THE International Aids Conference in Durban last week came out with some disturbing statistics. Of the 300,000 people who are infected with HIV in SA every year, almost a third are young women. This means that about 2,000 young women are infected with HIV every week.

These are big numbers. By comparison, Brazil, with the most severe HIV incidence in South America, has 43,000 new infections a year.

So we clearly need to try to reduce these new infections. The question is, how? In the past few years, there have been many studies showing that people on antiretroviral treatment (ART) were highly unlikely to transmit HIV to their sexual partners.

The effect is so dramatic that many scientists and policy makers started suggesting that the main thing we need to do to end AIDS is to put all infected people on treatment, as soon as they are diagnosed. This "treatment as prevention" approach seems attractive.

It means we can avoid difficult discussions about people’s sexual behaviour, and how to encourage and enable people at risk to make safer sexual choices.

But the truth is that we cannot treat our way out of this epidemic. HIV treatment is essential, and needs to be extended to as many people as possible, but it is not enough on its own to stop this ongoing tsunami of new infections.

There are a few important reasons for this.

First, it is very difficult to reach the level of coverage needed to see this prevention effect. You need to have about 80% of all HIV-infected people on ART.

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In SA, we have about 3.5-million people on treatment, out of an estimated 6.9-million infected people. This is an excellent result. But our health system is already struggling to manage and retain the number of HIV-positive patients on treatment. Enrolling another 2-million to 3-million is enormously challenging.

Just as a comparison, Canada has less than half of its HIV-positive population on treatment, and the US about a third. And these are wealthy and well-resourced countries.

Second, our neighbour, Botswana, has achieved high levels of ART coverage, but has not seen rates of new infections decreasing.

Last, new evidence coming out of the 2016 International AIDS Conference has suggested that randomised controlled trials of treatment as prevention have so far not shown a reduction in new HIV infections.

So, where does that leave us?

Obviously, the ideal solution is still an effective HIV vaccine, but while lots of new vaccine candidates are being tested, finding a good vaccine is still some way off.

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We can, however, already prevent every single HIV infection.

We have old tools, such as condoms and behaviour-change programmes, and newer tools, such as male circumcision and daily antiretroviral tablets (also called pre-exposure prophylaxis), which can prevent people from being infected.

These tools, used either individually or in combination, have proved to be very effective. But, we have not been very good at finding the people most at risk, and supporting them to use these different methods.

Yet, we know it can be done. We need to target the places and people where most new HIV infections are happening.

This does not occur uniformly around the country, and we waste resources trying to do everything for everyone, everywhere. We need to offer comprehensive services to young people.

Young women, especially, need less finger-wagging about "blessers", and more support to make their life choices more safely.

We may not be able to stop young women being in relationships with older men, but we need to make sure that these men are on antiretrovirals if they are HIV-infected, and that women can protect themselves from becoming infected.

• Johnson, a medical doctor and epidemiologist, is head of the health practice at Genesis Analytics. He writes in his personal capacity