The hepatitis A vaccine protects against a liver disease that's caused by the hepatitis A virus. The virus is generally spread from person to person (especially from unwashed hands) and can sometimes be transmitted through contaminated food or water.
Although hepatitis A is not as serious as hepatitis B or C (those infections turn into chronic illnesses, whereas a bout of hepatitis A usually clears up in a couple of months), it's still dangerous. Some people who become infected with hepatitis A have very mild or no symptoms, especially children under 6 (about 70 percent will have no symptoms at all). Those who do develop symptoms may experience fever, fatigue, nausea, loss of appetite, abdominal pain, dark urine, or a yellowing of the skin and eyes called jaundice. Mild cases usually go away on their own, but about 20 percent of people with hepatitis A infections have to be hospitalized. In very rare cases, the virus can cause liver failure and even death.
Before the hepatitis A vaccine was introduced in 1995, the virus infected around 25,000 people in the U.S. annually. Even though many cases went undiagnosed (because people didn't show symptoms) it's estimated that children under age 4 were most affected. Since the vaccine has become widely used, hepatitis A infections have decreased by nearly 90 percent.