So far, all of the swine flu viruses isolated from infected people have been virtually identical, which should simplify the development of a vaccine, an official from the Centers for Disease Control and Prevention said Friday.

"We are aggressively taking the very early steps that are necessary for vaccine manufacture should a decision be made to go ahead and ramp up for full-scale production," Nancy Cox, chief of the influenza branch at the National Center for Immunization and Respiratory Diseases, said at a CDC press briefing.

It takes months to develop and manufacture flu vaccine, so scientists have commenced working on one even though it's not yet clear who, if anyone, should be immunized.

"It's the right time," said Anne Schuchat, interim deputy director for the CDC's Science and Public Health Program, "because we won't have one if we don't talk about it now."

Viruses used in flu vaccines are grown in eggs. The first step toward a swine flu, or H1N1, vaccine is to grow enough of the virus, Cox said. To do that, scientists are combining genes from viruses known to grow well in eggs with H1N1, ending up with a high-growth virus that has H1N1 antigens, or proteins, on its surface.

"That process is going quite well," Cox said. "If the virus cooperates and grows well, we will soon be able to distribute a candidate vaccine virus to the manufacturers," probably in about three weeks. Manufacturers then will need eight to 10 weeks to produce pilot vaccine lots, which will be used to determine what dose is need to produce an adequate antibody respone in humans, she said.

A question "that's been on the minds of many" is whether manufacturers will be able to make sufficient numbers of doses of both seasonal flu vaccine and an H1N1 vaccine, Cox said. Each year, seasonal flu kills tens of thousands of Americans.

"The seasonal vaccine, as you know, is an extremely important tool," she said. Already, Cox said, manufacturers have produced a "great deal" of seasonal flu vaccine for the upcoming season, although some are further ahead than others.

"It's not an either/or situation," said Donna Cary, a spokeswoman for Sanofi Pasteur, which this past season made 50 million flu vaccine doses — 45% of the U.S. supply.

Sanofi Pasteur has a brand-new plant next to its older plant in Swiftwater, Pa., Cary said. Once the new plant was fully licensed, the company was going to shut it down for renovations, readying it for the possibility of a pandemic, she said.

But, Cox said, it could be used immediately for the manufacture of an H1N1 vaccine if necessary.

Sanofi Pasteur is in "constant contact" with federal public health officials, Cary said. "They haven't told us to do anything definitely yet."

Kevin Colgan, a spokesman for GlaxoSmithKline, said, "Until the health authorities determine the type of vaccine that's necessary, it's impossible to speculate how it will affect our seasonal flu vaccine production."

In a press release today, Colgan's company noted that the company "is continuing to produce and maximize supply of its seasonal influenza vaccine for use in the Southern hemisphere, as it enters the winter season, and for the Northern hemisphere later this year."