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The success of existing therapies and future vaccines for AIDS has been thrown into doubt by the case of an HIV-infected patient who has become 'super-infected' with a second strain of the virus.

Research published in this week's Nature suggests that HIV-infected patients can remain vulnerable to subsequent infection despite building up an immune response to the virus. Bruce Walker of Harvard Medical School, United States, and colleagues say that their results may have "important implications for public health and vaccine development".

The patient in question was treated with anti-retroviral drugs soon after becoming infected with HIV, using a 'stop-start' procedure where treatment is deliberately interrupted from time to time. Despite responding well initially, the patient was later found to have been infected with a similar strain of HIV, most probably through unprotected sex during the course of treatment.

The study was first announced in July at the International AIDS Conference in Barcelona where it stimulated scares that future vaccines may be ineffective. Ronald Desrosiers, a vaccine researcher at Harvard University, United States, was reported by the Wall Street Journal as saying that the research was a "huge blow to vaccine development".

But according to Andrew McMichael and Sarah Rowland-Jones of the Weatherall Institute of Molecular Medicine in Oxford, United Kingdom, this is not necessarily the case. In a separate Nature article, they say that the patient studied may not be representative, and that many people infected with HIV may be able to repel superinfection. In any case, they say, superinfection is likely to be rare.

The authors of the study also acknowledge that superinfection in an individual already infected with HIV is not the same as a healthy person developing an infection. This implies that the immunity generated by vaccination may not leave individuals open to the same susceptibility to infection as seen in this case study.

"This single-patient analysis raises many questions, but whether the news is bad, neutral or even good remains to be seen," say McMichael and Rowland-Jones. "Although causing a brief pause for thought, nothing here should slow or divert efforts to develop an HIV vaccine."

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