(Washington) U.S. authorities are pledging to eventually produce enough swine flu vaccine for everyone but the shots couldn’t begin until fall at the earliest.

Worries about the spread of the virus mounted as the nation’s swine flu caseload passed 100, and nearly 300 schools closed in communities across the country. Federal officials had to spend much of the day reassuring the public it’s still safe to fly and ride public transportation after Vice President Joe Biden said he wouldn’t recommend it to his family.

Clinics and hospital emergency rooms in New York, California and some other states are seeing a surge in patients with coughs and sneezes that might have been ignored before the outbreak.

Scientists were racing to prepare the key ingredient to make a vaccine against the never-before-seen flu strain – if it’s ultimately needed. But it will take several months before the first pilot lots begin required human testing to ensure 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.”

Until a vaccine is ready, the government has stockpiled anti-viral medications that can ease flu symptoms or help prevent infection. The medicines are proving effective.

Reassurances from top health officials didn’t stop the questions from coming.

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.

Although it is safe to fly, anyone with flu-like symptoms shouldn’t be traveling anywhere, unless they need to seek medical care.

The swine flu 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.

So far U.S. cases are mostly fairly mild with one death, a Mexican toddler who visited Texas with his family – unlike in Mexico where more than 160 suspected deaths have been reported. Most of the U.S. cases so far haven’t needed a doctor’s care, officials said.

Still, the U.S. is taking extraordinary precautions – including shipping millions of doses of anti-flu drugs to states in case they’re needed. The World Health Organization is warning of 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.

The CDC confirmed 109 cases Thursday, 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.