Immunization (vaccination) helps the body 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 vaccinated 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 vaccinated still may get the disease.
Vaccines have been very effective in preventing serious disease and in improving health worldwide. 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.
However, effective vaccines are not yet available for many important infections, including most sexually transmitted infections (such as HIV infection, syphilis, gonorrhea, and chlamydia), infections caused by ticks (such as Lyme disease), and many tropical diseases (such as Chagas disease).
As of October 2023, the World Health Organization (WHO) recommends widespread use of the RTS,S/AS01 (RTS,S) and R21/Matrix-M malaria vaccine among children in malaria-endemic areas with moderate to high Plasmodium falciparum malaria transmission. Malaria vaccination is an important intervention to prevent malaria, which causes hundreds of thousands of deaths each year, mostly in children in Africa. (See WHO: Malaria vaccine implementation programme.)
It is important to follow recommendations for vaccination. This is very important for a person's own health and for the health of their family and 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 effective, and side effects are rare.
Types of Immunization
There are 2 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). 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) that recognize and attack the specific bacteria or virus contained in the vaccine. Then whenever the person is exposed to that 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
Bacille Calmette-Guérin (BCG—for tuberculosis)
Chickenpox (varicella)
Cholera (certain vaccines given by mouth)
Influenza (only the nasal spray vaccine)
Polio (only the oral vaccine, which is no longer used in the United States)
Typhoid (only the oral vaccine)
Did You Know...
|
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 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.
Some vaccines are given routinely (that is, given to most people on a recommended schedule)—for example, the is given to adults, preferably every 10 years (see Centers for Disease Control and Prevention [CDC]: Adult Immunization Schedule by Age). Some vaccines are routinely given to children (see also CDC: Child and Adolescent Immunization Schedule by Age).
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 or other animal if the animal may be infected with rabies.
Vaccination Restrictions and Precautions
For many vaccines, the only reason for not being vaccinated is
A history of 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 trace amounts of egg antigens. According to the CDC, people with a history of egg allergy should receive the influenza vaccine, regardless of the severity of any previous reaction to egg. (See CDC: Summary: ‘Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices (ACIP)—United States, 2023-24’.) People should receive all vaccines, including the influenza vaccine, from health care professionals who have access to the proper equipment needed to recognize and treat a serious allergic reaction.
Vaccines that contain live organisms should not be used or should be delayed in people with certain conditions, such as
Use of medications that suppress the immune system (immunosuppressants), such as corticosteroids and chemotherapy agents
A disorder that weakens the immune system, such as HIV
Pregnancy
Development of Guillain-Barré syndrome within 6 weeks of a previous dose of the vaccine
In some cases, to prevent spreading infection to people with a weakened immune system, people who live with them should also not be given vaccines that contain live organisms.
If people stop taking the medications that suppress their immune system or if their weakened immune system recovers sufficiently, receiving vaccines that contain live virus may be safe.
Common Vaccinations in Children
Children in the United States typically are given a number of vaccines according to a standard schedule (see CDC: Vaccines for Your Children). 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 CDC: Recommendations for Ages 19 Years or Older). When advising adults about vaccination, a doctor considers the person’s age, health history, childhood vaccinations, occupation, geographic location, travel plans, and other factors.
Despite clinical guidelines and the burden and consequences of vaccine-preventable diseases, some adults do not receive the recommended vaccines. In the United States, vaccination coverage (the percentage of people who have received specific vaccines) among adults remains low for most vaccines. In addition, coverage for all vaccines differs by race and ethnicity with generally lower coverage among Black and Hispanic adults compared with White adults (see CDC: Vaccination Coverage among Adults in the United States, National Health Interview Survey, 2021).
Protecting Adults Through Vaccines
Disease* | Who Should Be Vaccinated |
|---|---|
People who have been exposed to anthrax People who may be exposed to anthrax, such as the following:
| |
Adults traveling to certain areas with chikungunya virus Laboratory workers in the United States | |
Chickenpox (varicella) | All adults who have not had the vaccine or chickenpox |
All people 6 months of age and older (vaccines made by Pfizer-BioNTech and Moderna) All people 12 years of age and older (vaccine made by Novavax) | |
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 | |
Adults in the United States who are at high risk of occupational exposure to Ebola because they are doing the following:
| |
Haemophilus influenzae type b (Hib) infections (such as meningitis) | Adults who have not been vaccinated and who are at increased risk, such as the following:
|
Adults who have not been vaccinated and who are at increased risk, such as the following:
| |
All adults 19–59 years of age and adults 60 years of age and older with risk factors for hepatitis B. Adults 60 years of age and older without known risk factors for hepatitis B may also receive the hepatitis B vaccine. Risk factors include the following:
| |
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 27 to 45 years old should talk with their doctor about whether they should be vaccinated People who have a weakened immune system, including people with HIV |
All people over age 6 months | |
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) | |
Certain people over age 55 People 16 to 23 years of age who want it People at increased risk, such as the following:
| |
All adults born in or after 1957 unless they have documentation of vaccination with one or more doses of the MMR vaccine or if laboratory tests detect evidence of mumps immunity Always given as a combination vaccine with measles and rubella (not available as a single vaccine) | |
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 |
Pneumococcal infections (such as meningitis and pneumonia) | All people 65 years old and over Adults 19 to 64 years old who have certain medical conditions or other risk factors such as the following:
|
Adults at increased risk, such as
| |
People who have been bitten by certain animals People who may be at increased risk of exposure to infected animals, such as
| |
Adults 60 years old and over, using shared clinical decision-making based on discussions with their health care professional Pregnant individuals at 32 to 36 weeks gestation for the prevention of lower respiratory tract disease caused by RSV in infants from birth through 6 months of age | |
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 if 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) |
Shingles (herpes zoster) | People 50 years old and over People 19 years old and over who have or will have a weakened immune system because of a disease or treatment for a disease |
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 | |
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 People who have a wound that poses an increased risk of tetanus if 5 years or longer have elapsed since their previous vaccine dose Pregnant women should receive Tdap during each pregnancy | |
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 | |
People traveling to certain parts of Africa and South America, where the infection is common | |
* Vaccines are available in the United States for these infections. | |
HIV = human immunodeficiency virus; Td = tetanus-diphtheria; Tdap = tetanus-diphtheria-pertussis. | |
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.
Vaccines usually cause no problems, although mild side effects, such as soreness or redness at the injection site, may occur. Sometimes parents are concerned about the safety 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 Vaccination Concerns in THE MANUAL and CDC's Vaccine Safety FAQs for Parents and Caregivers 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 English-language resources 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 by age
CDC: Travelers’ Health: Information about travel health notices and about what vaccines to get depending on destination
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 Organization (WHO): Malaria vaccine implementation programme
European Centre for Disease Prevention and Control (ECDC): Vaccine schedules in all countries in the EU/EEA
