WASHINGTON — Nearly 300 schools scattered around the country closed as the nation's swine flu caseload passed 100 today.

U.S. authorities pledged to eventually produce enough vaccine for everyone — but said shots couldn't begin until fall at the earliest.

The outbreak penetrated over a dozen states and even touched the White House, which disclosed that an aide to Energy Secretary Steven Chu apparently got sick helping arrange President Barack Obama's recent trip to Mexico but that the aide did not fly on Air Force One and never posed a risk to the president.

An estimated 12,000 people logged onto a Webcast where the government's top emergency officials sought to cut confusion by answering questions straight from the public: Can a factory worker handling parts from Mexico catch the virus? No. Can pets get it? No.

And is washing hands or using those alcohol-based hand gels best? Washing well enough is the real issue, answered Dr. Richard Besser, acting chief of the Centers for Disease Control and Prevention. He keeps hand gel in his pocket for between-washings but also suggested that people sing "Happy Birthday" as they wash their hands to make sure they've washed long enough to get rid of germs.

It is safe to fly, U.S. officials found themselves stressing after Vice President Joe Biden got off message today. Biden said he'd discourage family members from flying or even taking the subway. The White House insisted the vice president meant to say he was discouraging just nonessential travel to Mexico, the hardest-hit area.

"It is safe to fly. There is no reason to cancel flights," Transportation Secretary Ray LaHood said. Not just planes but "all modes of transportation are safe in America," he added.

But anyone with flu-like symptoms shouldn't be traveling anywhere unless they need to seek medical care — the same advice that doctors give during the winter when regular flu kills 36,000 Americans each year.

"If you're ill, you shouldn't get on an airplane or any public transport to travel," CDC's Dr. Anne Schuchat told a congressional hearing. "If you're sick, stay home. I can't tell you how many times I've said that this week."

So far U.S. cases are fairly mild for the most part, with one death, a Mexican toddler who visited Texas with his family — unlike in Mexico where more than 160 suspected deaths have been reported. In fact, Schuchat said most of the U.S. cases so far didn't need a doctor's care.

Still, the U.S. is taking extraordinary precautions, including shipping millions of doses of anti-flu drugs to states in case they're needed against what the World Health Organization has called an imminent pandemic, because scientists cannot predict what a brand-new virus might do. A key concern is whether this spring outbreak will resurge in the fall.

Remember, CDC's Besser cautioned, not every pandemic is like the disaster of 1918. "There are some pandemics that look very much like a bad flu season," he said.

Scientists are racing to prepare the key ingredient to make a vaccine against the never-before-seen flu strain, but it will take several months before the first pilot lots begin required human testing to make sure the vaccine is safe and effective. If all goes well, broader production could start in the fall.

"We think 600 million doses is achievable in a six-month time frame" from that fall start, Health and Human Services Assistant Secretary Craig Vanderwagen told lawmakers.

"I don't want anybody to have false expectations. The science is challenging here," Vanderwagen told reporters. "Production can be done, robust production capacity is there. It's a question of can we get the science worked on the specifics of this vaccine."

The number of closed schools more than doubled overnight to nearly 300 when the Fort Worth Independent School District in Texas announced it was closing its 140 schools, affecting about 80,000 students. High schools sports were suspended in Texas and Alabama.

"We do think it's very prudent to close schools when a case has been confirmed or is highly suspect," CDC's Schuchat told lawmakers today.

But closing a school alone won't stop community spread.

"If a school is closed, it's not closed so kids can go out to the mall or go out to the community at large," Homeland Security Secretary Janet Napolitano said. "Keep your young ones at home."

That means businesses will have to handle parents who miss work, Biden reiterated: "And the hope is that the employers will be generous in terms of how they treat that employee's necessary action of taking that child home and not being at work."

The CDC confirmed 109 cases today, and state officials confirm 22 more. Cases now are confirmed in: New York, Texas, California, South Carolina, Kansas, Massachusetts, Indiana, Ohio, Arizona, Michigan, Nevada, New Jersey, Delaware, Maine, Colorado, Georgia and Minnesota.