Overview of Immunization

Vaccines should be given exactly as recommended on the package insert. Noncompliance or other factors may result in changes in vaccine schedule for individual patients; for most vaccines, the interval between a series of doses may be lengthened without losing efficacy. If a vaccine series (eg, for hepatitis B or human papillomavirus) is interrupted, clinicians should give the next recommended dose the next time the patient presents, provided that the recommended interval between doses has passed. They should not restart the series (ie, with dose 1).

Injection vaccines are usually given intramuscularly into the midlateral thigh (in infants and toddlers) or into the deltoid muscle (in school-aged children and adults). Some vaccines are given subcutaneously. For details on vaccine administration, see the General Best Practice Guidelines for Vaccine Administration from the Advisory Committee for Immunization Practices (ACIP), Administering Vaccines to Adults from the Immunization Action Coalition, and CDC: Administer the Vaccine(s).

For patients with upper extremity lymphedema (eg, breast cancer patients), best practice is to use the other arm or an alternate location.

Shoulder injury related to vaccine administration (SIRVA) may be caused by the unintentional injection of a vaccine into tissues and structures under the deltoid muscle of the shoulder (1).

Clinicians should have a process in place to ensure that patient vaccination status is reviewed at each visit so that vaccines are given as per recommendations. Patients (or caregivers) should be encouraged to keep a history (written or electronic) of their vaccinations and share this information with new health care professionals and institutions to make sure that vaccinations are up to date.

Restrictions and precautions are conditions that increase the risk of an adverse reaction to a vaccine or that compromise the ability of a vaccine to produce immunity. These conditions are usually temporary, meaning the vaccine can be given later. Sometimes vaccination is indicated when a precaution exists because the protective effects of the vaccine outweigh the risk of an adverse reaction to the vaccine.

Contraindications are conditions that increase the risk of a serious adverse reaction. A vaccine should not be given when a contraindication is present.

In the United States, the safety of vaccines is ensured through two surveillance systems: the CDC's and the U.S. Food and Drug Administration's (FDA) Vaccine Adverse Event Reporting System (VAERS) and the Vaccine Safety Datalink (VSD).

VAERS is a safety program cosponsored by the FDA and the CDC; VAERS collects reports from individual patients who believe that they had an adverse event after a recent vaccination. Health care professionals are also required to report certain events after vaccination and may report events even if they are unsure the events are vaccine-related. VAERS reports originate all across the United States and provide a rapid assessment of potential safety issues. However, VAERS reports can show only temporal associations between vaccination and the suspected adverse event; they do not prove causation. Thus, VAERS reports must be further evaluated using other methods. One such method uses the VSD, which uses data from 11 large health care organizations. The data include vaccine administration (noted in the medical record as part of routine care), as well as subsequent medical history, including adverse events. Unlike VAERS, the VSD includes data from patients who have not received a given vaccine as well as those who have. As a result, the VSD can help distinguish actual adverse events from symptoms and disorders that occurred coincidentally after vaccination and thus determine the actual incidence of adverse events.

Nonetheless, many parents remain concerned about the safety of childhood vaccines and their possible adverse effects (particularly autism). These concerns have led some parents to not allow their children to be given some or all of the recommended vaccines (see Vaccine Hesitancy). As a result, outbreaks of diseases made uncommon by vaccination (eg, measles, pertussis) are becoming more common among unvaccinated children in North America and Europe.

One of the main parental concerns is that vaccines may increase the risk of autism. Reasons cited include

• A possible connection between the combination measles-mumps-rubella vaccine and autism (see MMR vaccine and autism)

• The possibility that thimerosal might cause autism (thimerosal is a mercury-based preservative used in some vaccines—see Thimerosal and autism)

• Use of multiple, simultaneous vaccines, given as recommended

In 1998, a brief report in The Lancet postulated a link between the measles virus in the MMR vaccine and autism. This report received significant media attention worldwide, and many parents began to doubt the safety of the MMR vaccine. However, The Lancet retracted the report because it contained serious scientific flaws; many subsequent, large studies have found no evidence to support an association between use of vaccines and risk of autism (1). The US Institute of Medicine Immunization Safety Review Committee reviewed epidemiologic studies (published and unpublished) to determine whether the measles-mumps-rubella vaccine and vaccines containing thimerosal cause autism and to identify possible biologic mechanisms for such an effect; based on the evidence, this group rejected a causal relationship between these vaccines and autism (2).

Some of the concerns regarding autism and childhood vaccines were regarding vaccine constituents. Thimerosal, a preservative, was the focus of some of these concerns, although the MMR vaccine never contained thimerosal. Currently, virtually every vaccine given to children in the United States is thimerosal-free. Small amounts of thimerosal continue to be used in multidose vials of influenza vaccine and in several vaccines intended for use in adults. For information about vaccines that contain low levels of thimerosal, see the Food and Drug Administration's web site (Thimerosal and Vaccines). Thimerosal is also in many vaccines used in resource-poor countries.

As with any treatment, clinicians should talk to their patients about the relative risks and benefits of recommended vaccines (3). In particular, clinicians must make sure that the parents of their patients are aware of the possible serious effects (including death) of vaccine-preventable childhood diseases such as measles, Hib infection, and pertussis, and clinicians should discuss any concerns parents may have about vaccinating their children. Resources for these discussions include the CDC's Talking with Parents about Vaccines for Infants and Vaccine Resources for Parents.

Immunizations may be required for travel to areas where infectious diseases are endemic (see table Vaccines for International Travel). The CDC can provide this information; a telephone service (1-800-232-4636 [CDC-INFO]) and web site (Travelers' Health) are available 24 hours/day.

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

1. Centers for Disease Control and Prevention: Current immunization schedules

2. Advisory Committee on Immunization Practices (ACIP): Vaccine-Specific Recommendations

3. ACIP: Recommended Adult Immunization Schedule, United States, 2024

4. ACIP: General Best Practice Guidelines for Vaccine Administration

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

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