⚠️

Important: VAERS reports alone cannot determine if a vaccine caused an adverse event. Reports may contain incomplete, inaccurate, or unverified information. Correlation does not equal causation.

VAERS Glossary

Plain-language definitions of vaccine safety terms and VAERS concepts. Understanding the terminology is essential for interpreting VAERS data correctly.

A B C D E G H L M O P R S T U V

A

Adverse Event (AE): Any health problem that occurs after vaccination, regardless of whether the vaccine caused it. An adverse event is a temporal association — something that happened after vaccination — not proof of causation.
Active Surveillance: A system that proactively monitors a defined population for health events. The Vaccine Safety Datalink (VSD) is an example. Active surveillance produces more reliable data than passive systems like VAERS.
Anaphylaxis: A severe, potentially life-threatening allergic reaction that occurs rapidly after exposure. Vaccination sites are equipped to treat anaphylaxis, and it typically resolves with epinephrine treatment.
ACIP (Advisory Committee on Immunization Practices): A group of medical and public health experts that advises the CDC on the use of vaccines in the U.S. population. ACIP reviews safety and effectiveness data — including signals detected through VAERS — and makes recommendations on who should receive which vaccines and when.

B

Background Rate: The rate at which a health event occurs in the general population regardless of vaccination. For example, heart attacks occur daily in the U.S. Some will happen shortly after vaccination purely by coincidence.

C

Causation vs Correlation: Correlation means two things happen around the same time. Causation means one thing actually caused the other. VAERS shows correlation only — a report filed after vaccination does not prove the vaccine caused the event. Learn more →
Clinical Information Statement (CIS): Required information that must be given to patients before vaccination, including known risks and side effects.
Causality: The determination that one thing actually caused another. Establishing causality between a vaccine and an adverse event requires controlled studies, biological plausibility, consistency across data sources, and evidence that the event occurs more often in vaccinated than unvaccinated people. A single VAERS report cannot establish causality.
Contraindication: A specific situation in which a vaccine should not be given because the risk outweighs the benefit — for example, a severe allergic reaction to a previous dose or a vaccine component. Contraindications are identified during clinical trials and refined through ongoing safety monitoring.
Confidence Interval: A range of values, derived from data, that is likely to contain the true value of a measure such as a risk estimate. Wider intervals mean more uncertainty. Confidence intervals help distinguish a real effect from statistical noise in vaccine safety studies.

D

Denominator Problem: VAERS counts reports (numerator) but doesn't track how many people were vaccinated (denominator). Without knowing the denominator, raw report counts are meaningless for calculating risk. Over 670 million COVID-19 doses were administered in the U.S. Learn more →
Dose Series: Which dose in a vaccination series a report refers to (1st dose, 2nd dose, booster, etc.). Different doses may have different side effect profiles. Learn more →
Denominator: The number of people (or doses) at risk — the bottom of a rate calculation. VAERS provides only numerators (report counts) and no denominator, which is why raw VAERS counts cannot be turned into rates or used to compare risk between vaccines. Learn more →

E

EUA (Emergency Use Authorization): A mechanism the FDA uses to allow the use of a medical product during a public health emergency before full approval, based on the best available evidence. The COVID-19 vaccines were first made available under EUA before receiving full licensure.
Epidemiology: The study of how diseases and health-related events are distributed in populations and what factors influence them. Epidemiological studies — comparing outcomes in vaccinated and unvaccinated groups — are what actually determine whether a vaccine causes a given adverse event, something VAERS alone cannot do.

G

Guillain-Barré Syndrome (GBS): A rare neurological disorder where the immune system attacks nerve cells. Has been associated with some vaccines at very low rates. Most patients recover fully. Learn more →

H

Healthcare Provider Report: Reports filed by doctors, nurses, pharmacists, or other medical professionals. These tend to be more clinically detailed than patient-filed reports. Learn more →

L

Lot Number: A manufacturer-assigned identifier for a specific batch of vaccines. In VAERS, lot numbers can be used to track reports by batch, but comparing raw counts between lots is misleading because lot sizes and distribution vary enormously. Learn more →
Life-Threatening Event: A VAERS outcome category indicating the reporter believed the person was at immediate risk of death from the adverse event at the time it occurred. Like other VAERS outcome flags, it reflects the reporter's assessment and does not confirm the vaccine as the cause.

M

MedDRA: Medical Dictionary for Regulatory Activities — the standardized medical terminology used in VAERS to code symptoms and diagnoses. Each symptom in VAERS is coded using MedDRA Preferred Terms.
Myocarditis: Inflammation of the heart muscle. Identified as a rare side effect of mRNA COVID-19 vaccines, primarily in young males after the second dose. Most cases are mild and resolve with treatment. Learn more →
Manufacturer Report: Reports filed by vaccine manufacturers. These are required by FDA regulations and often come from post-marketing surveillance programs. Learn more →

O

Onset Interval: The time between vaccination and the start of the adverse event (NUMDAYS field in VAERS). Helps distinguish likely vaccine reactions (typically 0-3 days) from coincidental events. Learn more →

P

Passive Surveillance: A system that relies on voluntary reports rather than actively seeking cases. VAERS is a passive system, meaning it depends on people choosing to file reports. This means VAERS may undercount events (underreporting) or include unrelated events. Learn more →
Post-Marketing Surveillance: The monitoring of vaccine safety after a vaccine has been approved and is being used in the general population. VAERS is one of several post-marketing surveillance tools.
PRR (Proportional Reporting Ratio): A statistical measure used in pharmacovigilance to detect disproportionate reporting. It compares how often a specific adverse event is reported for one vaccine versus all other vaccines. A high PRR flags an event for further review but, on its own, does not prove the vaccine caused it.
Pharmacovigilance: The science and activities involved in detecting, assessing, understanding, and preventing adverse effects of medical products, including vaccines. VAERS is a pharmacovigilance tool used for early signal detection.

R

Recovery Status: Whether the person recovered from the adverse event. In VAERS: Y (recovered), N (not recovered), or U (unknown). "Not recovered" often means symptoms were ongoing at time of report, not necessarily permanent. Learn more →
Reactogenicity: The expected, generally short-lived reactions to a vaccine that reflect the immune system responding — such as sore arm, fever, fatigue, and headache. Reactogenicity is a normal part of vaccination and is distinct from rare, serious adverse events.

S

Serious Adverse Event (SAE): An adverse event that results in death, a life-threatening condition, hospitalization, prolonged hospitalization, permanent disability, or a congenital anomaly/birth defect.
Signal Detection: The process of identifying potential safety concerns from surveillance data. VAERS is designed for signal detection — finding patterns that warrant further investigation — not for determining if a vaccine caused a specific event.
Stimulated Reporting: An increase in reports driven by media attention, public awareness campaigns, or regulatory requirements rather than an actual increase in adverse events. COVID-19 had massive stimulated reporting. Learn more →
Seroconversion: The point at which a person develops detectable antibodies in their blood in response to a vaccine or infection. Seroconversion is a measure of immune response and is used to evaluate whether a vaccine is generating the intended protection.

T

Thrombosis with Thrombocytopenia Syndrome (TTS): A rare condition involving blood clots and low platelet counts. Was identified as a very rare side effect of the J&J/Janssen COVID-19 vaccine.
Temporal Association: The observation that two events happened close together in time — for example, an adverse event occurring shortly after vaccination. VAERS captures temporal associations. A temporal association is a starting point for investigation, not proof that the vaccine caused the event.

U

Underreporting: The phenomenon where not all adverse events get reported to VAERS. Studies suggest VAERS captures only a fraction of events — estimates vary from 1% to 10% depending on the severity and type of event. Learn more →

V

VAERS: The Vaccine Adverse Event Reporting System — a passive surveillance system co-managed by the CDC and FDA that collects reports of adverse events following vaccination. Anyone can submit a report. Learn more →
VAERS ID: A unique identifier assigned to each report in VAERS. Can be used to look up individual reports in the VAERS database.
V-safe: A smartphone-based active surveillance program launched by the CDC for COVID-19 vaccines. Unlike VAERS, V-safe actively checks in with vaccinated individuals to collect health data.
Vaccine Safety Datalink (VSD): A collaboration between the CDC and several large healthcare organizations that uses electronic health records for active vaccine safety surveillance. Produces more reliable data than VAERS.

How to Use This Glossary

This glossary is designed to help you navigate VaccineWatch and interpret VAERS data correctly. Key concepts to understand before exploring the data:

Start Here

If you're new to VAERS, read the definitions for Adverse Event, Passive Surveillance, Causation vs Correlation, and Denominator Problem. These four concepts are essential for understanding everything else on this site.

For Deeper Analysis

Terms like PRR, Stimulated Reporting, Confidence Interval, and Epidemiology help you understand how researchers analyze VAERS data and why raw counts can be misleading. Many terms link to full analysis articles with charts and examples.

Explore More