(AP) - 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.

"We're not 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 30 states and counting, authorities say it's spreading just as easily as regular winter flu, but fortunately 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But the CDC says its own count is outdated almost as soon as it's announced.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.

More concerning 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.

"We're starting to see some encouraging signs and that's good but our approach has to be aggressive," Besser said.

Meanwhile, 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 can be ready to do both simultaneously. This isn't an either/or," she said of the two different vaccines.

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," Homeland Security Secretary Janet 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.

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" and NBC's "Meet the Press," with later interviews scheduled for CNN's "State of the Union" and "Face the Nation" on CBS.