In 1918-1919, a global flu epidemic, or pandemic, struck half the world's population and claimed the lives of 20 million. The challenge for scientists trying to protect us from the disease is that influenza viruses can change themselves, or mutate, to become different viruses. Scientists have classified flu viruses as types A, B and C. Type A is the most common and leads to the most serious epidemics. Type B can cause epidemics, but usually produces a milder disease than type A. Type C viruses have never been associated with a large epidemic. The vaccine is made from highly purified, egg-grown viruses that have been made noninfectious.
Vaccination is available to anyone who wants to reduce their chances of getting the flu.
Studies have shown the vaccine's effectiveness rate to be 70 to 90 percent in healthy young adults. In the elderly and in people with certain chronic illnesses, the vaccine sometimes doesn't prevent illness altogether, but it does reduce its severity and the risk of complications. Police, firefighters, and other community service providers may also find vaccination useful. The vaccine's most common side effect is soreness at the vaccination site for up to two days. Some people may experience post-shot fever, malaise, sore muscles, and other symptoms resembling the flu that can last for one to two days.