The flu vaccine is doing a poor job protecting older Americans and others against the strain of the virus that's causing most illnesses this season. Preliminary figures released Thursday suggest the vaccine is 36 percent effective overall in preventing flu illness severe enough to send a patient to the doctor's office.

There's only been one other time in the last decade when the flu vaccine did a worse job.

However, it appears to be more helpful for children. The estimates published by the U.S. Centers for Disease Control and Prevention show the vaccine reduces illness by 59 percent among children ages 6 months to 8 years old.

Most illnesses this winter have been caused by a nasty kind of flu called Type A H3N2. The vaccine was only 25 percent effective against that type. It was more effective against influenza B and an H1N1 strain, but those viruses were less prevalent this year.

Experts say the vaccine's poor performance is one reason why the United States has been suffering such an intense and deadly flu season. At least 63 children have died of flu nationwide so far this season.

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