THERE can be no surer way of averting a sexually transmitted infection such as AIDS than avoiding sex. That much is obvious. And it is also convenient for religious lobbyists who believe that premarital sex is a sin. But is it realistic? Those lobbyists argue that a popular alternative—known in the jargon as “abstinence-plus”—which recommends chastity but also explains how to use condoms, is likely to make things worse by encouraging earlier intercourse. “Abstinence-only” teaching, they reckon, should be more effective.

That, of course, is a possibility. But it is a testable possibility. And Kristen Underhill and her colleagues at the University of Oxford have, over the past few months, been testing it. Their conclusion is that it is wrong. Abstinence-only does not work. Abstinence-plus probably does.

Last month Dr Underhill published a review of 13 trials involving 16,000 young people in America. The trials compared the sexual behaviour of those given an abstinence-only education with that of those who were provided with no information at all or with whatever their schools normally taught. Pregnancies were as numerous in both groups. Sexually transmitted diseases were as widespread. The number of sexual partners was equally high and unprotected sex just as common.

Having thus discredited abstinence-only teaching, Dr Underhill and her colleagues decided to evaluate the slightly more complicated message of “abstinence-plus” using 39 trials that involved 38,000-odd young people from the United States, Canada and the Bahamas. Their results are published in the current issue of Public Library of Science Medicine.

This tuition—compared, as before, with whatever biology classes and playgrounds provide—reduced the number of pregnancies in three out of seven trials (the remaining four recorded no difference). Four out of 13 trials found that abstinence-plus-educated teenagers had fewer sexual partners, while the remainder showed no change. Fourteen studies reported that it increased condom use; 12 others reported no difference. Furthermore, in the vast majority of cases, abstinence-plus participants knew more about AIDS and HIV (the virus that causes the disease) than their peers did. And the tuition often reduced the frequency of anal sex (which brings a greater chance of passing on HIV than the vaginal option). In contrast to the fears of the protagonists of abstinence-only education, not one of the trials found that teenagers behaved in a riskier fashion in either the long or the short term after receiving abstinence-plus instruction.

Unfortunately (and surprisingly) only two of the studies addressed the question of disease transmission directly, and the numbers involved were too small to find a statistically significant difference between groups. Nevertheless, Dr Underhill's pair of reviews should make informative reading for policymakers. America's government earmarks money for abstinence-only teaching, which is matched by individual states. It should review that policy—which is clearly no better than the alternatives, and is probably worse. Its generosity to needy foreigners is similarly prescriptive. Of the $15 billion promised over five years by PEPFAR, President George Bush's personal anti-AIDS initiative, $1 billion is reserved for groups that intend to fight AIDS without mentioning condoms. Though Dr Underhill's results apply only to North America, they do suggest a need to investigate what happens elsewhere, in case PEPFAR's policy, too, needs to be reviewed.

A dose of prevention

Teaching people about what they might wear during intercourse is an important way of reducing the chance of them catching HIV. But teaching them, in addition, about what drugs they could take to reduce that risk may be added to the syllabus in the future. A vaccine is still a long way off, but four clinical trials—in Peru and Ecuador, Thailand, Botswana and also America—are assessing how well daily anti-retroviral pills, which are normally prescribed to control established HIV infections, prevent the virus infecting healthy people who do dangerous things. The results of these trials will be plugged into epidemiological computer models to assess the likely effect of various drug-distribution policies.

One model intended to do exactly that has already been built, by Ume Abbas and John Mellors of the University of Pittsburgh. It is designed to mimic a mature HIV epidemic in sub-Saharan Africa—which it did rather well when the researchers tested its output against data from Zambia, a country in which the epidemic has remained stable for a decade.

Writing in PLoS Medicine's sister journal, PLoS ONE, Dr Abbas and Dr Mellors describe what happened when they added prophylactic anti-retroviral drugs to the model. They experimented with different measures of drug efficacy and with different groups of people taking the pills.

Assuming that anti-retrovirals work 90% of the time and are taken by three-quarters of sexually active people, their model suggests that new HIV infections in sub-Saharan Africa would be cut by 74% over 10 years. Unfortunately, the idea of providing and delivering so many drugs to so many people is logistically implausible. And even if it could be done, it would cost about $6,000 per HIV infection averted—a lot of money in Africa.

However, giving the drug to the 16% of Africans who behave most riskily would be easier and could lead to a 29% reduction over a decade at only a tenth of that cost. A harsh calculation, but a realistic one—unlike expecting teenagers to give up sex because you tell them to.