How do vaccine adverse event reporting systems (like VAERS) work and why they cannot by themselves prove causation?
Executive summary
Vaccine adverse event reporting systems such as VAERS are national, passive surveillance tools designed to collect reports of health problems that occur after vaccination so experts can spot unusual patterns or “signals” that merit further study [1]. These systems are essential for early warning but, by design and limitation, cannot on their own prove that a vaccine caused a reported event—establishing causation requires follow-up epidemiologic, clinical and sometimes laboratory investigation [2].
1. What the system is and what it’s for
VAERS is the U.S. national post‑marketing surveillance system co‑managed by CDC and FDA to accept reports of possible adverse events after any licensed vaccine and to act as an early warning mechanism for rare or unexpected safety issues [1]. It exists because pre‑licensure trials cannot detect extremely rare events or events that only appear when millions are vaccinated, so passive reporting fills an ongoing surveillance need [3].
2. How reports get into VAERS and how data are processed
Anyone—patients, parents, clinicians, manufacturers—can submit a VAERS report, and federal law requires certain serious events be reported by providers and manufacturers, while the public-facing database makes initial reports available for analysis and transparency [4] [5]. CDC and FDA analyze incoming reports using statistical methods to detect unusual reporting patterns, and when a potential signal is found they follow up using other systems such as the Vaccine Safety Datalink (VSD), Clinical Immunization Safety Assessment (CISA), or FDA’s BEST system to validate and investigate [4] [1].
3. Strengths: why VAERS matters
As a hypothesis‑generating tool, VAERS can reveal new or rare adverse events that were not apparent in trials, prompting faster investigation and regulatory action when necessary; historical examples of signal detection using passive surveillance underline its role in the safety net [6] [3]. Its openness and ease of reporting broaden the net of surveillance and let public health agencies aggregate vast quantities of post‑licensure data that would otherwise be invisible [7].
4. Limitations: why VAERS cannot prove causation
VAERS is a passive system subject to underreporting, unverified or incomplete reports, inconsistent data quality, lack of dependable denominator data (the number vaccinated), and reporting biases—including spikes driven by media attention—so raw counts cannot be interpreted as evidence of causation, frequency, or severity without further study [8] [4] [7]. Because reports document temporal associations (something happened after vaccination) rather than proven causal links, each report may include multiple vaccines or unrelated health events, and VAERS does not adjudicate causality for routine entries [9] [6].
5. How causation is properly established after a signal
When VAERS flags a signal, agencies perform case reviews and turn to systems with linked medical records and denominators—like VSD—for epidemiologic studies that estimate relative risk, control for confounders, and establish statistical association; laboratory or clinical evidence and biological plausibility are additional steps required to move from association to a causal inference [2] [4]. Congressional, academic and CDC reviews consistently note that “without rigorous further study, it is not possible to determine if the vaccine caused the adverse health event” and that confirmation typically requires controlled epidemiologic or other studies [2] [3].
6. Why VAERS data get misused and the agendas behind it
Because VAERS data are public, raw and searchable, they are frequently cherry‑picked or taken out of context to suggest causation; anti‑vaccine actors exploit inherent limitations—unverified reports, duplicates, lack of denominators—to amplify fear, while public health authorities emphasize cautious interpretation and the need for corroborating studies [8] [4]. Media cycles and advocacy groups can push attention toward particular reports, which inflates reporting and complicates interpretation; CDC and FDA explicitly warn that report counts alone cannot establish safety problems [4] [1].
7. Bottom line
VAERS is indispensable as an early warning and transparency tool but is only the first step in a multi‑stage safety assessment: it raises hypotheses, not conclusions, and must be followed by rigorous case review, controlled epidemiologic studies, and clinical or laboratory evidence before causation can be assigned [1] [2] [3]. Where reporting systems succeed is in directing scarce investigative resources to signals that warrant those deeper, definitive inquiries [4].