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Vaccination is a method of immunization whereby elements of a disease are injected (or sprayed) into the body to stimulate the immune system to react. Usually, once the immune system has fought the vaccine, the body becomes resistant to the particular microbes that cause the disease. Vaccination is one of the most effective ways to prevent disease in individuals and eradicate diseases from communities.

A vaccine usually includes a weakened or dead microbe. When the microbe enters the body, antigens from the microbe alert the immune system to the presence of a foreign body. Antigens also tell the immune system what type of microbe is present. This allows the body to release antibodies that can fight the disease-causing microbes. After a period of time, the antibodies destroy the microbe and stimulate the creation of cells that will remain alert to that particular microbe. If the microbe is encountered again, the immune system will be able to fight it quickly before it causes disease in the body.

There are vaccines for many different diseases. Some must be administered during childhood and can protect people throughout their lives. Others can be administered later in life if a person is likely to encounter a particular disease (e.g., when people travel to a foreign country). Certain vaccines can be administered after infection. This is the case for rabies, measles, hepatitis B and tetanus because of the long incubation periods for these diseases.

Some vaccines may produce side effects that vary in severity from mild swelling or itching at the injection site to a more serious allergic reaction. However, most side effects of vaccines are mild and easily treatable.

In recent years, there has been speculation about a potential link between childhood vaccinations and an increase in the incidence of certain conditions, including autism and attention deficit hyperactivity disorder (ADHD). The speculation has centered on the number of recommended childhood vaccines, which has increased, and the use of preservatives such as thimerosal (a compound containing mercury) in some vaccines. However, numerous studies have addressed these issues and have found no evidence of links in either case. Thimerosal has been removed or reduced to trace amounts in all childhood vaccines except for the inactivated influenza vaccine as a precaution against unnecessary toxic exposure. According to the U.S. Food and Drug Administration, there is no convincing evidence of harm caused by the small amounts of thimerosal contained in influenza vaccines – other than minor symptoms of swelling or redness at the site of injection.

Many diseases that used to be common in the United States are no longer a threat because of rigorous vaccination campaigns. This has made some people reluctant about vaccinating themselves or their children because they think that the risk of side effects is more urgent than the chances of contracting the disease. However, the majority of side effects from vaccinations are mild, and there are rarely complications associated with vaccines. Measles has been essentially eradicated through vaccines in the United States, but isolated cases still occur among people who have not been vaccinated.

Smallpox has been eliminated worldwide through rigorous vaccination programs. The virus no longer exists naturally and only remains in controlled laboratory conditions. The general public no longer receives smallpox vaccines and has not for more than 30 years. Recently, concern has arisen that smallpox could be spread through bioterrorism. If terrorists acquired the virus, it could spread quickly in an unvaccinated population. However, the U.S. government still maintains large supplies of smallpox vaccine to fight such a threat.

Some common vaccinations include:

• Hepatitis B vaccine (HBV). Protects against hepatitis B, which infects the liver and can lead to long-term problems such as cirrhosis or cancer of the liver. The vaccine is given in three separate injections during childhood.
  
  
• Diphtheria, tetanus and pertussis vaccine (DTaP). Protects against diphtheria (serious throat infection), tetanus (serious nerve disease) and pertussis (respiratory illness sometimes known as “whooping cough”). DTaP is provided in a series of five injections from ages 2 months to 6 years. After the final dosage has been administered, tetanus and diphtheria booster shots are urged every 10 years. Diphtheria has been eradicated and tetanus has been nearly eradicated in the United States as a result of this vaccine. The vaccine is up to 80 percent effective in preventing pertussis infections.
  
  
• Hib vaccine. Immunizes patients against haemophilus influenzae type b bacteria, which was the leading cause of meningitis in children prior to the development of this vaccine. It is given in three doses and a booster from 2 months to 15 months. More than 90 percent of infants who receive at least three doses of the vaccine are protected against meningitis, pneumonia, inflammation of the membrane covering the heart (pericarditis), and certain infections of the blood, bones and joints.
  
  
• Inactivated polio vaccine (IPV). Prevents polio, a virus that can cause permanent paralysis. The vaccine is typically administered four times between ages 2 months and 6 years. More than 95 percent of children who are vaccinated will be protected against polio, which has been eradicated in the United States as a result this vaccine. Previously, children often received this vaccine in oral form. Today, it is given almost exclusively by injection, which eliminates the small risk of developing polio that was associated with the live oral vaccine.
  
  
• Pneumococcal conjugate vaccine (PCV). Protects against pneumococcal infections, which can lead to pneumonia, blood infections and bacterial meningitis. This infection is spread through direct contact with another person and is most dangerous to children under 2. Immunizations are given through four injections between ages 2 months and 15 months, and they provide three years of protection. PCV is also given to children who have sickle cell anemia, a damaged spleen, HIV/AIDS and diseases (e.g., diabetes, cancer) and medicines (e.g., steroids, chemotherapy) that affect the immune system.
  
  
• Measles, mumps and rubella vaccine (MMR). Protects against measles, mumps and rubella (also known as German measles). The vaccine is given in two doses between the ages of 12 months and 6 years. More than 95 percent of children who receive the vaccine will be protected from these diseases. Adults with certain conditions such as diabetes, heart, kidney or liver disease (other than cancer) or HIV should also receive the MMR vaccine if they did not do so as a child.
  
  
• Chickenpox (varicella) vaccine. Protects against chickenpox, a common childhood illness. The vaccine is typically given once, although older children may require two doses given one month apart. The vaccine prevents severe forms of chickenpox in 95 percent of those who are immunized, and prevents milder forms in 85 percent of those who are immunized.
  
  
• Influenza vaccine (flu shot). Protects against influenza (flu), one of the most common preventable infectious diseases in children and adults. It can be given as an injection or a nasal spray, although the nasal spray vaccine is not approved for children under 5 years, pregnant women or people with conditions that may weaken their immune system. The flu vaccine needs to be administered yearly because of the propensity of the flu virus to mutate over short periods of time and become resistant to previously used vaccines. While there has been some debate over the actual effectiveness of the influenza vaccine, studies show substantial benefit. Experts typically recommend this vaccine for children, older adults, and people with weakened immune systems.  

Some vaccines, such as the vaccine for hepatitis A, offer protection for life. However, vaccines such as tetanus and diphtheria require booster shots. A booster shot is an additional dose of the vaccine that is given either once or regularly (for example, every 10 years). It stimulates the immune system again and ensures that the body is adequately protected.