A disappointed and puzzled Brisbane biotech firm announced Sunday that its experimental AIDS vaccine failed to protect white and Latino volunteers against HIV infections, while inexplicably shielding two-thirds of the black, Asian and other non-Latino minority participants.

Officials of VaxGen planned to hold a briefing this morning to elaborate on the unexpected results of the company's three-year study of an experimental vaccine tested primarily on gay men in the United States and Europe.

VaxGen vice president for research Phillip Berman, who began work on the vaccine in 1984, put the best spin on the divided findings.

"This is the first time we have specific numbers to suggest that a vaccine has prevented HIV infection in humans," Berman said, adding "we're not sure yet why certain groups have a better immune response."

As the world's largest and most advanced AIDS vaccine study, VaxGen's clinical trial has been closely watched by researchers worldwide looking for ways to blunt an epidemic that has claimed about 25 million lives since AIDS was first recognized in the early '80s.

Now scientists must figure out why, among the 4,511 white and Latino volunteers, the infection rates were virtually the same among those who received the vaccine and those who took the placebo.

However, among the 314 blacks in the study, those who received the vaccine were 78 percent less likely to contract HIV than those in the placebo group. When another 184 Asian and other non-Latino minority volunteers were added to the African American category, the effectiveness of the vaccine declined slightly to 67 percent -- a figure that still impressed UCSF AIDS researcher Tom Coates.

"This is an intriguing and potentially important finding that needs to be followed with further study," said Coates, adding that because the number of black and non-Latino volunteers was relatively small there's a chance the findings are a fluke.

SIGNIFICANT DIFFERENCE

However, VaxGen said its statistical analysis indicates the divided findings represent a genuine difference in the efficacy of the experimental vaccine and not a quirk.

"VaxGen's conduct of this trial lived up to the highest standards of scientific integrity," said Walter Dowdle, a former deputy director of the U.S.

Centers for Disease Control and Prevention and chairman of an independent data monitoring committee that oversaw the conduct of the study.

By design, VaxGen recruited mostly gay male volunteers who were HIV- negative at the start of the trial, but who were thought to be at higher than average risk of contracting HIV because they engage in anal intercourse with men.

Two-thirds of the volunteers were given a series of injections with the experimental vaccine Berman developed. The rest were given a placebo, and the two groups were ultimately compared to see whether they had differing rates of infection.

VACCINE THEORY

The theory behind the vaccine was simple.

The virus is a string of genetic letters similar to DNA. This malicious genetic package comes wrapped inside a harmless protein coat. If HIV were an M&M, the chocolate would be the deadly viral material while the candy shell would be the protein coat.

What VaxGen did was to strip off that harmless protein coat and inject that coat into patients -- thus, in theory, alerting their immune systems to be on guard for any foreign invader that looked like HIV.

UCSF's Coates said he found it encouraging that VaxGen reported that blacks and non-Latino minorities produced large numbers of antibodies against HIV, while whites and Latinos did not seem to produce these neutralizing antibodies. Though at a loss to explain this fact, Coates called the difference in antibody output "a very interesting development."

VaxGen said it planned "to confirm if there was a direct correlation between the level of antibodies and the prevention of infection."

RISKY BEHAVIOR REDUCED

In one other positive development, VaxGen reported that an independent study by the CDC found that volunteers in the study did not increase risky behavior just because they might have been vaccinated. AIDS watchers had been concerned that volunteers might get a mistaken feeling of invulnerability and be more likely to engage in unprotected anal sex.

But VaxGen said such risk-elevating behaviors actually declined overall, presumably because the volunteers also got counseling every six months in addition to the shots.

As the findings are digested by AIDS activists, scientists and government regulators, the question will be what comes next. In a statement released late Sunday, VaxGen chief executive Lance Gordon said the company planned to push ahead despite what must be viewed as a setback.

FUTURE RESEARCH

"We intend to continue development of the vaccine . . . including additional studies as necessary, for use in groups in which the vaccine demonstrated a significant reduction in infection," Gordon said, adding that the company will also work in parallel to make its product more useful to whites and Latinos.

But any move to push for U.S. Food & Drug Administration approval of the current vaccine for targeted racial or ethnic groups is almost sure to be controversial.

"Should we go out and vaccinate all African Americans today?" asked UCSF's Coates rhetorically. "Not without further study."

Pat Christen, executive director of the San Francisco AIDS Foundation praised VaxGen for its "remarkable and historic" study, but said, "It is completely premature to determine whether or not (the experimental vaccine) is truly effective for particular populations."