A new analysis of more than 50 strains of the H1N1 influenza virus at the center of the global outbreak concludes they are closely related and can be fought with a single vaccine.

"We see much less variation among these new H1N1 viruses than we do for typical seasonal influenza viruses," said Dr. Nancy Cox, the senior author of a study released yesterday by the journal Science.

That will make the job of creating a pandemic flu vaccine "much, much easier," she said.

Cox, who heads the influenza division at the Centers for Disease Control and Prevention in Atlanta, and colleagues from the U.S., Mexico and England also found that exposure to the normal seasonal flu strains provides little protection against the novel H1N1 that was identified in April.

Those findings could boost the likelihood that the U.S. will move forward with an H1N1 vaccine that would be offered separately from the seasonal flu vaccine.

The CDC is testing two potential seed stock candidates for an H1N1 vaccine to see whether they provoke an effective immune response.

The candidates combine portions of the H1N1 strain with other flu viruses that are known to grow efficiently in chicken eggs, a key step in the vaccine-making process, Dr. Anne Schuchat, interim deputy director for the CDC's Science and Public Health Program, said yesterday.

The agency expects to deliver a suitable seed stock to vaccine manufacturers by the end of the month, Schuchat said.

In a related development, the Department of Health and Human Services announced yesterday that it has set aside about $1 billion for development of an H1N1 vaccine. Some of the money will fund clinical trials of a vaccine over the summer, and some will be used to ramp up production of key vaccine ingredients in case they are needed, said Secretary Kathleen Sebelius.

The CDC has identified 6,552 cases of confirmed and probable H1N1 flu in the U.S., including nine deaths.

For each patient identified, public health officials estimate there are another 20 people with mild symptoms.

"We're estimating more than 100,000 people probably have this virus now in the U.S.," Schuchat said.

Worldwide, there have been 11,168 cases in 42 countries, including 86 deaths, the World Health Organization said yesterday.

The report in Science traced the origin of all eight genes that make up the H1N1 virus and found that in each case, the closest known relative is from a swine flu.

That suggests the virus had been circulating among pigs for some time before recently jumping to humans.

Scientists, however, have yet to find this particular strain of H1N1 in any pig outside of Alberta, Canada, where an infected farmworker transferred the virus to a herd.