* This is the Consumer Version. *
Overview of Immunization
Immunization enables the body to better defend itself against diseases caused by certain bacteria or viruses. Immunity may occur naturally (when people are exposed to bacteria or viruses), or doctors may provide it through vaccination. When people are immunized against a disease, they usually do not get the disease or get only a mild form of the disease. However, because no vaccine is 100% effective, some people who have been immunized still may get the disease.
In communities and countries where vaccines are widely used, many diseases that were once common and/or fatal (such as polio, measles, and diphtheria) are now rare or under control. One disease, smallpox, has been completely eliminated by vaccination. Vaccines have been very effective in preventing serious disease and in improving health worldwide. However, effective vaccines are not yet available for many important infections, including Ebola virus infection, most sexually transmitted diseases (such as HIV, syphilis, gonorrhea, and chlamydial infections), and many tropical diseases (such as malaria).
Following recommendations for vaccination is very important for people's own health and for the health of their family and the people in their community. Many of the diseases prevented by vaccination are still present in the United States and remain common in other parts of the world. These diseases can spread rapidly among unvaccinated children, who, because of the ease of modern travel, can be exposed even if they live in areas where a disease is not common.
Vaccines available today are highly reliable, and most people tolerate them well. They rarely have side effects.
There are two types of immunization:
In active immunization, vaccines are used to stimulate the body’s natural defense mechanisms. Vaccines are preparations that contain one of the following:
The body’s immune system responds to a vaccine by producing substances (such as antibodies and white blood cells) that recognize and attack the specific bacteria or virus contained in the vaccine. Then whenever the person is exposed to the specific bacteria or virus, the body automatically produces these antibodies and other substances to prevent or lessen illness. The process of giving a vaccine is called vaccination, although many doctors use the more general term immunization.
Vaccines that contain live but weakened organisms include
Not all the vaccines that contain live organisms are available in the United States.
In passive immunization, antibodies against a specific infectious organism are given directly to a person. These antibodies are obtained from several sources:
The blood (serum) of animals (usually horses) that have been exposed to a particular organism or toxin and have developed immunity
Blood collected from a large group of people—called pooled human immune globulin
People known to have antibodies to a particular disease (that is, people who have been immunized or who are recovering from the disease)—called hyperimmune globulin—because these people have higher levels of antibodies in their blood
Antibody-producing cells (usually taken from mice) grown in a laboratory
Passive immunization is used for people whose immune system does not respond adequately to an infection or for people who acquire an infection before they can be vaccinated (for example, after being bitten by an animal with rabies).
Passive immunization can also be used to prevent disease when people are likely to be exposed and do not have time to get or complete a vaccination series. For example, a solution containing gamma globulin that is active against chickenpox virus can be given to a pregnant woman who does not have immunity to the virus and has been exposed to it. The chickenpox virus can harm the fetus and cause serious complications (such as pneumonia) in the woman.
Passive immunization lasts for only a few days or weeks, until the body eliminates the injected antibodies.
Vaccines and antibodies are usually given by injection into a muscle (intramuscularly) or under the skin (subcutaneously). Antibodies are sometimes injected into a vein (intravenously). One type of influenza vaccine is sprayed into the nose.
More than one vaccine may be given at a time—in one combination vaccine or in separate injections at different injection sites (see Use of several vaccines at the same time).
Some vaccines are given routinely—for example, the tetanus toxoid is given to adults, preferably every 10 years. Some vaccines are routinely given to children (see Figure: Vaccinating Infants and Children).
Other vaccines are usually given mainly to specific groups of people. For example, the yellow fever vaccine is given only to people traveling to certain parts of Africa and South America. Still other vaccines are given after possible exposure to a specific disease. For example, the rabies vaccine may be given to a person who has been bitten by a dog.
For many vaccines, the only reason for not being vaccinated is
A serious, life-threatening allergic reaction (such as an anaphylactic reaction) to the vaccine or to one of its components
Some vaccines, including most influenza vaccines, contain very small amounts of material from eggs. Many people are allergic to eggs, but only those who have had severe allergic reactions to eggs should avoid the influenza vaccine. If people have had less severe reactions to eggs (such as a rash), doctors may give them influenza vaccine that contains inactivated virus, but not influenza vaccine that contains live virus.
Vaccines that contain live virus, should not be used or should be delayed in people with certain conditions, such as
A weakened immune system due to a disorder, such as AIDS, or to drugs that suppress the immune system (immunosuppressants), such as corticosteroids and chemotherapy drugs
Some progressive nervous system disorders, such as Guillain-Barré syndrome
If people stop taking the drugs that suppress the immune system or if their weakened immune system recovers sufficiently, giving them vaccines that contain live virus may be safe.
Children typically are given a number of vaccines according to a standard schedule (see Figure: Vaccinating Infants and Children and Centers for Disease Control and Prevention: Immunization Schedules ). If vaccines are missed, most can be given later, according to a catch-up schedule.
Protecting Children Through Vaccines
Adults may also be advised to receive certain vaccines (see also Centers for Disease Control and Prevention: Immunization Schedules for Adults ). When advising adults about vaccination, a doctor considers the person’s age, health history, childhood vaccinations, occupation, geographic location, travel plans, and other factors.
Protecting Adults Through Vaccines
In the United States, the Centers for Disease Control and Prevention (CDC) monitors the safety of vaccines. Doctors must report certain problems that occur after routine vaccination to the CDC's Vaccine Adverse Event Reporting System [VAERS], Vaccine Safety Datalink [VSD]—see Childhood Vaccinations : Vaccine Safety). For additional information about the safety of individual vaccines, see Vaccine Safety at the CDC web site.
Vaccines usually cause no problems, although mild side effects, such as soreness or redness at the injection site, may occur. Nonetheless, many parents remain concerned about the safety and possible adverse effects of childhood vaccines. One of parents' main concerns has been that certain vaccines, such as the measles-mumps-rubella (MMR) vaccine or vaccines that contain thimerosal (a mercury-based preservative), may increase the risk of autism. Many different groups of scientists have studied these concerns and have completely disproved the supposed relationship between vaccines and autism (see Childhood Vaccinations : Vaccination Concerns in The MerckManual and FAQs About Vaccine Safety at the CDC web site.
Nevertheless, most manufacturers have developed thimerosal-free vaccines for use in infants and adults. Information about vaccines that currently contain low levels of mercury or thimerosal is available at the Institute for Vaccine Safety web site.
Residents of the United States may be required to receive specific vaccines before traveling to areas that have infectious diseases not normally found in the United States (see Table: Vaccines for International Travel* ,†,‡). Recommendations change frequently in response to disease outbreaks.
The CDC provides the most up-to-date information on vaccination requirements in their Travelers’ Health section. Also, the CDC has a 24-hour telephone service (1-800-232-4636 [CDC-INFO]) that provides information.
* This is the Consumer Version. *