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Overview of Immunization

By

Margot L. Savoy

, MD, MPH, Lewis Katz School of Medicine at Temple University

Last full review/revision Oct 2020| Content last modified Oct 2020
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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, many tropical diseases (such as malaria), and newer infections, for example, coronavirus infections (such as COVID-19).

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). 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

Did You Know...

  • Some vaccines contain a weakened but living form of the microorganism that they protect against.

Passive immunization

In passive immunization, antibodies against a specific infectious 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).

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

  • 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 US. 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.

Vaccines that contain live organisms should not be used or should be delayed in people with certain conditions, such as

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 Centers for Disease Control and Prevention: Immunization Schedules). If vaccines are missed, most can be given later, according to a catch-up schedule.

Table
icon

Protecting Children Through Vaccines

Disease

When Do Vaccinations Typically Start

Chickenpox (varicella)

Age 12–15 months

Age 2 months

Age 2 months

Age 12–23 months

Birth

Age 11–12 years (can start at age 9 years)

Age 6 months

Age 12–15 months

Age 11–12 years

Age 2 to 9 months for children at high risk

Age 12–15 months

Age 2 months

Age 2 months

Age 2 months

Age 2 months

Rubella (German measles)

Age 12–15 months

Age 2 months

Common Vaccinations in Adults

Adults may also be advised to receive certain vaccines (see also Centers for Disease Control and Prevention: Immunization Schedule for Adults). When advising adults about vaccination, a doctor considers the person’s age, health history, childhood vaccinations, occupation, geographic location, travel plans, and other factors.

Table
icon

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:

  • Some military personnel

  • Some laboratory workers

  • Veterinarians

Chickenpox (varicella)

All adults who have not had the vaccine or chickenpox

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 who have not been vaccinated and who are at increased risk, such as the following:

  • People who do not have a functioning spleen

  • People who have a weakened immune system (such as those with AIDS)

  • People who have had chemotherapy for cancer

  • People who have had stem cell transplantation

Adults who have not been vaccinated and who are at increased risk, such as the following:

  • People who travel to or work in areas where the infection is common

  • People who use illegal drugs (such as methamphetamine)

  • Men who have sex with men

  • People who have a chronic liver disorder or high levels of certain liver enzymes in their blood

  • Healthy adults 40 years or under who have recently been exposed to hepatitis A virus

  • People who anticipate close contact with an adopted child during the first 60 days after the child arrives in the United States from an area where hepatitis A is common

  • People who are homeless

  • Pregnant women who are identified to be at risk of getting hepatitis A infection during pregnancy (such as women who are international travelers, who use illicit drugs [injected or not], who may be exposed at work, who anticipate close personal contact with an international adopted child, or who are homeless) or who are at risk of getting very sick or dying of hepatitis A virus infection (such as women who have chronic liver disease or HIV infection)

Adults who have not been vaccinated and who are at increased risk, such as the following:

  • Health care, custodial, or public safety workers

  • Travelers to areas where the infection is common

  • People with a chronic liver disorder or high levels of certain liver enzymes in their blood

  • People with kidney failure, including those who need dialysis

  • People who inject illegal drugs

  • People who have several sex partners

  • People who need to be evaluated or treated for a sexually transmitted disease

  • Men who have sex with men

  • Sex partners and household contacts of people known to be carriers of hepatitis B

  • People with HIV infection

  • People who are under 60 and have diabetes and sometimes people who are age 60 and older who have diabetes

  • People who have spent time (as patients, residents, or employees) in correctional facilities or in facilities that provide services for people at high risk of hepatitis B (such as drug-abuse treatment centers, sites for injection drug use, hemodialysis centers, institutions for developmentally disabled people, and places where people with sexually transmitted diseases or HIV infection are tested and treated)

  • Pregnant women if they are at risk of getting the infection or of getting very sick or dying of hepatitis B infection (such as women who have chronic liver disease or HIV infection)

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

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:

  • People who do not have a functioning spleen (including sickle cell disease)

  • People with HIV infection

  • People with certain immunodeficiency disorders

  • People who take eculizumab or ravulizumab (drugs that block the complement system)

  • Microbiologists who are routinely exposed to the bacteria

  • Adolescents if they have not already been vaccinated

  • First-year college students who live in dormitories who are 21 years old or younger and who have not been given a dose of the vaccine on or after their 16th birthday

  • Military recruits

  • Travelers to or residents of areas where the infection is common

  • People who have been exposed during a meningitis outbreak

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)

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 aged 65 years and over

Adults at increased risk, such as the following:

  • People with a chronic heart, lung (including asthma and emphysema), kidney, or liver disorder

  • People with diabetes

  • People with a cerebrospinal fluid leak

  • People with a weakened immune system (including those with HIV infection, leukemia, lymphoma, or cancer, those who take drugs that suppress the immune system [immunosuppressants], and those who have had certain organ transplants)

  • People who do not have a functioning spleen (including those with sickle cell disease)

  • People who have a cochlear implant

  • People who have an alcohol use disorder

  • Adults who smoke cigarettes

Adults at increased risk, such as

  • Travelers to areas where polio is common

  • Laboratory workers who work with the polio virus

  • People who treat people who may have polio

People who have been bitten by certain animals

People who may be at increased risk of exposure to infected animals, such as

  • Veterinarians and animal handlers

  • Laboratory workers who handle animals that may be rabid

  • People who explore bat caves

  • People who live or stay more than 30 days in developing countries where rabies in dogs is widespread

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)

Shingles (herpes zoster)

People aged 50 and over

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

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.

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

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.

  • Centers for Disease Control and Prevention (CDC): Child and adolescent immunization schedule for ages 18 years or younger

  • CDC: Adult immunization schedule for ages 19 years and older

  • CDC: FAQs about vaccine safety for parents and caregivers

  • 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

Drugs Mentioned In This Article

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DESOXYN
ADRENALIN
SOLIRIS
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