For the first time ever, scientists have created a vaccine to prevent an epidemic that hasn't yet happened.

Why? The disease is bird flu -- the very lethal H5N1 strain of avian influenza virus. It's killed 57 of the 112 humans known to have caught it. So far, it's very unusual for one person to catch it from another. But that could change. If the bird flu virus mutates or recombines with a human flu virus, it could result in a strain that could easily spread from human to human.

If that happens -- and many experts say it's just a matter of time -- millions of people will die unless vaccinated ahead of time.

Early test results show the vaccine almost surely works, says study leader John Treanor, MD, professor of medicine and director of the vaccine and treatment evaluation unit at the University of Rochester, N.Y.

"We now have shown you can make a vaccine that is well tolerated and that, in humans, makes immune responses known to be protective against influenza," Treanor tells WebMD. "The results are quite robust."

The U.S. is getting ready to order millions of vaccine doses in addition to the 2 million doses it's already bought from drug maker Sanofi-Pasteur, a subsidiary of Aventis and a WebMD sponsor. The U.S. has also ordered bird flu vaccine from Chiron Corp., but tests of this vaccine have not yet begun.

Read WebMD's "Fears of Global Bird Flu Outbreak Increase"

No Need -- Yet -- to Seek Bird Flu Vaccination

It's not yet time to line up for bird flu shots, warns Anthony S. Fauci, MD, director of the National Institute of Allergy and Infectious Diseases.

"This vaccine does not need to be deployed right now. We may never have to use this vaccine," Fauci tells WebMD. "We only made it, and we hope this never occurs, in case the current H5N1 bird flu now circulating in Asia develops the capability of spreading from person to person in a sustained manner. Then -- and only then -- would we think of deploying this vaccine."

If and when a bird flu pandemic occurs, a vaccine would be the "bedrock" of control efforts, Fauci says. But many other measures are under way. Greatly improved surveillance is discovering human bird flu infections faster than ever before. This would give public health authorities the chance to contain or at least slow the virus with tools such as patient isolation, antiviral medicines, and, if necessary, quarantine.

Read WebMD's "Bird Flu May Be More Contagious Than Thought"

Bird Flu Mutant Might Mean New Vaccine

The new bird flu vaccine is based on a bird flu virus isolated directly from an infected person in Vietnam. The good news, Treanor says, is that this bird flu virus is very much like the virus seen in people with more recent bird flu infections. This means the vaccine likely would work against current bird flu virus infection.

However, Treanor and Fauci both say there's no guarantee that this vaccine will work against a bird flu virus that makes the genetic changes needed to spread easily among humans.

"If the virus does evolve, this current vaccine may not be protective," Fauci says. "You would have to substitute for the currently circulating virus. But even if you have to change vaccines later on, we now know how to do it. This puts us months ahead. This is not the end. But it is an important first step."

Read WebMD's "Warnings Grow Dire on Bird Flu Threat"

How Much Time Needed to Prepare Vaccine?

Treanor estimates that if a pandemic flu virus did appear, it would take about six months to come up with a vaccine. Why so long? Flu vaccines are still made using a decades-old process requiring millions of sterile chicken eggs. Newer technologies exist but have not yet been fully tested or licensed.

"It is important that we have been able to go through the process of making an H5N1 bird flu vaccine," Treanor says. "It is a dry run. Going through every step lets us encounter all the unforeseen difficulties you might encounter in a pandemic situation. We are far ahead of where we would have been if we were starting from scratch."

The promising early findings are based on 113 people vaccinated at the University of Rochester, UCLA, and the University of Maryland. An additional 350 people have also received the vaccine or placebo at three other test centers: Baylor College of Medicine, Cincinnati University Children's Hospital Medical Center, and Saint Louis University Health Sciences Center.

One drawback to the new vaccine is that it takes two doses given four weeks apart. That's no surprise. It takes two flu shots to vaccinate children for the first time. During flu season one shot annually is recommended because the flu shot is really a booster vaccination for those previously vaccinated or who have had the flu. Since few humans have ever had H5N1 flu infections before, full immunity will take two doses.

And doses are also a problem. Vaccination against bird flu is going to take a higher vaccine dose than is usual for flu vaccines. This means that a scarce vaccine supply can't be diluted to make more doses. Treanor says that researchers are testing vaccine ingredients called adjuvants -- alum, for example -- that boost immune responses and might stretch limited vaccine supplies.

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: The National Institute of Allergy and Infectious Diseases. Anthony S. Fauci, MD, director, National Institute of Allergy and Infectious Diseases, Bethesda, Md. John Treanor, MD, professor of medicine and director, vaccine and treatment evaluation unit, University of Rochester, N.Y.