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 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 most sexually transmitted diseases (such as HIV infection, syphilis, gonorrhea, and chlamydial infections), infections caused by ticks (such as Lyme disease), 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 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
In active immunization, vaccines are used to stimulate the body’s natural defense mechanisms (the immune system). 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
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Bacille Calmette-Guérin (BCG—for tuberculosis)
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Chickenpox (varicella)
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Cholera (certain vaccines given by mouth)
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Influenza (only the nasal spray vaccine)
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Polio (only the oral vaccine, which is no longer used in the United States)
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Typhoid (only the oral vaccine)
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Shingles (only one of the two available vaccines)
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:
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The blood (serum) of animals (usually horses) that have been exposed to a particular organism or toxin and have developed immunity
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Blood collected from a large group of people—called pooled human immune globulin
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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
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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 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).
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).
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.
Vaccination Restrictions and Precautions
For many vaccines, the only reason for not being vaccinated is
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A serious, life-threatening allergic reaction (such as an anaphylactic reaction) to the vaccine or to one of its components
Egg allergy is common in the United States. Some vaccines, including most influenza vaccines, 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) 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
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Use of drugs that suppress the immune system (immunosuppressants), such as corticosteroids and chemotherapy drugs
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A disorder that weakens the immune system, such as AIDS
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Pregnancy
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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 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.
Protecting Children Through Vaccines
Disease |
When Do Vaccinations Typically Start |
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Chickenpox (varicella) |
Age 12–15 months |
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Age 12 years |
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Age 2 months |
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Haemophilus influenzae type b infections (such as meningitis) |
Age 2 months |
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Age 12–23 months |
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Birth |
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Human papillomavirus (HPV) |
Age 11–12 years (can start at age 9 years) |
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Age 6 months |
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Age 12–15 months |
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Age 11–12 years Age 2 to 9 months for children at high risk |
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Age 12–15 months |
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Age 2 months |
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Age 2 months |
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Age 2 months |
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Age 2 months |
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Rubella (German measles) |
Age 12–15 months |
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Age 2 months |
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.
Protecting Adults Through Vaccines
Disease* |
Who Should Be Vaccinated |
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People who have been exposed to anthrax People who may be exposed to anthrax, such as the following: |
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Chickenpox (varicella) |
All adults who have not had the vaccine or chickenpox |
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All people 12 years of age and older (Pfizer-BioNTech vaccine) All adults 18 years of age and older (Moderna and Johnson & Johnson vaccines) |
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All adults (usually as a combination vaccine with tetanus as Td or also with pertussis as Tdap) if they have not already been vaccinated Pregnant women should receive Tdap during each pregnancy |
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All adults in the United States who are at high risk of occupational exposure to Ebola because they are doing the following: |
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Haemophilus influenzae type b infections (such as meningitis) |
Adults who have not been vaccinated and who are at increased risk, such as the following:
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Adults who have not been vaccinated and who are at increased risk, such as the following:
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Adults who have not been vaccinated and who are at increased risk, such as the following:
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Human papillomavirus (HPV) |
All males and females who have not been previously vaccinated (typically at age 11 or 12 years) through age 26 years All adults aged 27 to 45 years should talk with their doctor about whether they should be vaccinated People who have a weakened immune system, including people with HIV |
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All people over age 6 months |
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All adults born in or after 1957 unless they have documentation of vaccination with one or more doses of the measles, mumps, and rubella (MMR) vaccine or laboratory tests detect evidence of measles immunity Always given as a combination vaccine with mumps and rubella (not available as a single vaccine) |
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Certain people over age 55 People 16 to 23 years of age who want it People at increased risk, such as the following:
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All adults born in or after 1957 unless they have documentation of vaccination with one or more doses of the MMR vaccine or laboratory tests detect evidence of mumps immunity Always given as a combination vaccine with measles and rubella (not available as a single vaccine) |
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Pertussis (whooping cough) |
All adults (usually given as a combination vaccine with tetanus and diphtheria as Tdap) if they have not already been vaccinated Pregnant women during each pregnancy |
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Pneumococcal infections (such as meningitis and pneumonia) |
All people aged 65 years and over Adults at increased risk, such as the following:
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Adults at increased risk, such as |
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People who have been bitten by certain animals People who may be at increased risk of exposure to infected animals, such as |
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Rubella (German measles) |
All adults born in or after 1957 unless they have documentation of vaccination with one or more doses of the MMR vaccine or laboratory tests detect evidence of rubella immunity Women who are planning on becoming pregnant and do not have immunity to rubella Always given as a combination vaccine with measles and mumps (not available as a single vaccine) |
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Shingles (herpes zoster) |
People aged 50 and over |
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Not currently recommended except for people at high risk of being exposed to the smallpox virus, such as laboratory workers who directly handle the virus and related materials |
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All adults every 10 years (usually as a combination vaccine with tetanus and diphtheria as Td or also with pertussis as Tdap) if they have not already been vaccinated Pregnant women should receive Tdap during each pregnancy |
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People traveling to areas where the infection is common People who have close contact with a typhoid carrier Laboratory workers who work with the bacteria that cause typhoid fever |
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People traveling to certain parts of Africa and South America, where the infection is common |
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* Vaccines are available in the United States for these infections. |
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HIV = human immunodeficiency virus; Td = tetanus-diphtheria; Tdap = tetanus-diphtheria-pertussis. |
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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 and their possible side effects.
One of parents' main concerns has been
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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 Vaccination Concerns 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). 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.
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Centers for Disease Control and Prevention (CDC): Child and adolescent immunization schedule for ages 18 years or younger, 2021
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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
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Vaccine Adverse Event Reporting System (VAERS): Where and how to report side effects of vaccines
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Vaccine Safety Datalink (VSD): A collaborative organization that monitors and evaluates the safety of vaccines
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Children's Hospital of Philadelphia: Vaccine Education Center
