"Importantly, our study doesn't say that a vaccine is not possible," said Walker. "It says that the cross-protective immunity generated by natural infection is not that great." Particularly striking, said Walker, was that even though the two strains of virus were only about 12 percent different genetically, the difference in the surface proteins targeted by the immune cells was 50 percent. This broader immunity-related difference was what rendered the new virus unrecognizable by the existing immune response, he said.

"This has important basic research implications, because investigators have long believed in a phenomenon of 'innocent bystander activation,' in which if you boost immunity to one viral strain, it will boost immunity to others," he said.

While there had been previous reports of superinfection of people by different strains of HIV from disparate regions of the world, the report of superinfection from two relatively close North American strains of HIV is especially cautionary, said Walker. "This study emphasizes the importance of achieving broad cross-reactive immunity and incorporating immune responses to as much of the virus as possible to counteract these problems," he said.

Although the emergence of documented cases of superinfection suggests that no single HIV vaccine is likely to be completely effective for all strains of the virus, "if patients develop even partially effective cellular immunity, they may end up with lower viral load than they would otherwise have and will do better," said Walker.

Walker said that strategies to develop HIV vaccines may need to take a cue from influenza, where efforts to develop new vaccines are ongoing because new strains of the virus are emerging continually. "On the other hand," said Walker, "we're learning an incredible amount about the immune system and how it deals with chronic viruses. I th
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Contact: Jim Keeley
keeleyj@hhmi.org
301-215-8858
Howard Hughes Medical Institute
27-Nov-2002