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 Chikungunya disease).
In 2021, the World Health Organization (WHO) recommended widespread use of the RTS,S/AS01 (RTS,S) malaria vaccine among children in sub-Saharan Africa and in other regions with moderate to high Plasmodium falciparum malaria transmission. This was an important new 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 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). 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 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
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)
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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 (see Use of several vaccines at the same time).
routinely given to children (see also Centers for Disease Control and Prevention [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 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. Many people with egg allergy can safely receive vaccines that contain very small amounts of material from eggs, such as influenza vaccines. 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 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.
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 (immunosuppressants), such as corticosteroids and chemotherapy drugs
A disorder that weakens the immune system, such as AIDS
Pregnancy
Development of Guillain-Barré syndrome within 6 weeks after 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 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 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.
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
