Originally Published on NPR.org May 1, 2009 – By Joanne Silbernerand Nell Greenfieldboyce

Scientists looking at the genes of the new swine flu virus say they don't see certain markers of virulence that made the 1918 pandemic strain so deadly, the Centers for Disease Control and Prevention said Friday.

"We do not see the markers for virulence that were seen in the 1918 virus," said Dr. Nancy Cox , CDC flu chief.

"However, we know that there is a great deal that we do not understand about the virulence of the 1918 virus or other influenza viruses," that caused serious illnesses, she said. "So we are continuing to learn."

The global flu epidemic early last century was possibly the deadliest outbreak of all time. The virus was an H1N1 strain — different from the H1N1 strain involved in the current outbreak — and struck mostly healthy young adults. Experts estimate it killed about 40 million to 50 million people worldwide.

Progress Toward Development Of Vaccine

Development of a vaccine against the new swine flu virus is rapidly moving forward, even though the vast majority of cases outside of Mexico so far appear to be mild and scientists aren't certain whether the virus is a type that kills a lot of the people it infects.

The World Health Organization said Friday that it is coordinating efforts with a few dozen vaccine manufacturers in countries from China to Brazil to Hungary, as the number of confirmed swine flu cases continued to rise.

As of Friday afternoon, 320 cases had been reported worldwide, including a Mexican tourist in Hong Kong who became Asia's first confirmed case.

"Unless really there is very, very soon a signal that this might not continue, it seems most likely that the manufacturers will proceed and we will certainly support them," said Marie-Paule Kieny, WHO director of the Initiative for Vaccine Research.

Producing A Vaccine

The first step in the process, isolating the virus, is already done.

Kieny said places such as the Centers for Disease Control and Prevention in Atlanta are now working to tweak the virus in ways that will make it better for vaccine production. She added that this work should be completed by the end of May.

After that, the virus preparation goes to manufacturers, who further adapt it to their own production processes. Factories will grow the virus in chicken eggs, kill and purify it, and then formulate the vaccine, which must be tested in clinical trials for safety in humans.

The first doses may be ready in months. From the time a potentially pandemic virus is identified, Kieny said, "it takes between four and six months to have the first doses of vaccine coming out of a factory and being available for immunizing people."

A vaccine tailored for swine flu is needed because tests suggest that the seasonal flu vaccine already in production most likely would not offer protection against the swine flu virus.

"We have no doubt that making a successful vaccine is possible within a relatively short period of time," says Kieny.

She says the cost should be similar to that of other vaccines already used in developing countries, and that the WHO already knows of donors who might help with the costs of getting the vaccine to poorer countries.

Anne Schuchat, the interim deputy director for the science and public health program at the Centers for Disease Control and Prevention, said, "We haven't made the decision yet that we will manufacture large numbers of doses of vaccine, and we haven't made the decision yet that we will use a vaccine if we produce a lot of it."

Even if a vaccine is created, it's not clear how widespread — or even whether — it would be used.

Long-Term Vigilance Emphasized

For a full week, the threat of an imminent swine flu pandemic has dominated headlines around the globe.

No one knows where the virus first emerged.

In March and early April, Mexican public health officials began to see increased reports of people with flulike illness. By April 23, several of those cases had been confirmed to be a swine flu virus. Additional testing revealed it was the same swine flu strain that was detected in two children living in California.

On Friday, a top Mexican health official accused the WHO of responding too slowly to the country's April 16 warning of an unusually late rash of flu and pneumonia cases in Mexico. The WHO disputes that claim.

Early reports from Mexico linked the virus to hundreds of deaths among healthy adults. Since then, however, only a few of those suspected cases have been confirmed by laboratory tests.

In the United States, the vast majority of cases so far have been mild, like seasonal flu. The number of confirmed U.S. swine flu cases stood at 153 in 21 states, including the first cases reported in Florida.

After a Cabinet meeting Friday, President Obama said, "I'm optimistic that we're going to be able to manage this [flu outbreak] effectively."

He said the swine flu, also called H1N1, may just run its course like ordinary seasonal flu viruses, "in which case, we will have prepared and we won't need all these preparations." But he stressed the need for long-term vigilance, noting that even if the flu is relatively mild now, "it could come back in a more virulent form during the actual flu season" in the fall.

Decoding The Swine Flu Virus

Some scientists have speculated that more than one new swine flu virus may be circulating. But Kieny says all the viruses taken from patients and analyzed are more than 99 percent identical. "So we have no reason to think that these viruses are different," she says.

Experts still lack critical information about the virus. They don't know how frequently it causes severe disease or death, because they don't know how many people have actually been exposed to it in Mexico and other places around the world.

But scientists do know the virus has a surprising pedigree. "It's got pieces from a lot of different species from different parts of the world," says Arnold Monto, an epidemiologist at the University of Michigan.

These include pigs, birds and humans, Monto says. And he says bits of genetic material in the new virus can be traced to Europe, Asia and North America. They probably got mixed together when a cell in an animal or person became infected by different viruses that swapped genetic parts.

This kind of thing probably happens a lot, says Monto. "But the virus typically is not fit enough to transmit."

In this case, the virus has been able to jump from person to person.

Health Officials Trying To Strike A Balance

Besides developing a vaccine against the virus, governments around the world have been mobilizing to distribute anti-flu drugs.

The U.S. released 11 million courses of flu-fighting drugs from a national stockpile on Sunday, shipping them to states around the country, and announced it would be purchasing 13 million more to replenish the stockpile. The government also sent 400,000 courses to Mexico. The WHO also will begin to mobilize its stockpile of anti-flu drugs, which contains 5 million doses.

Public health officials are preparing for the worst, just in case.

"It is early days, and with influenza, we always want to be humble and know that things can change and it can be unpredictable," said the CDC's Schuchat.

Still, she said, there's no need for alarm. "We're trying to strike a balance and be prudent, but not have people panic."

Schuchat also batted down remarks like those of Vice President Biden, who said on NBC's Today Show that he would tell his family not to travel in confined spaces like planes or subways. Biden's office later clarified his remarks, saying only people who are sick should avoid public transportation.

If people have no symptoms of illness, she said, they should not avoid public transportation like airplanes or subways out of fear of catching the virus.

"We don't think that's the appropriate step to take right now," Schuchat says. "We think it is safe to travel, and I took an airplane back home just last night."

Material from NPR wire services was used in this report.