An experimental vaccine appears to be effective against a strain of flu virus that experts fear could spark a devastating pandemic, offering the first evidence that any inoculation could provide a powerful weapon against the deadly microbe, a federal health official said yesterday.
Two doses of the vaccine produced an immune system response potent enough to neutralize the virus in tests on 113 volunteers who were injected as part of a federally sponsored study being conducted at three U.S. universities.
"This is very good news," said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. "This is the first vaccine that anybody has that has been tested to show that you can actually produce a robust immune response."
Public health authorities are alarmed by a strain of flu virus known as H5N1, which has been spreading primarily in birds across Asia and in Russia. It also has infected more than 100 humans in the past 18 months, killing about half of them. If the virus starts to spread efficiently among humans, experts fear it could trigger a global pandemic that could kill millions.
In response, millions of birds throughout Asia have been slaughtered to try to stem the spread of the virus, governments and the World Health Organization have been stockpiling antiviral drugs, and scientists have been scrambling to produce an effective vaccine.
Much more testing will be needed to determine exactly how the vaccine could be used, and other hurdles remain, including being able to produce and distribute large quantities of vaccine in the event of a pandemic, Fauci said. But he said the results represent a crucial milestone.
"The data are pretty solid," Fauci said. "They're preliminary, but solid. Having a vaccine in case we had a pandemic flu is a very important part of a comprehensive response to a pandemic."
Other experts agreed that the tests marked a key step in the world's attempt to prepare for a possible pandemic.
"These are very important studies," said Michael T. Osterholm of the University of Minnesota. "This is confirmation of what we hoped would be the case. We all had anticipated that two doses were going to work. If it didn't, we would be in trouble."
But Osterholm said the world would still be woefully unprepared if a pandemic occurred. The capability to produce and distribute a large amount of vaccine quickly, for example, is far from adequate.
"That's the Achilles' heel of our public health response," Osterholm said. "These data are encouraging and supportive, but in the end, a lot of people would be left unvaccinated in the event of a pandemic."
The new vaccine results stem from a study being conducted at the University of Maryland at Baltimore, the University of Rochester in New York and the University of California at Los Angeles. Researchers tested a vaccine made by Sanofi Pasteur, a French company, using a version of the virus originally isolated in Vietnam that has had a gene removed to make it harmless.
The vaccine was administered in four dosages to 452 healthy adults beginning in April. They received a booster shot four weeks later. Blood samples showed the level of antibodies produced by the subjects' immune systems rose in direct proportion to the dose they received, with the strongest dose producing what is considered an adequate response, Fauci said.
"That put the antibody level in the range that you would predict would be protective. Obviously, you never know until you test it in the field. But generally you can pretty much gauge that if you get the antibody level up to a certain value, that pretty much guarantees protection," Fauci said.
The federal government has purchased 2 million doses of the vaccine and plans to buy more. The results will help officials determine how to package the vaccine for use if necessary.
"These results will guide us to the appropriate dose and dose regimen," he said.
After further studies in healthy adults, including testing another booster shot, researchers next plan to begin evaluating the vaccine in other groups, such as the elderly and children -- two groups that tend to be particularly vulnerable to the flu, Fauci said.
"When we get this nailed down, then we will move on to the elderly, probably in the next month or so," Fauci said.
Several other countries are also trying to develop vaccines against the virus, and the government plans to test another vaccine produced by Chiron Corp., but the Sanofi Pasteur vaccine is the furthest along, Fauci said.
The one drug that seems able to prevent infection and reduce the virus's spread is sold as Tamiflu. More than two dozen countries and the WHO are stockpiling it. But supplies are so limited that it would be impossible to treat everyone in the event of an outbreak.
Last week, two groups of scientists reported the results of the most sophisticated analyses of how a pandemic could be stopped. Computer models indicated that a pandemic could be stifled within about a week with drugs and quarantines. But experts said that many uncertainties remained and that an effective vaccine could be invaluable.