LONDON - A British company reported Wednesday it had achieved the best results ever seen on an experimental human vaccine for bird flu and said mass production might be possible by 2007. A global health official called GlaxoSmithKline's early results "an exciting piece of science." If future tests are as promising, it would be a major step in the frustrating campaign to protect people from a possible deadly flu pandemic. The U.S. government's chief infectious disease scientist also was very optimistic. "The data are really very impressive," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "It changes the whole complexion of the issue that we have to face of getting enough vaccine for people who might need it in a pandemic." Glaxo's results came from tests on 400 people in Belgium, most of whom developed strong immune responses from very low doses of the prototype vaccine. Success from wider tests of the vaccine could intensify competition with Sanofi-Aventis SA, whose vaccine unit, Sanofi Pasteur, reported disappointing results in March on its experimental product. It protected only about half of those who got two shots with a very high dose - 90 micrograms of the key ingredient. Glaxo said two shots of its vaccine provoked strong responses in more than 80 percent of people tested at lower doses than other experimental bird flu vaccines are using. Some received as little as 3.8 micrograms, said Fauci, who has seen the test results on the vaccine. "It's pretty strong," he said. The Glaxo vaccine includes an immune-system booster that allows it to use less of the main active ingredient, meaning that greater quantities could be produced if the H5N1 bird virus mutates into a form that spreads easily among people and causes a global epidemic. The vaccine uses an inactivated version of the newer strain of H5N1, which was isolated in Indonesia last year. "It's a good and exciting piece of science," said Dr. David Nabarro, the United Nations' coordinator for avian and pandemic influenza. "But as with all new discoveries, quite a lot of work has now got to be done to establish its place in public health and pandemic preparedness." Sanofi and another vaccine maker, Chiron Corp., also have been experimenting with ingredients called adjuvants to boost effectiveness. Glaxo's results, which were announced by the company but have not yet been published in a medical journal, are the best success reported so far with this approach. "This is very significant," said Dr. Albert Osterhaus, head of the virology department at Erasmus University in the Netherlands. "With this adjuvant added to the vaccine, provided the rest of the tests are OK, you could make 10 times as much vaccine." Glaxo's chief executive officer, J.P. Garnier, said the preliminary findings validate this approach, and that the company expects to seek regulatory approval "in the coming months." While cautioning that it's still early in the testing, some pandemic flu experts are optimistic that this may ultimately lead to production of many more doses of pandemic vaccine. However, Dr. Klaus Stohr, a World Health Organization flu vaccine adviser, said it would have been better if the adjuvant was a substance widely available to other companies rather than a Glaxo company product. "Access for other companies to use it will most likely be limited," he said. More than 20 clinical trials involving potential H5N1 vaccines are being underaken by more than 30 companies. Glaxo's success also would not guarantee that all people and countries would be protected in the event of a flu pandemic. Flu viruses mutate so readily that it may ultimately be a different strain of the virus that threatens people. "It's a risk judgment for those potentially purchasing vaccine," said Dr. Angus Nicoll, influenza coordinator at the European Center for Disease Prevention and Control. Because it is impossible to predict which influenza strain will spark the next pandemic, "it's a very difficult decision for a country to decide whether to invest in pandemic vaccines," he said. In May, the U.S. government awarded more than $1 billion to five companies, including Glaxo, which are developing faster ways to mass produce vaccine in case of a pandemic. The government also has ordered millions of dollars worth of Aventis' experimental vaccine to stockpile in case bird flu starts spreading more easily from person to person.