Merck Manual

Please confirm that you are a health care professional

honeypot link

Childhood Vaccination


Michael J. Smith

, MD, MSCE, Duke University School of Medicine

Reviewed/Revised Jul 2023

Childhood Vaccination Schedules

In the United States, vaccination of children and adolescents follows a schedule recommended by the Advisory Committee on Immunization Practices (ACIP) of the Centers for Disease Control and Prevention (CDC):

The schedules also are available as a free mobile app for health care providers. Additionally, clinicians should consult the relevant vaccine-specific Advisory Committee on Immunization Practices (ACIP) recommendations.

Vaccine Efficacy and Safety

Vaccination has been profoundly effective in preventing serious disease. Given their modest cost (particularly in comparison to medications that must be taken long-term), vaccines are one of the most cost-effective medical interventions. Vaccines have been so effective that many health care professionals currently in practice have seen few or no cases of diseases that were once extremely common and often fatal.

Because the diseases that vaccines prevent have typically become rare in the United States and because vaccines are given to otherwise healthy children, vaccines must have a high safety profile to be acceptable to patients and caregivers.

Before licensure, vaccines (like any medical product) are tested in randomized controlled trials (RCTs) that compare the new vaccine to placebo (or a previously existing vaccine if one exists). Such pre-licensing RCTs are designed primarily to assess vaccine efficacy and to identify common adverse events (eg, fever; local reactions such as injection site redness, swelling, and pain). However, some adverse events occur too rarely to be detected in an RCT of any practical size and may not appear until after a vaccine enters routine use. Thus, two surveillance systems, the Vaccine Adverse Event Reporting System (VAERS) and the Vaccine Safety Datalink (VSD), were created to monitor vaccine safety post-licensure.

VAERS is a safety program cosponsored by the U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (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 from all across the country 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 13 sites that provide clinical, methodological, and data expertise; 11 are data-providing sites (1 Reference Vaccination has been extremely effective in preventing serious disease and in improving health worldwide. Because of vaccines, infections that were once very common and/or fatal (eg, smallpox... read more ). 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 Vaccine Hesitancy Despite the rigorous vaccine safety systems in place in the United States, some parents remain concerned about the safety of the use and schedule of vaccines in children. These concerns have... read more ). 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.


  • 1. CDC: Vaccine Safety Datalink (VSD). Accessed June 28, 2023.

More Information

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

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!