The United States could begin mass production of an experimental avian flu vaccine by this September.

Tests have been conducted on 450 healthy adults. Results from 115 of those people have shown a high enough immune response to protect against the avian flu spreading amongst birds in Asia and now Russia.

Dr. Anthony S. Fauci, hed of the U.S. National Institute of Allergy and Infectious Diseases said Sunday that he expects the other 335 patients to show a good response too.

If this vaccine turns out to be effective, it will be a major step forward in preventing what many think will be the next pandemic flu.

Dr. Donald Low, a Toronto microbiologist, told CTV Newsnet on Sunday that while the development was important, it was only one step.

There's a "number that have to be played out over the next several months and even years," he said.

"How much do you put in each dose?  How many does do you give? How long does it last for? Do you have to add some other things to make it a really good stimulator of the immune system and to protect us."

U.S. officials say more testing has to be done -- on young and elderly subjects, for example.

One thing they have found is the current avian flu vaccine formulation requires a higher dose than seasonal flu vaccines in order to trigger an immune response.

The elderly will require even higher doses.

However, they have already ordered two million doses from a French manufacturer -- although those can't be changed if testing indicates the formulation should be changed.

Last week, Mike Leavitt, the U.S. Health and Human Services Secretary, said the U.S. federal government wants to stockpile 20 million doses of the vaccine.

Currently, the pharmaceutical companies make enough flu vaccine to cover the most vulnerable members of society: The elderly, children and the sick.

Companies might not be able to make the seasonal flu vaccine and the bird flu vaccine at the same time.

Production of regular vaccine ends by mid-September.

"That's a big problem here, is being able to ramp up the industry to make these things. Because the industry is not there to make the kind of volumes we need for a pandemic influenza."

The last pandemic, the Spanish Flu of 1918, killed 50 million people worldwide -- and that was in an era before easy global travel. Another thing to consider, however, is the Spanish flu mainly killed the elderly and young; avian flu has so far killed one of every two people infected with it.

"There will be no stopping it at the gates," Low said.

"If a pandemic evolves in Asia, for example, it's just a matter of time before it ends up on our doorstep, so we have to be prepared."

The fact the avian flu is showing up in different countries is worrisome, he said.

With a report from CTV's Paula Newton