US authorities are pledging to eventually produce enough swine flu vaccine for everyone but say that the earliest the shots will be available is the northern autumn.

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

Federal officials had to spend much of the day reassuring the public it was 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 ensure the vaccine is safe and effective. If all goes well, broader production could start in the northern autumn.

“We think 600 million doses is achievable in a six-month time frame” from that northern autumn start, Health and Human Services Assistant Secretary Craig Vanderwagen told lawmakers on 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 antiviral medications that can ease flu symptoms or help prevent infection. The medicines are proving effective.

Reassurances from top health officials did not stop the questions from coming.

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