PARIS — Evidence is growing that "superinfection" (search) with more than one strain of HIV may be more common than previously thought, which could complicate efforts to make a vaccine, experts said Monday at an international AIDS conference.

Scientists reported three new cases of HIV-infected people who initially were doing well without drugs but became sick years later after contracting a second strain of the AIDS virus (search).

"Superinfection is sobering," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (search), the chief U.S. AIDS research agency. He was not involved in the studies.

"That means that although you can mount an adequate response against one virus, the body still does not have the capability to protect you against new infection, which tells you that the development of a vaccine is going to be even more of a challenge."

Fauci said it is too early to tell how big a problem superinfection will become but that he does not believe superinfections are the reason patients on treatment can suddenly deteriorate.

None of the patients in the three cases discussed at the conference were being treated for HIV, which can become resistant to drugs over time.

At the meeting, Dr. Luc Perrin, a professor of clinical virology at the University of Geneva in Switzerland, reported finding superinfections in two Swiss intravenous drug users. In the study, Perrin followed 136 drug users with HIV and found that the amount of HIV in the blood of five patients suddenly shot up after years of control without drugs.

Tests confirmed that two of the five had a superinfection, Perrin said.

"I think superinfection most of the time is transient and is not detected," he said. "It may be that you are more frequently infected than you think but that frequently, you are able to take care of it."

In another study, Dr. Harold Burger of Albany Medical College in Albany, N.Y., said genetic tests on a superinfected woman showed the two viruses mixed and produced a hybrid that took over from the original virus.

Although the development of a hybrid was not surprising — scientists estimate there are 14 mixed strains circulating — the report is the first documented case of two HIV strains, or subtypes, combining in one person to form a third strain.

"The issue is can you get a vaccine that will cover all subtypes?" said Dr. Anton Pozniak, an AIDS specialist at Chelsea and Westminster Hospital in London, who was not connected with the research.

"Say you do. Imagine somebody with a subtype 'C' has sex with someone with subtype 'A' and the two viruses then, circulating in the blood, combine in some way and suddenly some vaccine, because the infection is an 'A/C,' won't work," he said. "Or, perhaps an 'A/C' is more virulent and will attack the immune system in a much more aggressive way than either the 'A' or the 'C' — These are all theoretical possibilities."

Surveys of HIV patients have found many mixtures of virus strains. Scientists suspect they occur when two viruses mix in the bloodstream, but this is the first time they've proven it can happen that way, Pozniak said.

"We just don't know how common it is. People say it's rare but we just don't know," Pozniak said.

"This reinforces the message that we've got to stop HIV today so that we can deal with what we have now and not generate a whole load of new mutants that wouldn't have been there otherwise," Pozniak said.