updated 4/26/2009 5:16:53 PM ET 2009-04-26T21:16:53

U.S. health officials say they are "very pessimistic" that the seasonal flu vaccine protects against the unique swine flu infecting people in the United States, Mexico and other countries.

A U.S. Centers for Disease Control and Prevention official said Sunday afternoon that tests of the seasonal vaccine and the new virus show no cross-reaction, suggesting that people who got the vaccine have no added protection against the new bug.

It's possible that people who have been exposed to flu viruses every year — especially older people, with a greater exposure history — may have some natural immunity, the CDC official said in a call with reporters.

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