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Advisory Guide to the Interpretation of VAERS Data

When evaluating data from VAERS, it is important to note that for any reported event, no cause and effect relationship has been established. VAERS is interested in all potential associations between vaccines and adverse events. Therefore, VAERS collects data on any adverse event following vaccination, be it coincidental or truly caused by a vaccine. The report of an adverse event to VAERS is not documentation that a vaccine caused the event.

VAERS data contains coincidental events and those truly caused by vaccines.

Over ten million vaccinations per year are given to children less than one year old, usually between 2 months and 6 months of age. At this age, infants are at greatest risk for certain medical events, including high fevers, seizures, and sudden infant death syndrome (SIDS). Some infants will by coincidence experience such an event shortly after a vaccination.

These coincidences make it difficult to know whether a particular adverse event resulted from a concurrent condition or from a vaccination. Therefore, doctors and other vaccine providers are encouraged to report all adverse events following vaccinations, whether or not they believe that the vaccination was the cause.

VAERS data have a number of limitations you should remember:

The coding of VAERS reports also requires careful interpretation: The purpose of VAERS is to detect possible signals of adverse events associated with vaccines. Additional scientific investigations are almost always required to properly validate signals from VAERS and establish a cause and effect relationship between a vaccine and an adverse event. For example, potential concerns raised by VAERS are investigated through a CDC project called the Vaccine Safety Datalink (VSD). VSD is a large-linked database and includes information on more than six million people, allowing for planned vaccine safety studies as well as timely investigations of hypotheses.


Source:  http://vaers.hhs.gov/info.htm accessed on April 25, 2006.
This page last reviewed: Friday, June 26, 2009
This information is provided as technical reference material. Please contact us at cwus@cdc.gov to request a simple text version of this document.