 2000, with an even sharper increase in young men from 33 to 66% - which also suggests a decline in the social desirability for young men of having sex.

In Zimbabwe,⁴ significant falls in HIV incidence (from 4.8% in 1992 to 3.4% in 1999 in pregnant women in Harare, for instance, and from 3.0% in 1994-95 to 1.3% in 2002-03 in male factory workers) do seem to have been at least partly caused by, or at least accompanied by, falls in sexual risk behaviour. For instance, one study⁵found that the proportion of young men aged 17 to 24 who had started having sex fell by nearly 50% - from 45% in 1998-2003 to 27% in 2001-2003; the proportion of young women aged 15 to 24 who started having sex fell from 21% to 9%.

A synthesis of national surveys from Zimbabwe,⁶ however, found that age at sexual debut did not show any significant trend. What did make a difference, it appears, was a change in the number of sexual partners people had, and specifically a reduction in casual sex amongst men, of all ages. There was a striking fall in the proportion of men reporting sex with ‘non-regular’ partners in national surveys from 60% in 1999 to 20% in 2003. This was supported by a 20% increase in condom use. It has been suggested that a decline of this magnitude in casual sex was as much to do with Zimbabwe’s economic crisis and a resultant reduction in the money men had to spend on pursuing sex as any conscious decision to reduce their sexual risk. Nonetheless, UNAIDS concluded that the reductions in HIV incidence in Zimbabwe were real and at least partly driven by reductions in risk behaviour.

For more data on the decline of prevalence in Zimbabwe, see Being faithful.

However, another study from the rural province of Rakai in Uganda⁷ found that abstinence rates in teenagers had declined from 60 to 50% in women and 32 to 28% in men between 1990 and 2002. The same study found that an accompanying decline in HIV prevalence from 17.6 to 11.4% was largely due to mortality: the peak in HIV incidence in the late 1980s was now seeing its consequence in a peak in AIDS deaths, and the main driver of a reduction in the number of people with HIV was that more people were dying from it than becoming infected with it.