Vaccination has been profoundly effective in preventing serious disease. Given their modest cost (particularly in comparison to drugs that must be taken long-term), vaccines are one of the most cost-effective medications. Vaccines have been so effective that many health care practitioners 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 US 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 is used to collect reports from individual patients who believe that they had an adverse event after a recent vaccination. Health care practitioners 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 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 9 large managed care organizations representing more than 9 million people. 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.
For specific adverse effects of specific vaccines, see Overview of Immunization Overview of Immunization Immunity can be achieved Actively by using antigens (eg, vaccines, toxoids) Passively by using antibodies (eg, immune globulins, antitoxins) A toxoid is a bacterial toxin that has been modified... read more .
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.
CDC, FDA, and agencies of the U.S. Department of Health and Human Services (HHS): Vaccine Adverse Event Reporting System (VAERS)
CDC and the FDA: Vaccine Safety Datalink (VSD)