The CDC is understating the potential of Guillain-Barre' Syndrome (GBS) from the H1N1 (swine flu) vaccine.  GBS is a rare disorder in which the person's immune system attacks its own nerve cells, causing loss of muscle control, weakness and possibly peripheral paralysis leading to respiratory failure.  In its worst form, GBS is life-threatening.   The 1976 swine flu vaccine program was halted because of an association with a slightly increased risk for GBS and a less than aggressive swine flu, with only limited cases.  Rest assured that if GBS rates go up due to the vaccine, the CDC will be monitoring closely in a retro-preventive kind of way.  See http://www.cdc.gov/h1n1flu/vaccination/gbs_qa.htm