The best defense against influenza is the flu vaccine, which is 70 percent to 90 percent effective in preventing illness, according to the Centers for Disease Control and Prevention. In higher-risk groups, the vaccine may be less effective in actually preventing the flu, but it will protect against the mores severe aspects of the illness and lessen the serious complications and risk of death. This year's vaccine supply will be delivered in stages as only three drug companies are manufacturing it; delays are usually caused by the complicated procedures used to determine the strain, the amount needed and then production problems surrounding egg supplies, and actually growing the virus.