Mass production of a new vaccine that scientists believe can protect against an avian flu (search) outbreak could begin as early as mid-September, the director of the National Institute of Allergy and Infectious Diseases (search) said Sunday.

Dr. Anthony S. Fauci (search) said the government is ready to move ahead with ordering significantly more than the 2 million doses it acquired from a French vaccine maker before testing began earlier this year to jump start the U.S. vaccine stockpile in case the tests were successful.

Additional tests of the vaccine are being conducted on the elderly and children.

Preliminary data from the first 115 of the initial tests on 450 healthy adults showed an immune response that scientists believe is strong enough to protect against the avian influenza that's spreading among birds in Asia and Russia. Fauci said he expects analysis of data from the other 300 tests will show similar results.

"We're now, given these results, going to move ahead with ordering from the company additional dozes," Fauci said in a telephone interview. "I can't tell you exactly how many; that's going to depend on the production capability, but certainly it will be significantly more than the 2 million doses."

For the past year, government health officials have been hurrying to develop the vaccine because of fears that the avian influenza strain could change into one that could spread rapidly among humans throughout the world. While the strain has killed millions of birds, only about 50 humans have died from it and so far there has been no widespread transmission of the virus from one human to another.

Fauci predicted that the Food and Drug Administration could approve the new vaccine fairly quickly, since it's similar to prior seasonal flu vaccines the agency approves each year. The bigger problem is the lack of manufacturing capacity to produce the number of doses that may be needed.

An influenza pandemic similar to the one in 1918 that killed 50 million people would require hundreds of millions of doses of the vaccine, Fauci said, and the handful of companies that now make influenza vaccines cannot produce the new bird flu vaccine and the regular seasonal flu vaccine at the same time.

Production of next winter's seasonal flu vaccine will end later this month, meaning it will be mid-September at the earliest before mass production of the bird flu vaccine can get underway, he said.

"It's less a regulatory issue than a production capacity issue," Fauci said.

The next step in the testing process is to try out the vaccine on a group of volunteers over age 65, followed by tests on children. Fauci said trials on the over-65 volunteers will begin within a month, and will take four to six months to complete. Tests on children will follow immediately.

In each case, Fauci said, scientists will determine if there are safety issues associated with giving the vaccine to those more vulnerable groups and what the appropriate dosage level should be for each group.

The initial tests of the 450 healthy adults found that higher doses than normally given in the seasonal flu vaccine were needed to produce the required immune response. Even larger doses likely would be needed to induce a similar immune response in the elderly.

Fauci said more testing is needed to determine the optimal dosage level. Until that is done, the 2 million doses already purchased by the government in bulk form cannot be repackaged for use if there is an emergency.

Health and Human Services Secretary Mike Leavitt said last week the government wants to stockpile 20 million doses of the new vaccine. He estimated that it would take four to six months to mass produce the vaccine.

"We're in the process of doing aggressive contingency planning to determine how we can ratchet up production dramatically," Leavitt said.