Guillain Barre Syndome. An auto immune response, often to a virus or vaccine, where the immune system goes into overdrive and strips the protective covering from the peripheral nerves (think toes & fingers to spinal cord) and leaves one weak and sometimes paralyzed. There was an 'outbreak' after the last round of swine flu vaccines back in the 1970s. Hopefully times have changed. This time they're actually educating health care professionals about it and will be reporting cases to a centralized body, but it is still a very rare response.
And you shouldn't take it if you have severe allergies to eggs, etc:
"Anyone who has a severe (life-threatening) allergy to eggs or to any other substance in the vaccine should not get the vaccine. People should always inform their immunization provider if they have any severe allergies, if they’ve ever had a severe allergic reaction following flu vaccination, or if they have ever had GBS."
Also be aware that there is a PRESERVATIVE-FREE version available for pregnant women and children:
"There is no evidence that thimerosal (used as a preservative in vaccine packaged in multi-dose vials) is harmful to a pregnant woman or a fetus. However, because some women are concerned about exposure to preservatives during pregnancy, manufacturers will produce preservative-free seasonal and 2009 H1N1 influenza vaccines in single dose syringes for pregnant women and small children. CDC recommends that pregnant women may receive influenza vaccine with or without thimerosal"