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Overview of Immunization

ByMargot L. Savoy, MD, MPH, Lewis Katz School of Medicine at Temple University
Reviewed/Revised Modified Jul 2025
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Immunization is the process of becoming protected against a disease. There are 2 main ways this process takes place:

  • Immunity

  • Vaccines

Immunity is the immune system's way of defending the body against diseases caused by foreign invaders such as bacteria, fungi, cancers, or viruses. People are born with innate immunity, which fights invaders before people are exposed to them, and later develop acquired immunity, which fights invaders after people are exposed to them.

Vaccines are medical treatments that help the immune system recognize diseases caused by certain bacteria (such as the ones that causes pertussis) and viruses (such as the one that causes the flu) and defend the body against them. The process of giving a vaccine is called vaccination, although many health care professionals use the more general term immunization.

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 the following:

Vaccines are 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 people, 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.

The World Health Organization (WHO) recommends widespread use of the RTS,S/AS01 (RTS,S) and R21/Matrix-M malaria vaccine among children in areas where malaria is common and where spread of malaria caused by the parasite Plasmodium falciparum is moderate to high. 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.)

Types of Immunization

There are 2 types of immunization:

  • Active immunization

  • Passive immunization

Active immunization

In active immunization, vaccines are given to stimulate the body’s natural defense mechanisms (the immune system). 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.

Vaccines work in different ways to trigger active immunization and there are several kinds:

  • Inactivated vaccines: Contain a piece of the bacteria or virus that does not cause infection

  • Live-attenuated vaccines: Contain a weakened (attenuated), live, whole microorganisms that do not cause infection

  • mRNA vaccines: Contain messenger RNA of the virus, which triggers the body's immune system to remember the messenger RNA and fight future infections

  • Subunit, recombinant, polysaccharide, and conjugate vaccines: Contain only a specific piece of the virus or bacterium (such as its protein)

  • Toxoid vaccines: Contain a harmless substance (called a toxoid) that has been modified from a harmful substance (called a toxin) produced by a bacterium

  • Vector vaccines: Contain genetic material of the virus, which triggers the body's immune system to remember the genetic material and fight future infections, and also contain a different, harmless virus that helps get the genetic material into cells

  • Adjuvanted vaccines: Contain an additional chemical (the adjuvant) that boosts the effectiveness of the vaccine by stimulating a stronger and longer response from the body's immune system

Active immunity takes time (usually several weeks) to develop, but, because it involves the creation of immunologic memory, active immunity lasts a long time.

Vaccines that contain live organisms include the following:

Did You Know...

  • Vaccines work by stimulating the body’s natural defense mechanisms (the immune system).

Passive immunization

In passive immunization, antibodies against a specific infectious organism or the toxin produced by an organism are given directly to a person.

Antibodies for passive immunization are obtained from several sources:

  • The blood 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 regardless of whether or not they have had a disease (called pooled human immune globulin)

  • People known to have higher blood levels of antibodies to a particular disease because they have received a vaccine for it or are recovering from the disease (called hyperimmune globulin)

  • 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 person 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 pregnant person.

Passive immunization develops immediately but 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 also injected into a vein (intravenously) or under the skin (subcutaneously). One type of influenza vaccine is sprayed into the nose. The mpox vaccine may be administered directly into the skin (intradermally).

More than one vaccine may be given at a time. For example, some vaccines are combined into one or separate injections of different vaccines can be given at different injection sites. Many childhood vaccines are recommended for simultaneous administration, particularly in the first year of life. In general, vaccines are designed to be given according to recommended schedules that optimize their efficacy.

Some vaccines are given routinely to adults based on a recommended schedule (see Centers for Disease Control and Prevention [CDC]: Recommended Immunizations for Adults Aged 19 Years and Older, United States, 2025), and some vaccines are routinely given to children (see also CDC: Child and Adolescent Immunization Schedule by Age).

Other vaccines are not routinely given but may be given 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 (substances that the immune system can recognize and can respond to). 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: Flu Vaccines and People with Egg Allergies.) 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 given to or should be delayed in people who:

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 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: Immunization 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, the percentage of adults who have received specific vaccines remains low for most vaccines. In addition, the percentage differs by race and ethnicity with generally lower percentages among Black and Hispanic adults compared with White adults.

Table
Table

Vaccine Safety and Hesitancy

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 spectrum disorder.

Many different groups of scientists have studied these concerns and have completely disproved the supposed relationship between vaccines and autism spectrum disorder (see Childhood Vaccination Concerns and CDC: About Common Vaccine Safety Questions and Concerns).

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 (see CDC: 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.

More Information

The following English-language resources may be useful. Please note that The Manual is not responsible for the content of these resources.

  1. Children's Hospital of Philadelphia: Vaccine Education Center

  2. European Centre for Disease Prevention and Control (ECDC): Vaccine schedules in all countries in the EU/EEA

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