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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:

  • VAERS data are derived from a passive surveillance system and represent unverified reports of health events, both minor and serious, that occur after vaccination.
  • In certain cases VAERS requests additional information from reporters, healthcare providers and other parties. The status, event description and medical codes associated with a report are subject to change upon receipt of such additional information.
  • When multiple reports of a single case or event are received, only the first report received is included in the publicly accessible dataset. Subsequent reports may contain additional or conflicting data, and there is no assurance that the data provided in the public dataset is the most accurate or current available.
  • Such data are subject to limitations of under-reporting, simultaneous administration of multiple vaccine antigens (making it difficult to know to which of the vaccines, if any, the event might be attributed), reporting bias, and lack of incidence rates in unvaccinated comparison groups.
  • While some events reported to VAERS are truly caused by vaccines, others may be related to an underlying disease or condition, to medications being taken concurrently, or may occur by chance.
  • VAERS occasionally receives case reports from US manufacturers that were reported to their foreign subsidiaries. Under FDA regulations, if a manufacturer is notified of a foreign case report that describes an event that is both serious and unexpected (in other words, it does not appear in the product labeling), they are required to submit it to VAERS. It is important to realize that these case reports are of variable data quality and completeness, due to the many differences in country reporting practices and surveillance system quality. For this reason they are provided as separate files.
  • In some media reports and on some web sites on the Internet, VAERS reports are presented as verified cases of vaccine deaths and injuries. Statements such as these misrepresent the nature of the VAERS surveillance system.

The coding of VAERS reports also requires careful interpretation.

  • All narrative text taken from VAERS reports are coded and entered into the VAERS database using the Medical Dictionary for Regulatory Activities (MedDRA), which are key words representing medical condition(s) described in the case report.

Establishing causal relationships between vaccines and adverse events requires additional scientific investigation.

  • The CDC and FDA take into account the complex factors mentioned above, and others, when monitoring vaccine safety and analyzing VAERS reports.
  • 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.
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For additional information, please call VAERS toll-free 1-800-822-7967 or e-mail

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1 Passive Surveillance - Passive surveillance systems rely on health professionals or vaccinees to voluntarily submit reports of illness following vaccination. Limitations of passive surveillance systems include variability in reporting standards, reporter bias and significant under-reporting of events.

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