Half of patients on anti-HIV medication in sub-Saharan Africa die or discontinue their treatment within two years of starting it, research suggests.

For many patients, treatment centres are too far away or the cost of drugs remains too high, while others start anti-retroviral therapy too late, or are deterred by the side effects, say the US and South African researchers in a review in the Public Library of Science Medicine journal. Their review of the literature suggests that around 40% cent of patients were lost to treatment programmes after two years but they estimate however, that the true figure is likely to be around 50%.

Lead author Sydney Rosen of the Boston University School of Public Health said: "For those who have struggled to launch and expand treatment programmes in resource-constrained settings, reaching a 60% patient retention after two years' of treatment in just a few years' time is an extraordinary achievement.” However, "at the same time, losing up to half of those who initiate therapy within two years is cause for concern."

The review notes that many people starting anti-retroviral therapy in Sub-Saharan African already have very advanced HIV disease – and that stopping treatment in these circumstances could prove fatal very quickly. The drop-out rates vary hugely, however, and the authors say it is vital for the treatment centres with the worst retention rates to learn lessons from the best performing ones.

A leading South African HIV specialist, Prof Glenda Gray from the University of Witwatersrand, said that having 40% or more of patients drop out "was simply not good enough". She claimed: "It's not enough just supplying the drugs. People need to be reminded that they have to keep on taking them. I suspect that many people start to feel better once they've taken the drugs for a while. And once this happens they stop taking them. We need to get better at supporting and educating patients."

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The PloS article suggests that some people abandon treatment programmes for fear that their HIV status will be disclosed – putting them at risk of social exclusion or even physical danger, while Unicef, the United Nations Children's Fund, has warned that greater political leadership is needed to challenge the stigma that still surrounds the disease in South Africa.

The global fight against AIDS had already suffered a major setback last month with news that Merck & Co had halted trials on its HIV vaccine – generally considered one of the most promising in development. The adenovirus vaccine had conspicuously failed to prevent infection in a study of 3,000 high-risk volunteers around the globe.

A follow-up trial to be conducted in South Africa by Prof Gray was also cancelled. “That was a big blow," she said. "It's going to be hard to move forward after that." Last year 2.9 million people died from AIDS, 2.1 million of them in sub-Saharan Africa. Over 20 million people in the region are currently thought to be infected with the virus.