Effectiveness and safety of childhood vaccination

[Jessica]

Vaccination has been profoundly effective in preventing serious disease (see Table: Case Rates of Some Diseases Preventable by Vaccines). Given their modest cost (particularly in comparison to drugs that must be taken long-term), vaccines are one of the most cost-effective pharmaceutical products. 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 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.
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 FDA and the CDC; VAERS collects 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 originate 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 9 large managed care organizations (MCOs) 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.