ATLANTA (AP) -- The first doses of swine flu vaccine may all be the nasal spray version, government health officials said Friday.

The government has said a trickle of vaccine will be available in early October, but on Friday they defined the size of that trickle -- an estimated 3.4 million doses.

Currently it looks like all of them will be a nasal spray vaccine that is approved only for healthy people ages 2 to 49, said Dr. Jay Butler, an official with the Centers for Disease Control and Prevention.

The nasal spray, called FluMist, is not recommended for some of the people most in danger of severe swine flu complications. That includes pregnant women, children younger than 2, and people with asthma and other chronic respiratory diseases.

However, it's possible that some vaccine shots will become available by the first week of October as well, said Butler, chief of the CDC's swine flu vaccine task force.

Flu shots are made of killed influenza virus, while FluMist is a live but weakened strain. The nasal spray is only approved in the United States, and is made by the Maryland-based MedImmune, an AstraZeneca PLC subsidiary. Four other companies are making flu shots for the U.S.

The initial vaccine doses will go to up to 90,000 sites, including schools and clinics, across the U.S. State health departments will determine which offices and clinics get the shots, and whether health care workers or others get the first doses, Butler said at a CDC press conference Friday.

The government has ordered 195 million doses but may order more if there's enough demand, health officials have said. Typically fewer than 100 million Americans get a flu vaccine every year, and it's unclear whether swine flu will prompt much more demand.

A recent Associated Press-GfK poll found 57 percent of people said they were likely to get it.

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