NEW YORK (AP) — The flu vaccine is doing a poor job protecting older Americans and others against the bug that's causing most illnesses.

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.

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.

Experts say the vaccine's poor performance is one reason why the United States has been suffering a surprisingly intense flu season.

The Centers for Disease Control and Prevention published the estimates.