Scientists in the United States report developing revolutionary experimental vaccines that offer hope of blocking the spread of bird flu among people. Their novel product can be made quickly and induced a strong immune response in animals.

An effective vaccine against the bird flu virus, known as H5N1, is an urgent global public health priority. But the traditional method of making flu vaccines could be inadequate if a human pandemic breaks out.

This techniques grows the virus in chicken eggs before being harvested, chemically inactivated, and used to produce vaccine.

When the vaccine containing the inactivated virus is injected into people, the immune system attacks it, training the body to recognize the real enemy.

But a single vaccine dose requires one or two eggs, so many millions are needed to make large quantities of vaccine. The process is slow, taking at least six months. Since poultry is susceptible to bird flu, it might not be possible to ensure enough healthy eggs to make vaccine for a population. And viruses can mutate quickly into different forms that can evade a vaccine created for them.

Now, two groups of researchers have independently tested a new method that appears to overcome these obstacles -- at least in animals. They have devised experimental vaccines that can be made quickly and work against a variety of bird flu strains.

University of Pittsburgh physician Andrea Gambotto led one of the teams, which reports its findings in the Journal of Virology. "We have proven that this vaccine is 100 percent effective in both chickens and mice," he said.

According to a study in the journal Lancet, a second team from Purdue University and the U.S. Centers for Disease Control got the same result in mice alone with their version of the vaccine.

Instead of growing the entire virus in eggs, they inserted a gene from the H5N1 virus into a modified common cold virus, which translated the gene into a single H5N1 surface protein called hemagglutinin. The cold virus in which the hemagglutinin was grown served as the delivery vehicle for the vaccine.

Both avian flu vaccines avoid the pitfalls of traditional vaccine production. Each was made in only days to weeks and is not dependent on a supply of healthy chickens, as Centers for Disease Control researcher Mary Hoelscher points out. "If we do have this virus killing off chickens, with this strategy we would not have to worry about the supply of vaccine because we don't rely on eggs for producing the vaccine," he said.

The new vaccines have another benefit over traditional ones. They can be used against several H5N1 bird flu strains, while the egg-grown variety using a single inactivated flu virus is useful against only the single strain it includes. Hoelscher's team showed this when the hemagglutinin gene their vaccine used, which came from the 1997 Hong Kong bird flu strain, brought about a strong immune response in mice injected with 2003 and 2004 strains. Hoelscher's colleague at Purdue University, Suresh Mittal of India, says this is because the hemagglutinin protein is common to all H5N1 bird flu varieties, so it is effective even if the virus mutates.

The result, says Mittal, is that vaccine could be stockpiled ahead of a potential pandemic without having to know the exact H5N1 strain causing it. "Otherwise, [with] the traditional approach, we can have a vaccine for the pandemic virus once it happens. Making that type of vaccine, it will require six to nine months, and by that time the majority of the damage will be done in the world," he said.

The only drawback is that the vaccines would not be widely available for several years, since long human trials are ahead. But vaccine expert Gary Nabel of the U.S. government's National Institutes of Health says they have promise for the future. "We are now moving forward with a lot of new approaches to flu. The good news here is that we have identified another one that appears to work," he said.