Another common type of vaccine is an "inactivated vaccine." These vaccines contain dead viruses or bacteria. These do not cause the immune system to react as strongly as live vaccines. Because of this, people may need "booster shots" - extra doses of the vaccine, given at certain times, so their immune system can "learn" how to fight off the infection. Examples of inactivated vaccines include vaccines for pertussis (whooping cough), rabies, and hepatitis B.