WASHINGTON, D.C. - Although the number of potential swine flu cases in Washington state hit 25 on Sunday, U.S. health officials are cautiously optimistic that the new swine flu isn't as dangerous as first feared, but urged people to keep taking commonsense precautions - and they can't predict if it will roar back in the fall.

"The good news is when we look at this virus right now, we're not seeing some of the things in the virus that have been associated in the past with more severe flu. That's encouraging, but it doesn't mean we're out of the woods yet," said Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention.

With swine flu, or the H1N1 flu as the government prefers to call it, now in 21 states and counting, authorities say it's spreading just as easily as regular winter flu. But, as Besser made the rounds of the five Sunday talk shows with the president's health and homeland security chiefs, they said that doesn't seem to cause as severe disease as it did in Mexico, where there are signs the outbreak is waning.

But the CDC says its own count is outdated almost as soon as it's announced. More cases are being confirmed daily. About one-third so far are people who had been to Mexico and probably picked up the infection there. Many newly infected people are getting the illness in the U.S., and the CDC says it probably still is spreading.

"It's a rapidly evolving situation and it's still one that is cloaked in uncertainty," Besser said. "But each day we're getting more information ... and we're starting to see encouraging signs."

Yet he warned that with a new flu strain, one that people do not have immunity to, "you would expect that there are going to be hospitalizations and, unfortunately, there will be more deaths."

A big concern is whether the virus will return, perhaps harder, when regular influenza begins its march here. Flu season in the Southern Hemisphere is about to begin, and U.S. authorities will watch how the swine flu circulates there over the coming months as they prepare the first vaccine and then decide whether to order that large amounts of it be produced in the fall.

"The early news seems to be cautiously optimistic about where we are right now," said Health and Human Services Secretary Kathleen Sebelius. But, she added, "We certainly can't get complacent. We need to get prepared."

Production of regular winter flu vaccine is going full-tilt, "to make sure we kind of clear the decks," Health and Human Services Secretary Kathleen Sebelius said.

"We are testing the virus strain for H1N1 virus so that we're ready to go into production later, in a month or two, when we make sure that we have the right dosage and the right tests. So we'll be ready for both," she said.

People need to get ready, too, said Homeland Security Secretary Janet Napolitano.

"The danger is that it could come back in the fall. It could come back in a more virulent form. And so all of the work we're doing now not only was designed to get ahead of this outbreak of the flu, but there's a lot of planning that individuals and families and businesses and government is going to have to do over the course of the summer as we prepare for the next flu season," she said.

Even if the new virus doesn't prove as potent as authorities feared, Besser said that doesn't mean the U.S. and World Health Organization overreacted in racing to prevent a pandemic, or worldwide spread, of a virus never before seen.

With a new infectious disease, "you basically get one shot, you get one chance to try to reduce the impact," Besser said. "You take a very aggressive approach and as you learn more information you can tailor your response."

It was just over a week ago that authorities learned the new flu CDC had detected in a handful of people in California was causing a large outbreak and deaths in Mexico, triggering global alarm.

"We didn't know what its lethality was going to be. We had to move. Once you get behind flu, you can't catch up," Napolitano said.

Had this started as a small outbreak in a more distant land, stricter border measures might have bought the U.S. an extra two weeks to three weeks, Besser said. As it was, the CDC first discovered a new virus in California before the Mexican situation even was known.

Napolitano said the government relied on health experts in deciding not to close the border with Mexico. "The epidemiologists say, the scientists say, it would have done no good at all and come at enormous cost," she said.

If the new flu proves no more lethal than regular winter flu, consider that those garden-variety strains hospitalize 200,000 Americans and kill 36,000 every year. That is why Besser keeps warning that as swine flu spreads to enough people, more severe illness is likely to follow.

Besser, Sebelius and Napolitano appeared on "Fox News Sunday," ABC's "This Week," NBC's "Meet the Press," CNN's "State of the Union" and "Face the Nation" on CBS.

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