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Overview of Immunization

by William D. Surkis, MD, Jerome Santoro, MD

Immunization enables the body to better defend itself against diseases caused by certain bacteria or viruses. Immunity 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, measles, 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 Ebola virus infection, most sexually transmitted diseases (such as HIV, syphilis, gonorrhea, and chlamydial infections), 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 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. 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 nasal vaccine

  • Measles-mumps-rubella

  • Polio (only the oral vaccine)

  • Rotavirus

  • Typhoid (only the oral vaccine)

  • Shingles (zoster)

  • Yellow fever

Not all the vaccines that contain live organisms are available in the United States.

Did You Know...

  • Some vaccines contain a weakened but living form of the virus 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 days or 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: Vaccinating Infants and Children).

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

Some vaccines, including most influenza vaccines, contain very small amounts of material from eggs. Many people are allergic to eggs, but only those who have had severe allergic reactions to eggs should avoid the influenza vaccine. If people have had less severe reactions to eggs (such as a rash), doctors may give them influenza vaccine that contains inactivated virus, but not influenza vaccine that contains live virus.

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

  • A weakened immune system due to a disorder, such as AIDS, or to drugs that suppress the immune system (immunosuppressants), such as corticosteroids and chemotherapy drugs

  • Pregnancy

  • Some progressive nervous system disorders, such as Guillain-Barré syndrome

If people stop taking the drugs that suppress the 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: Vaccinating Infants and Children and Centers for Disease Control and Prevention: Immunization Schedules ). 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

Chickenpox (varicella)

Age 12–15 months

Diphtheria

Age 2 months

Haemophilus influenzae type b infections (such as meningitis)

Age 2 months

Hepatitis A

Age 12–23 months

Hepatitis B

Birth

Human papillomavirus

Age 11–12 years

Influenza

Age 6 months

Measles

Age 12–15 months

Meningococcal meningitis

Age 11–12 years

Age 2 months for children at high risk

Mumps

Age 12–15 months

Pertussis

Age 2 months

Pneumococcal infections

Age 2 months

Polio

Age 2 months

Rotavirus

Age 2 months

Rubella (German measles)

Age 12–15 months

Tetanus

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 Schedules 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.

Protecting Adults Through Vaccines

Disease*

Who Should Be Vaccinated

Anthrax

People who may be exposed to anthrax, such as the following:

  • Some military personnel

  • Some laboratory workers

Chickenpox (varicella)

All adults who have not had the vaccine or the disease

Diphtheria

All adults

  • As a combination vaccine with tetanus and pertussis if they have never received the combination

  • As a combination booster vaccine with tetanus every 10 years

Haemophilus influenzae type b infections (such as meningitis)

If not vaccinated during childhood, adults 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

Hepatitis A

Adults at increased risk, such as the following:

  • Travelers to areas where the disease is common

  • People who inject illegal drugs

  • Men who have sex with men

  • People who have a chronic liver disorder

  • People who are treated with blood clotting factors

Hepatitis B

Adults at increased risk, such as the following:

  • Health care workers

  • Travelers to areas where the disease is common

  • People with a chronic liver disorder

  • People with kidney failure, including those who need dialysis

  • People who inject illegal drugs

  • People who have several sex partners

  • 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

Human papillomavirus

All females aged 11–26 years

All males aged 11–21 years

All males aged 22–26 years if they have not been previously vaccinated and have sex with men or have HIV infection

Influenza

All people over age 6 months

Measles

All adults born in or after 1957 who have not had the infection or two doses of the vaccine

Health care workers if laboratory tests do not detect evidence of measles immunity

Always given as a combination vaccine with mumps and rubella (not available as a single vaccine)

Meningococcal meningitis

People at increased risk, such as the following:

  • People who do not have a functioning spleen

  • People with certain immunodeficiency disorders

  • Microbiologists who are routinely exposed to the bacteria

  • Adolescents entering high school if they have not already been vaccinated

  • All first-year college students who live in dormitories and are aged 21 or under if they have not already been vaccinated on or after their 16th birthday

  • All military recruits if they have not already been vaccinated

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

Mumps

All adults born in or after 1957 who have not had the infection or two doses of the vaccine

Health care workers if laboratory tests do not 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) if they have not already been vaccinated

Pregnant women

Pneumococcal infections (such as meningitis and pneumonia)

Adults at increased risk, such as the following:

  • People aged 65 and over

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

  • People with diabetes

  • People with a cerebrospinal fluid leak

  • People with a weakened immune system

  • People who do not have a functioning spleen

  • Alcoholics

  • Adults who smoke cigarettes

  • Residents of nursing homes or long-term care facilities

Polio

Adults at increased risk, such as travelers to areas where polio is common and laboratory workers who work with the virus

Rabies

People who have been bitten by or who are exposed to certain animals (such as veterinarians)

Rubella (German measles)

All adults born in or after 1957 who have not had the infection or two doses of the vaccine

Health care workers if laboratory test do not 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 60 and over

Smallpox

Not currently recommended except for laboratory workers who directly handle the virus and related materials

Tetanus

All adults (boosters every 10 years after the primary series, which is usually given during childhood as a combination vaccine with diphtheria and pertussis)

Tuberculosis (bacille Calmette-Guérin, or BCG)

Not used in the United States

Typhoid

People traveling to areas where the disease is common

Yellow fever

People traveling to certain parts of Africa and South America, where the disease is common

*Vaccines are available In the United States for these diseases.

HIV = human immunodeficiency virus.

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], Vaccine Safety Datalink [VSD]—see Childhood Vaccinations : Vaccine Safety). For additional information about the safety of individual vaccines, see Vaccine Safety at the CDC web site.

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 and possible adverse effects 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 Vaccinations : Vaccination Concerns in The M erck M anual 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 mercury or thimerosal is available at the Institute for Vaccine Safety web site.

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.

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