Complications of influenza, such as pneumonia, can cause prolonged disability or death. For those with chronic illnesses, developing the flu is potentially life threatening.
Worldwide epidemics occur when there is a major shift in influenza A virus.
In 1918-1919, over 20 million people died during an influenza epidemic. Influenza immunizations can help anyone reduce the risk of getting this infection. This immunization is especially important for those who are at high risk for developing complications of infection, or those who may transmit influenza to persons at high risk. In the United States, flu season peaks between late December and early March. Influenza immunizations are needed each year before the flu season because immunity from a prior year's immunization lasts only for 3 to 6 months, and the strains of influenza change from year to year.