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[BANGKOK] More than a quarter of the HIV-positive Tanzanian women enrolled in an ongoing study have been infected more than once, with different strains of the HIV virus, according to results presented today at the International AIDS Conference in Bangkok. The phenomenon is known as 'superinfection'.

A second study, on 78 men and women in the United States, reports superinfection in as many as five per cent of patients per year. Together, the findings raise concerns that superinfections may hamper future efforts to control HIV/AIDS with either vaccines or drugs.

The African study is following 600 women to see how many become infected with HIV more than once, and what the consequences of superinfection are. Until now, only sporadic reports of superinfection have appeared in the scientific literature, with little indication of how often it occurs, or its effects over time.

Francine McCutchan of the US-based Henry M. Jackson Foundation is leading the research in collaboration with the Tanzanian ministry of health and the University of Munich in Germany. The study follows women, including commercial sex workers, whose lifestyles place them at high risk of HIV infection and re-infection.

Unpublished results show that, three years into the study, 68 per cent of the women have been infected at least once with HIV. Looking more closely at 98 of the infected women, McCutchan and her team found that 28 per cent had been infected at least twice with different viral strains.

McCutchan concludes that whatever immune defences the women muster against the first infection are failing to protect them against a second infection. Researchers have previously raised the concern that superinfection, if widespread, could pose a challenge to the use of vaccines against HIV: if a vaccine only works against one or a few strains, it may not protect against other strains.

The jury is still out as to what superinfection means for vaccine design, says McCutchan, but she hopes that future investigation may help to provide useful clues.

"We need to study co-infection together with immune responses to see what is protective and what allows superinfection," she says.

She also notes that the lifestyles of some of the Tanzanian women under study make them more vulnerable to superinfection with HIV than the population as a whole, and no one knows how widespread the phenomenon really is.

In the second study, due to be published in the Journal of the American Medical Association, David Smith of the University of California, San Diego, revealed that some of the cases of superinfection in 78 individuals in the United States involve drug-resistant forms of HIV. This suggests that a patient's initial course of treatment may suddenly fail to work if the patient becomes infected with a second, drug-resistant HIV strain.

This has big implications for developing countries where plans are now underway to make antiretroviral drugs more widely available. Even in the United States, patients are not always followed closely to monitor the appearance of drug resistant mutations, says Smith, and superinfection could cause a sudden unexpected downturn in a patient's wellbeing.

In resource-poor countries in Africa, where resistance testing is mostly an unaffordable luxury, the problem could be magnified considerably if superinfection occurs with drug-resistant forms of HIV. "As we start to use more medications there, this will be a continuing emerging problem", Smith predicts. 

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