U.S. authorities are pledging to eventually produce enough swine flu vaccine for everyone but say that the shots couldn't begin until fall at the earliest.

Worries about the spread of the virus mounted Thursday as the U.S. swine flu caseload passed 100, and nearly 300 schools closed. 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.

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 make the vaccine is safe and effective. If all goes well, broader production could start in the fall.

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"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 Thursday.

"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.

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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.

Where Did Virus Originate?

Medical detectives have not pinpointed where the outbreak began. Scientists believe that somewhere in the world, months or even a year ago, a pig virus jumped to a human and mutated, and has been spreading between humans ever since.

China has gone on a rhetorical offensive to squash any suggestion it's the source of the swine flu after some Mexican officials were quoted in media reports in the past week saying the virus came from Asia and the governor of Mexico's Veracruz state was quoted as saying the virus specifically came from China.

One of the deaths in Mexico directly attributed to swine flu was that of a Bangladeshi immigrant, said Mexico's chief epidemiologist Miguel Angel Lezana.

Lezana said the unnamed Bangladeshi had lived in Mexico for six months and was recently visited by a brother who arrived from Bangladesh or Pakistan and was reportedly ill. The brother has left Mexico and his whereabouts are unknown, Lezana said. He suggested the brother could have brought the virus from Pakistan or Bangladesh.

By March 9, the first symptoms were showing up in the Mexican state of Veracruz, where pig farming is a key industry in mountain hamlets and where small clinics provide the only health care.

The earliest confirmed case was there: a 5-year-old boy who was one of hundreds of people in the town of La Gloria whose flu symptoms left them struggling to breathe.

Neighbors of the inspector, Maria Adela Gutierrez, said Wednesday that she fell ill after pairing up with a temporary worker from Veracruz who seemed to have a very bad cold. Other people from La Gloria kept going to jobs in Mexico City despite their illnesses, and could have infected people in the capital.

Emergency Response

There is no vaccine to prevent infection but U.S. health officials aim to have a key ingredient for one ready in early May, the big step that vaccine manufacturers are awaiting. But even if the World Health Organization ordered up emergency vaccine supplies — and that decision hasn't been made yet — it would take at least two more months to produce the initial shots needed for human safety testing.

"We're working together at 100 miles an hour to get material that will be useful," Dr. Jesse Goodman, who oversees the Food and Drug Administration's swine flu work, told The Associated Press.

Obama asked Congress for $1.5 billion in emergency funds to help build more drug stockpiles and monitor future cases, as well as help international efforts to avoid a full-fledged pandemic.

Obama said his administration has made sure that needed medical supplies are on hand and he praised the Bush administration for stockpiling 50 million doses of antiviral medications

Swine Flu vs. Seasonal Influenza

“The illness is consistent with seasonal influenza and has generally the same symptoms of seasonal influenza,” Fukuda said. “Our information to date shows that the infection can range from very mild, requiring no hospitalization and recovering after a few days, to fatal, which is also in keeping with seasonal influenza."

Fukuda said there is no evidence that the virus is spreading from pigs or that it is unsafe to eat pork.

Authorities sought to keep the crisis in context: Flu deaths are common around the world. In the U.S. alone, the CDC says about 36,000 people a year die of flu-related causes.

Children, especially those younger than age 5, are particularly vulnerable to flu and its complications, and every year children die from seasonal flu.

According to the CDC, more than 20,000 children younger than age 5 are hospitalized every year because of seasonal flu. In the 2007-08 flu season, the CDC received reports that 86 children nationwide died from flu complications.

As of April 11, CDC had received reports of 53 seasonal flu-related deaths in children during the current seasonal flu season.

Still, the CDC calls the new strain a combination of pig, bird and human viruses for which people may have limited natural immunity.

Hence the need for a vaccine.

The Associated Press and Reuters contributed to this report.

Related:

Swine Flu Timeline

Swine Flu Q & A

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