What the Vaccine Adverse Event Reporting System is, what it isn't, and how to interpret its data correctly
The Vaccine Adverse Event Reporting System (VAERS) is one of the most frequently cited — and most frequently misunderstood — sources of vaccine safety data. It is a passive surveillance system co-managed by the CDC and FDA that collects reports of adverse events occurring after vaccination.
Understanding what VAERS is designed to do, and what it cannot tell us, is essential for interpreting any data drawn from it correctly.
VAERS is designed to detect potential safety signals — new or unexpected patterns of adverse events that warrant further investigation. It is not designed to prove causation.
Reports are submitted voluntarily by patients, caregivers, healthcare providers, and manufacturers. Unlike active surveillance, VAERS does not seek out cases — it receives them.
All VAERS data is publicly available and searchable at vaers.hhs.gov. This transparency is a deliberate feature enabling independent researchers to analyze trends.
Healthcare providers and vaccine manufacturers are required by law to report certain adverse events to VAERS. Patients and caregivers may also submit reports voluntarily.
Anyone can submit a VAERS report — patients, parents, caregivers, healthcare providers, and vaccine manufacturers. Healthcare providers are legally required to report:
Epidemiologists look for unexpected patterns or clusters of events that occur more frequently than background rates would predict. This triggers formal investigation, not automatic conclusions.
VAERS data generates hypotheses tested using active surveillance systems like the Vaccine Safety Datalink (VSD) and the BEST System, which can establish causation.
Tracking changes in report rates over time can reveal emerging signals — for example, an unusual increase in a specific event type following a new vaccine rollout.
Peer-reviewed researchers use VAERS as one data source among many, always with appropriate methodological controls and caveats about its limitations.
VAERS captures only a fraction of actual adverse events. A frequently cited study estimated that fewer than 1% of adverse events are reported to passive surveillance systems (Lazarus et al., 2010). This means:
A critical limitation of VAERS data is that a report of an adverse event following vaccination does not establish that the vaccine caused the event. Establishing causality requires a structured scientific framework. The most widely used is the Bradford Hill Criteria, developed by epidemiologist Sir Austin Bradford Hill in 1965 — the same methodology used to prove the link between cigarette smoking and lung cancer.
The nine viewpoints scientists use to assess whether association equals causation:
How large is the statistical association? A stronger association is more likely to be causal.
Has the association been observed repeatedly by different researchers, in different populations and settings?
Is the association specific to a particular exposure and outcome, with no other plausible explanation?
Did the exposure precede the outcome? The vaccine must come before the adverse event. This is the only non-negotiable criterion.
Is there a dose-response relationship? Does higher exposure produce stronger effects?
Is there a biologically plausible mechanism that could explain the association?
Does the association fit with what we know about the natural history of the disease?
Does removing the exposure reduce the outcome? For example, does withdrawal of a vaccine reduce the adverse event rate?
Are there similar, established cause-and-effect relationships that support the proposed causal link?
"None of these nine viewpoints can bring indisputable evidence for or against causation. What they can do is help determine whether there is any other explanation equally or more likely than cause and effect." — Sir Austin Bradford Hill
Applied to vaccines: Regulatory agencies and independent researchers apply these criteria — alongside tools like the WHO Global Advisory Committee on Vaccine Safety (GACVS) causality assessment framework — when evaluating whether a reported adverse event is causally linked to a vaccine. This is why scientific consensus on vaccine causality is based on the totality of evidence, not individual VAERS reports.
Evidence Label: Established Scientific MethodologySee also: Known Adverse Events | Global Reporting Systems
CDC. "VAERS — Vaccine Adverse Event Reporting System." Centers for Disease Control and Prevention. https://www.cdc.gov/vaccinesafety/ensuringsafety/monitoring/vaers/index.html
FDA. "Vaccine Adverse Event Reporting System (VAERS)." U.S. Food and Drug Administration. https://www.fda.gov/vaccines-blood-biologics/vaccine-adverse-events
Shimabukuro TT, et al. "Safety monitoring in the Vaccine Adverse Event Reporting System (VAERS)." Vaccine. 2015;33(36):4398-4405. https://doi.org/10.1016/j.vaccine.2015.07.035
Lazarus R, et al. "Electronic Support for Public Health–Vaccine Adverse Event Reporting System (ESP:VAERS)." Agency for Healthcare Research and Quality. 2010. https://digital.ahrq.gov/sites/default/files/docs/publication/r18hs017045-lazarus-final-report-2011.pdf
Iskander JK, et al. "The role of VAERS in monitoring vaccine safety." Pediatric Annals. 2004;33(9):599-606. https://doi.org/10.3928/0090-4481-20040901-10