HIV photos may bode ill for vaccine.

There is good news and bad news in the quest for an AIDS vaccine. The federal government has given the green light for wide-scale testing of a vaccine produced by VaxGen, a biotech firm in the San Francisco Bay area, but some researchers say the vaccine is probably based on faulty assumptions.

The trials for the vaccine, known as AIDSvax, will involve 5,000 volunteers at multiple sites around the country. The three-year study will determine how safe the vaccine is and whether it prompts an effective immune response. The vaccine does not use the virus itself but instead is based on a particular protein found on the surface of HIV. In theory that protein will produce antibodies that will attack the virus if it enters the body. The vaccine has undergone limited testing so far, producing an antibody response in 99% of those injected with it.

However, two weeks after the vaccine was given clearance, scientists using advanced technology managed for the first time to photograph HIV attacking a cell. The study, by researchers at the Dana-Farber Cancer Institute and New York's Columbia University, was quickly hailed as a landmark by David Baltimore, head of a government advisory committee on AIDS vaccines. The photos provided a wealth of information, including how the virus eludes detection by the body with such techniques as cloaking itself in sugar molecules commonly found in the body's proteins.

More important, the pictures illustrated that the virus uses not one but two probes to invade CD4 cells. The virus docks first at one site, then quickly reveals a hidden antenna to latch on to the second site before the cell can repel it with an immune response. Some researchers believe the data shows that AIDSvax has little chance of working because the virus manages to evade the kind of direct immune attacks the vaccine is meant to produce. VaxGen executives, though, said they remained confident that the vaccine would be effective.