Grim findings presented Wednesday at the 14th International Conference on AIDS put a successful AIDS vaccine even further off on the horizon. Harvard Medical School's Dr. Bruce Walker startled scientists with word of an unusual case of a patient who, despite building up an immune response to HIV, acquired a second HIV infection from a closely related virus and suffered a major setback. Scientists could be heard cursing and gasping as Walker presented his data.

Over two years, Walker treated 14 patients with HIV drugs immediately after infection for a few weeks, then took them off the medicine to allow their immune systems a chance to detect the viruses as they surged out of hiding. One of those patients, a gay Boston man, went through two rounds of on/off medication and seemed to be doing extraordinarily well.

But in one month's time the virus's replication surged, and when genetically analyzed proved to be 12 percent different from the type of HIV in the patient just 30 days earlier. The patient's immune system was suddenly helpless in the face of this apparently new HIV.

The patient said he had recently had unprotected sex with a male partner. The patient was superinfected with another virus, 88 percent identical to the first, Walker said. "He never got a new response against the second virus, and he declined clinically," Walker said. "The public health implication of this is that it is possible to become infected with a second strain of HIV, even a very closely related one."

Vaccine researchers assumed that an effective vaccine would have to be made up of samples of each of the major seven or eight classes of HIV now circulating around the world. But Walker's patient was infected with two viruses from the same class, genetically very similar. "The strength of the [patient's] cellular immune response at the time of exposure was substantially greater than you get from vaccination, but it could not prevent infection," said John Moore, a Cornell University AIDS vaccine researcher. "This case, albeit anecdotal, has shattering implications for the development of a prophylactic vaccine."

A protective vaccine potentially containing hundreds or thousands of viral samples would be impossible to test and manufacture and may not be tolerated by human beings. Dr. Margaret Johnson, head of the National Institutes of Health's AIDS vaccine program, noted that nobody really knows how common superinfection may be.

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