By Julie Steenhuysen

CHICAGO (Reuters) - A top U.S. health official says the first weeks of October are going to be "a little bumpy" as the government distributes the supply the swine flu vaccine ready next week -- knowing it will not be enough.

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said on Friday his agency faced a dilemma because most of the U.S. H1N1 vaccine will be unavailable until mid- to late October.

"The choice was having vaccine stack up in a warehouse ... or getting it out as soon as it became available. We feel the only right answer is to get it out as rapidly as possible," Frieden told reporters in a telephone briefing.

But that move will add to the strain on state and local health departments that likely will have to turn people away.

"It's going to be a little bumpy," said Frieden.

The U.S. government said this week it will have 6 million to 7 million doses of swine flu vaccine ready next week, and another 40 million does by mid- to late October.

Production was expected to continue at a rate of 10 million to 20 million doses per week to a total of more than 250 million by year end.

Most of the vaccine that will be ready early will be the nasal spray vaccine made by AstraZeneca's MedImmune unit. It is approved for people aged 2-49, but is not recommended for people with certain underlying health problems, who have a higher risk of becoming severely ill if infected with the flu.

'BATCH BY BATCH'

The United States has ordered vaccine from five companies -- MedImmune, Sanofi-Aventis, Australia's CSL, GlaxoSmithKline and Novartis.

Frieden said the problem arose from working with different manufacturers. "It doesn't come out in one big drop. It comes out batch by batch," he said.

Frieden said vaccine was the best tool for fighting swine flu, which was declared a pandemic in June and is spreading widely in most of the United States.

Frieden urged Americans not to underestimate influenza, noting that many people mix up flu with mild respiratory viruses.

"Flu makes you pretty sick. You feel bad. If you've got an underlying health problem or you are unlucky, you can get very sick. You need to take the flu seriously," Frieden said.

Frieden addressed concerns about an unpublished study reported by Canadian media suggesting that people in Canada who were vaccinated against seasonal flu last spring had double the risk of becoming infected with pandemic H1N1 flu.

Frieden said the CDC has analyzed data from New York and other parts of the United States and data from Australia.

"Nothing we've seen suggest that is likely to be a problem," he said. "If there is preliminary data, we'd like to see it."

(Editing by Maggie Fox and Xavier Briand)

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