Statistics show that most people recover within two weeks. However, in some people, the infection becomes severe. Potentially life-threatening complications, such as pneumonia, may develop. Statistics also show that influenza is responsible for about 20,000 deaths annually in the United States and a significantly higher number of hospitalizations. Although anyone is at risk for more serious complications, this risk is highest in the elderly and those with chronic health problems.
Influenza viruses change continually.
Each year, the major strains of influenza change slightly, thereby allowing them to evade human immune defense systems. When a strain changes significantly, as has happened at least three times this century, virtually no one in the population can resist infection and a worldwide epidemic results. Each year the vaccine is re-developed using a dead virus from the new strain. Because it is a dead virus, recipients of the vaccine do not acquire the flu. The most common side effect is soreness at the injection site, which affects less than one-third of those who receive the vaccine.