Vaccines are safe and dependable, but they are not perfect. For example, up to 10 percent of all the people who are vaccinated against a particular disease may not be protected. In addition, vaccines occasionally produce harmful reactions in people. Sabin polio vaccine, for instance, may cause paralysis. This reaction occurs on an average of about 1 in every 2.7 million people who are immunized.
Passive immunization involves the injection of serum into the body.
A serum contains antibodies that have been formed in another person or an animal. It provides almost immediate protection from infection. But this immunity lasts only a few months because the antibodies gradually disappear. Physicians give serums to people who already have been exposed to such diseases as hepatitis, measles, rabies, and tetanus. Vaccines may work too slowly to help these patients.