The SARS vaccine entered a phase I clinical trial last December and enrollment was completed in May, said Dr. Gary Nabel, director of NIH's Vaccine Research Center.

So far, 15 people have been given the vaccine.

"I don't think we've seen any adverse events, so I think the news so far is good," Nabel said, noting that the data are still being collected and analyzed and the agency will not draw any firm conclusions until that process is completed.

"I would think ... by sometime early next year we will have the results," Nabel added.

That would give NIH officials an idea of the safety profile of the vaccine and whether it induced immune responses in the study participants that would protect against SARS infection, he said.

SARS first emerged in China in late 2002, and the virus spread rapidly around the world, infecting more than 8,000 people and killing 774.

The vaccine being developed by the NIH targets the spike protein, which the SARS virus uses to bind to and infect human cells. The vaccine, which Nabel helped develop, contains a piece of DNA that encodes for the spike protein and is designed to elicit the body's immune system to produce antibodies against the virus to help ward off infection - if the person is ever exposed to the real thing.

The vaccine is being manufactured for the trial by Vical Inc. in San Diego, but the company has not indicated any interest in developing the vaccine, Nabel said.

"At this point, it's unlikely industry is going to step in and develop a SARS vaccine" because the potential market is too small to be worth the investment, he said. "This would be the kind of vaccine the government would have to decide is of sufficient high priority to move it forward under Project Bioshield."

The final stages of development will likely entail animal testing, Nabel said. The Food and Drug Administration enacted a rule in 2002 that allow studies in animal models to substitute for human trials to show efficacy of a new medication in situations where studies on people would be impossible or unethical.

"This is really invaluable because there's just no other way," Nabel said.

It would be unacceptable to intentionally expose people to SARS to test the vaccine, and there is not a natural outbreak occurring anywhere where those likely to become infected could be inoculated.

Even if the vaccine does become available for widespread use, a vital question may still remain.

"As the SARS virus has evolved, it has acquired mutations that seem to make it act differently," Nabel said. "So we can make a vaccine for the highly lethal form that originally broke out, but the question is whether the virus is evolving in a way that might make it resistant to antiviral drugs or the vaccine itself."

A team of researchers led by Stephen Harrison of Children's Hospital and Harvard Medical School reported in the Sept. 16 issue of the journal Science that they had obtained the first molecular images of how the SARS spike protein binds to human cells.

Harrison said a detailed understanding of this process may enable researchers to predict whether a new variant is infectious.

"One of the critical issues in a SARS epidemic would be to predict whether a given variant of the virus will jump species or move laterally from one human to the other," he said in a statement. "Understanding the structure of this complex will help us understand what these mutations in the spike protein mean in terms of infectivity."

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