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But there are other arguments for getting a flu shot. For one thing, the vaccine is only about 70 percent effective to begin with -- and that's in a good year, when the vaccine is well-calibrated to circulating strains. That's not good enough to protect the U.S. population for one year, let alone two, CDC officials say.
But the main argument now is one of waning immunity. CDC officials believe that a year after someone gets the flu shot, antibody levels -- an indicator of immunity -- can fall by two-thirds or more. Some key studies indicate the resulting levels are not strong enough to be protective, said Nancy Cox, head of the CDC's chief of the CDC's flu division.
However, other studies are less clear. Some have suggested that a flu vaccination can provide sufficient protection for more than a year in adults, and perhaps two or three years in children.
Cox said some of those conflicting studies are outdated and flawed, and noted more recent U.S. studies that found large drops in children's immunity in just one year.
Other researchers sound less decided about which studies were right and which were wrong on this question.
"Nobody really, really knows," said Dr. John Treanor, a flu vaccine researcher at the University of Rochester School of Medicine.
But even if the question is unsettled, Treanor and other experts said the CDC's position is probably the wisest course.
"The bottom line is, with our current knowledge, we believe it is better to be re-vaccinated. And getting another shot is certainly not going to harm you," said Dr. Arnold Monto, an esteemed University of Michigan flu expert.
Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
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