Important note: Information in this article was accurate in 2002. The state of the art may have changed since the publication date.
BBC News - Thursday, 5 September, 2002
It follows the case of a 38-year-old man who was infected with a second strain of the virus - two years after he had been originally diagnosed as HIV positive.
The case was reported in this week's New England Journal of Medicine by doctors in Switzerland.
They said the case highlighted the need for people with the disease not to engage in risky sex.
They also warned that the case could have implications for scientists trying to develop a vaccine to fight the disease.
Similar cases of re-infection have been reported in the United States and in Thailand.
Diagnosed again
The 38-year-old man was first diagnosed with HIV in 1998. He was subsequently enrolled in a Swiss study to examine the effects of treating the virus early.
The man was successfully treated for more than two years and was taken off the drugs after getting an experimental vaccine intended to boost his immune system.
However, in April 2001 a few weeks after he had unprotected sex with men, his virus level jumped.
Doctors discovered that he had been infected with a second strain of the virus.
The man has since resumed taking medication and is responding well.
Dr Bernard Hirschel of the University of Geneva who is involved in the study said the case highlighted doctors' lack of knowledge about HIV.
"It just shows how little we understand what's happening with HIV-related immunity," he said.
"What would have really have helped to clinch this case is if we had found the source," he said.
"But he had a number of anonymous sex partners in Brazil and it was hopeless."
Vaccine fears
In an editorial in the journal, Dr Bruce Walker and Dr Philip Goulder of Massachusetts General Hospital in Boston said the case showed that people with HIV should avoid reinfection.
They warned that contracting a second strain of HIV could make the disease even harder to treat.
"Superinfection may precipitate more rapid progression of the disease," they said.
Until now, doctors had hoped that infection with one strain of HIV would protect the body from other strains, which would make it easier to develop a vaccine.
More than a dozen strains of the virus have been detected around the world.
Dr Walker said that there was concern that even if an effective vaccine is developed it will not protect against all strains of the disease.
Scientists are already working on vaccines for specific geographic areas.
Risks vary
But Jack Summerside, head of Living Well with HIV services at the UK's Terrence Higgins Trust said the issue was complex.
"It adds to the body of work suggesting the possibility that people with HIV can risk subsequent re-infection with different strains of HIV."
But he added: "There appear to be very specific circumstances where this has been demonstrated.
"These include whether or not the individual is taking anti-HIV treatment, and the degree of difference of HIV sub-type between partners.
"It would be misleading to translate these into overly simplistic health information for people with HIV regarding condom use with HIV positive partners."
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