Vaccine is a medicine used to protect the body against disease. Vaccines provide immunity by causing the body to manufacture disease-fighting substances called antibodies. This process is a type of immunization.
A vaccine contains substances that stimulate the body’s immune system to produce antibodies against a particular infectious disease. These antibodies provide protection if the person is exposed to the actual disease-causing organism.
Vaccines contain substances that are powerful enough to trigger antibody production but that do not actually cause disease. Many vaccines consist of disease-causing bacteria or viruses that have been killed. Others consist of the live germs, but in a weakened form that does not cause the disease. Vaccines known as toxoids are made from poisons produced by disease-causing organisms. These poisons are chemically treated so that they provide immunity without causing disease. Still other vaccines are made from parts of disease-causing organisms. Another group consists of live organisms that resemble disease-causing ones.
A new group of vaccines works by using genetic material rather than pieces of a disease-causing organism. The genetic material instructs the body’s own cells to create proteins that cause the desired immune response. Several vaccines developed to prevent the respiratory disease COVID-19 use this method. The genetic material used in these COVID-19 vaccines is called messenger RNA or mRNA (see RNA).
Vaccines have been developed against many diseases, including chickenpox, COVID-19, diphtheria, influenza, measles, meningitis, mumps, pneumococcal pneumonia, poliomyelitis, rabies, rubella (German measles), tetanus, whooping cough, and yellow fever. Most vaccines are injected into the body. Sabin polio vaccine is taken orally.
A single dose of some vaccines provides lifelong protection against infection. Other vaccines require several doses to produce immunity. Some of these must be reinforced at regular intervals with booster doses. Toxoids for diphtheria and tetanus are generally combined with whooping cough vaccine. Protection against measles, mumps, and rubella also may be provided in one vaccine. Most vaccines begin to provide immunity about two weeks after they are administered.
In the United States, most physicians recommend that children be vaccinated against 11 diseases: chickenpox, diphtheria, human papillomavirus, measles, meningitis caused by the bacterium Haemophilus influenzae type b, mumps, polio, rubella, tetanus, whooping cough, and hepatitis (types A and B). Influenza vaccine (commonly called the “flu shot”) is given routinely to people of all ages during flu season.
Other vaccines, such as those for cholera and yellow fever, are given only to persons who plan to travel to places where the disease is widespread. Vaccines for some rare diseases are given only to those who may be exposed to them.
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 in an average of about 1 in every 2.7 million people who are immunized.
Scientists and doctors around the world are always working to develop and test new vaccines. Some scientists work on creating vaccines for diseases that have no existing vaccine. When a vaccine is already available for a disease, scientists may engineer one that is more effective, more convenient to administer, or cheaper to manufacture.
