Overview of Immunization

There are two types of immunization:

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 Use of several vaccines at the same time Despite the strong vaccine safety systems in place in the United States, some parents remain concerned about the use and schedule of vaccines in children. These concerns can lead some parents... read more ).

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 Routine Vaccinations for Infants, Children, and Adolescents ).

Other vaccines are usually given mainly to specific groups of people. For example, the yellow fever vaccine Yellow Fever Yellow fever is a mosquito-borne viral disease that occurs mainly in the tropics. Yellow fever occurs only in the tropical areas of Central Africa, southern Panama, and South America. Some people... read more 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 Prevention Rabies is a viral infection of the brain that is transmitted by animals and that causes inflammation of the brain and spinal cord. Once the virus reaches the spinal cord and brain, rabies is... read more may be given to a person who has been bitten by a dog.

For many vaccines, the only reason for not being vaccinated is

Egg allergy is common in the United States. Some vaccines, including most influenza vaccines Influenza Vaccine The influenza virus vaccine helps protect against influenza. Two types of influenza virus, type A and type B, regularly cause seasonal epidemics of influenza in the United States. There are... read more , 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 Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more ) 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. 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

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.

Children typically are given a number of vaccines according to a standard schedule (see figure Routine Vaccinations for Infants, Children, and Adolescents Routine Vaccinations for Infants, Children, and Adolescents and see Centers for Disease Control and Prevention: Recommended Child and Adolescent Immunization Schedule for ages 18 years or younger, United States, 2021). If vaccines are missed, most can be given later, according to a catch-up schedule.

Adults may also be advised to receive certain vaccines (see also Centers for Disease Control and Prevention: Recommended Adult Immunization Schedule for ages 19 years or older, United States, 2021). When advising adults about vaccination, a doctor considers the person’s age, health history, childhood vaccinations, occupation, geographic location, travel plans, and other factors.

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 Vaccine Safety Vaccination protects children against many infectious diseases. Vaccines contain either noninfectious components of bacteria or viruses or whole forms of these organisms that have been weakened... read more 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 Childhood Vaccination Concerns Despite the strong vaccine safety systems in place in the United States, some parents remain concerned about the use and schedule of vaccines in children. These concerns can lead some parents... read more 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).

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 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.

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.