Immunization enables the body to better defend itself against diseases caused by certain bacteria or viruses. Immunity (the ability of the body to defend itself against diseases caused by certain bacteria or viruses) 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 Lyme Disease Lyme disease is a tick-transmitted infection caused by Borrelia species, primarily by Borrelia burgdorferi and sometimes by Borrelia mayonii in the United States. These spiral-shaped bacteria... read more 
and diphtheria Diphtheria Diphtheria is a contagious, sometimes fatal infection of the upper respiratory tract caused by the gram-positive, rod-shaped bacteria (see figure How Bacteria Shape Up) Corynebacterium diphtheriae... read more 
) are now rare or under control. One disease, smallpox Smallpox Smallpox is a highly contagious, very deadly disease caused by the variola virus. The disease is now considered eliminated. There have been no cases of smallpox since 1977. People can acquire... read more 
, has been completely eliminated by vaccination. On October 6, 2021, the World Health Organization (WHO) recommended widespread use of the RTS,S/AS01 (RTS,S) malaria Malaria Malaria is infection of red blood cells with one of five species of Plasmodium, a protozoan. Malaria causes fever, chills, sweating, a general feeling of illness (malaise), and sometimes diarrhea... read more vaccine among children in sub-Saharan Africa and in other regions with moderate to high Plasmodium falciparum malaria transmission. (See WHO recommends groundbreaking malaria vaccine for children at risk.) 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 most sexually transmitted diseases (such as HIV infection Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and can cause acquired immunodeficiency syndrome (AIDS). HIV is transmitted... read more 
, syphilis Syphilis Syphilis is a sexually transmitted disease caused by the bacteria Treponema pallidum. Syphilis can occur in three stages of symptoms, separated by periods of apparent good health. It begins... read more 
, gonorrhea Gonorrhea Gonorrhea is a sexually transmitted disease caused by the bacteria Neisseria gonorrhoeae, which infect the lining of the urethra, cervix, rectum, and throat or the membranes that cover the front... read more 
, and chlamydial infections Chlamydial and Other Nongonococcal Infections Chlamydial infections include sexually transmitted diseases of the urethra, cervix, and rectum that are caused by the bacteria Chlamydia trachomatis. These bacteria can also infect the membranes... read more 
), infections caused by ticks (such as Lyme disease Lyme Disease Lyme disease is a tick-transmitted infection caused by Borrelia species, primarily by Borrelia burgdorferi and sometimes by Borrelia mayonii in the United States. These spiral-shaped bacteria... read more ), and many tropical diseases (such as dengue Dengue Dengue is a mosquito-borne viral infection that causes fever, generalized body aches, and, if severe, external and internal bleeding (called dengue hemorrhagic fever). About 50 to 100 million... read more ).
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 easily spread from person to person. Many of them 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.
Types of Immunization
There are two types of immunization:
Active immunization
Passive immunization
Active immunization
In active immunization, vaccines are used to stimulate the body’s natural defense mechanisms (the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more ). Vaccines are preparations that contain one of the following:
Noninfectious fragments of bacteria or viruses
A usually harmful substance (toxin) that is produced by a bacteria but has been modified to be harmless—called a toxoid
Weakened (attenuated), live whole organisms that do not cause illness
The body’s immune system responds to a vaccine by producing substances (such as antibodies Antibodies One of the body's lines of defense (immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more 
and white blood cells White blood cells The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more ) 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
Polio Polio Vaccine The polio vaccine protects against polio, a very contagious viral infection that affects the spinal cord and brain. Polio can cause permanent muscle weakness, paralysis, and sometimes death... read more (only the oral vaccine, which is no longer used in the United States)
Passive immunization
In passive immunization, antibodies against a specific infectious organism (or the toxin produced by an 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 Chickenpox Chickenpox is a highly contagious viral infection with the varicella-zoster virus that causes a characteristic itchy rash, consisting of small, raised, blistered, or crusted spots. Chickenpox... read more 
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 weeks, until the body eliminates the injected antibodies.
Vaccine Administration
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 Use of several vaccines at the same time Despite the strong vaccine safety systems in place in the United States, some parents remain concerned about the use and schedule of vaccines in children. These concerns can lead some parents... read more ).
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 Routine Vaccinations for Infants, Children, and Adolescents Routine Vaccinations for Infants, Children, and Adolescents Most doctors follow the vaccination schedule recommended by the Centers for Disease Control and Prevention (CDC—see the schedule for infants and children and the schedule for older children... read more ).
Other vaccines are usually given mainly to specific groups of people. For example, the yellow fever vaccine Yellow Fever Yellow fever is a mosquito-borne viral disease that occurs mainly in the tropics. Yellow fever occurs only in the tropical areas of Central Africa, southern Panama, and South America. Some people... read more 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 Prevention Rabies is a viral infection of the brain that is transmitted by animals and that causes inflammation of the brain and spinal cord. Once the virus reaches the spinal cord and brain, rabies is... read more may be given to a person who has been bitten by a dog.
Vaccination Restrictions and Precautions
For many vaccines, the only reason for not being vaccinated is
A serious, life-threatening allergic reaction (such as an anaphylactic reaction Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more ) to the vaccine or to one of its components
Egg allergy is common in the United States. Some vaccines, including most influenza vaccines Influenza Vaccine The influenza virus vaccine helps protect against influenza. Two types of influenza virus, type A and type B, regularly cause seasonal epidemics of influenza in the United States. There are... read more , contain very small amounts of material from eggs. Thus, there is concern about using such vaccines in people who are allergic to eggs. However, the Centers for Disease Control and Prevention (CDC) states that although mild reactions may occur, serious allergic reactions (anaphylaxis Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more ) are unlikely. Recommendations for the influenza vaccine vary according to the severity of the allergic reaction to eggs and the vaccine. If people who had a severe, life-threatening allergic reaction after they were given the influenza vaccine or eggs, they should not be given the influenza vaccine. If people had only a rash after exposure to eggs or the vaccine, they may be given the vaccine. If people had a more serious reaction, such as facial swelling, difficulty breathing, or dizziness, or reactions that required an injection of the drug epinephrine or other emergency treatment, they should get the vaccine in a medical setting supervised by a clinician who has experience recognizing and managing severe allergic reactions. Experts generally think these CDC recommendations also are appropriate in regard to other egg-derived vaccines besides influenza.
Vaccines that contain live organisms should not be used or should be delayed in people with certain conditions, such as
Use of drugs that suppress the immune system Some Drugs That Can Cause Immunodeficiency Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more (immunosuppressants), such as corticosteroids and chemotherapy drugs
A disorder that weakens the immune system Disorders That Can Cause Immunodeficiency Immunodeficiency disorders involve malfunction of the immune system, resulting in infections that develop and recur more frequently, are more severe, and last longer than usual. Immunodeficiency... read more , such as AIDS
Pregnancy
Development of Guillain-Barré syndrome within 6 weeks after a previous dose of the vaccine
If people stop taking the drugs that suppress their immune system or if their weakened immune system recovers sufficiently, giving them vaccines that contain live virus may be safe.
Common Vaccinations in Children
Children typically are given a number of vaccines according to a standard schedule (see figure Routine Vaccinations for Infants, Children, and Adolescents Routine Vaccinations for Infants, Children, and Adolescents Most doctors follow the vaccination schedule recommended by the Centers for Disease Control and Prevention (CDC—see the schedule for infants and children and the schedule for older children... read more and see Centers for Disease Control and Prevention: Recommended Child and Adolescent Immunization Schedule for ages 18 years or younger, United States, 2021). If vaccines are missed, most can be given later, according to a catch-up schedule.
Common Vaccinations in Adults
Adults may also be advised to receive certain vaccines (see also Centers for Disease Control and Prevention: Recommended Adult Immunization Schedule for ages 19 years or older, United States, 2021). When advising adults about vaccination, a doctor considers the person’s age, health history, childhood vaccinations, occupation, geographic location, travel plans, and other factors.
Vaccine Safety
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) and Vaccine Safety Datalink (VSD). If any health problem happens after vaccination, anyone—doctors, nurses, or any member of the general public—can submit a report to VAERS. VAERS reports cannot determine whether a health problem was caused by the vaccine.
Before a new vaccine can be licensed, it, like any medical product, is tested in controlled clinical trials. Such trials compare the new vaccine to a placebo or to a previously existing vaccine for the same disease. Such trials show whether the vaccine is effective and identify common side effects. However, some side effects are too rare to be detected in any reasonably sized clinical trial and do not become apparent until after a vaccine is used routinely in many people. Thus, a surveillance system called the Vaccine Adverse Event Reporting System was created to monitor the safety of vaccines that are used in the general public. VAERS collects reports from people who believe that they had a side effect after a recent vaccination and from health care practitioners who identify certain possible side effects after a vaccine was given, even if they are unsure the effects are related to the vaccine. Thus, the existence of a VAERS report is not proof that a vaccine caused a certain side effect. VAERS is simply a system for collecting data about things that might be side effects. Then, the Food and Drug Administration (FDA) can further evaluate the concern by comparing how often the possible side effect occurred in people who were vaccinated to how often it occurred in people who were not vaccinated.
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 of childhood vaccines Vaccine Safety Vaccination protects children against many infectious diseases. Vaccines contain either noninfectious components of bacteria or viruses or whole forms of these organisms that have been weakened... read more and their possible side effects.
One of parents' main concerns has been
That certain vaccines, such as the measles-mumps-rubella (MMR) vaccine Measles, Mumps, and Rubella Vaccine The measles, mumps, and rubella (MMR) vaccine is a combination vaccine that helps protect against these three serious viral infections. The vaccine contains live but weakened measles, mumps... read more or vaccines that contain thimerosal (a mercury-based preservative), may increase the risk of autism Autism Spectrum Disorders Autism spectrum disorders are conditions in which people have difficulty developing normal social relationships, use language abnormally or not at all, and show restricted or repetitive behaviors... read more
Many different groups of scientists have studied these concerns and have completely disproved the supposed relationship between vaccines and autism (see Childhood Vaccination Concerns Childhood Vaccination Concerns Despite the strong vaccine safety systems in place in the United States, some parents remain concerned about the use and schedule of vaccines in children. These concerns can lead some parents... read more in THE MANUAL 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 thimerosal is available at the Food and Drug Administration's web site (Thimerosal and Vaccines).
Vaccination Before Foreign Travel
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 Vaccines for International Travel*,† Travel preparation is crucial, even for healthy people. Proper preparations are inexpensive relative to the costs of getting sick or injured while away from home. Prior to travel, people and... read more ). 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.
More Information
The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
Centers for Disease Control and Prevention (CDC): Child and adolescent immunization schedule for ages 18 years or younger, 2021
Food and Drug Administration (FDA): Thimerosal and Vaccines: Comprehensive information about thimerosal—what it is, why it is used in vaccines, why it is safe, and how many vaccines are now made without it
Vaccine Adverse Event Reporting System (VAERS): Where and how to report side effects of vaccines
Vaccine Safety Datalink (VSD): A collaborative organization that monitors and evaluates the safety of vaccines
Children's Hospital of Philadelphia: Vaccine Education Center
World Health Organaization (WHO): WHO recommends groundbreaking malaria vaccine for children at risk
