VAERS Data Explorer

Accessing VAERS Data

The CDC WONDER system provides public access to VAERS data through an interactive online interface. This guide walks you through accessing, filtering, and interpreting the data responsibly.

Step 1: Accessing CDC WONDER

Navigate to VAERS Data (CDC WONDER). The system provides two types of data summaries:

Request Reports: Individual case details with symptoms, demographics, and outcomes
Summary Tables: Aggregated counts by category (vaccine, manufacturer, symptom, outcome)

Step 2: Filtering Your Query

Key filters available in CDC WONDER include:

Vaccine Type: Filter by specific vaccines (e.g., COVID-19, influenza, HPV)
Date Range: Select report submission date or event date
Age Group: Filter by demographic categories
Serious vs. Non-Serious: Focus on reports meeting FDA serious criteria
Outcome: Filter by death, hospitalization, life-threatening illness, or recovery

Important Limitation

VAERS data cannot determine if a vaccine caused an adverse event. Reports alone cannot establish causality, and raw counts must be interpreted in context of vaccine exposure.

Understanding MedDRA Codes

VAERS uses the Medical Dictionary for Regulatory Activities (MedDRA) to code reported symptoms. MedDRA is a highly granular system with over 80,000 terms organized in a hierarchical structure:

System Organ Class (SOC): Highest level (e.g., "Cardiac disorders")
Preferred Term (PT): Specific medical term (e.g., "Myocarditis")
Lower Level Term (LLT): Synonyms and sub-terms

When searching VAERS, be aware that searching for a broad term (e.g., "cardiac") will return more results than a specific term (e.g., "myocarditis").

Background Rate Comparison

To assess whether reported events represent true vaccine-associated risks, analysts compare VAERS counts to expected "background rates" — the incidence of events in the unvaccinated population. Key considerations:

Background rates vary by age, sex, and geographic region
Conditions with low background rates are easier to detect as signals
Conditions that are common (e.g., headache, fatigue) will have many coincidental reports
Published epidemiological studies provide more reliable risk estimates than raw VAERS counts

Documented Limitations

Underreporting

Not all adverse events are reported. The completeness of reporting varies by severity and public awareness.

Causality Cannot Be Determined

Reports indicate temporal association only. Events may be coincidental or have alternative causes.

Data Quality Varies

Reports contain unverified information submitted by diverse reporters with varying medical expertise.

No Denominator Data

Raw counts don't account for number of vaccine doses administered, making interpretation difficult.

Best Practices for Analysis

Use peer-reviewed studies for risk estimates rather than raw VAERS counts
Compare to background rates when assessing whether reported events exceed expected levels
Consider biological plausibility and known mechanisms of vaccine-related events
Look for signals across multiple data sources (VAERS, VSD, published literature)
Consult FDA and CDC interpretations for established causal relationships