CDC WONDER Database: What It Contains and How to Use It

What CDC WONDER actually is

CDC WONDER (Wide-ranging ONline Data for Epidemiologic Research) is the CDC's public query system for accessing national health statistics. It covers mortality, natality, population estimates, environmental health, and infectious disease data.

Unlike openFDA's REST API, WONDER is primarily a web-based query tool. The interface is dated (think 1990s form submissions), but the underlying data is some of the most authoritative public health information available in the United States.

Key datasets available

Dataset	Coverage	Updated
Underlying Cause of Death	1999-2024	Annually
Multiple Cause of Death	1999-2024	Annually
Natality (birth data)	2016-2023	Annually
Infant Deaths	2007-2022	Annually
Population Estimates	1990-2023	Annually
Sexually Transmitted Infections	2000-2022	Annually
Vaccine Adverse Events (VAERS)	1990-present	Weekly
COVID-19 Provisional Deaths	2020-present	Weekly

Mortality data: the flagship dataset

The most-used WONDER dataset is the mortality file. It provides counts and rates of deaths by:

• Cause of death (ICD-10 codes)
• Demographics (age, sex, race, ethnicity)
• Geography (state, county, census region)
• Year and month

For example, you can query: "How many opioid-related deaths occurred in Ohio in 2023, broken down by age group?" WONDER answers that directly.

How to query WONDER

The primary interface is web-based at wonder.cdc.gov. For each dataset, you:

1. Select the dataset from the main menu
2. Set grouping variables (how you want results organized)
3. Set filter criteria (location, time period, demographics)
4. Choose specific cause-of-death codes if applicable
5. Submit and receive a results table

Results can be exported as tab-delimited text files for analysis.

Programmatic access limitations

WONDER was built before REST APIs became standard. Programmatic access is limited:

• No official REST API for most datasets
• SODA API exists for some CDC datasets at data.cdc.gov (separate system)
• Bulk downloads available for some datasets in compressed text format
• Automated queries possible through HTTP POST to the WONDER forms, but fragile

The CDC SODA (Socrata Open Data Access) endpoint at data.cdc.gov provides API access to some datasets that overlap with WONDER, but not all of them.

SODA API for CDC data

For datasets available through data.cdc.gov, the SODA API is the developer-friendly option:

# Weekly COVID-19 death counts by state
curl "https://data.cdc.gov/resource/r8kw-7aab.json?$limit=10&$where=state='California'"

SODA supports filtering, pagination, aggregation, and JSON/CSV output formats. Rate limits are 1,000 requests per hour without a token, higher with a free app token.

Use cases for health data developers

Drug overdose surveillance: WONDER mortality data coded with ICD-10 codes T36-T50 (poisoning by drugs) enables time-series analysis of overdose trends by substance, geography, and demographics.

Maternal mortality research: Natality and mortality files combined allow analysis of maternal death rates by hospital type, insurance status, and geographic access to care.

Vaccine safety monitoring: VAERS data in WONDER complements the openFDA VAERS endpoint, providing a different query interface over the same underlying data.

Environmental health: The environmental dataset links health outcomes to specific geographic exposures, useful for health equity research.

Data quality considerations

WONDER data has specific limitations to account for:

• Suppressed cells: Any count between 1-9 is suppressed to protect privacy. This makes county-level analysis difficult for rare conditions.
• Provisional data: Recent years may show lower counts because death certificates take time to process. The CDC flags which years are provisional.
• Coding changes: ICD-10 replaced ICD-9 in 1999. Trend analysis across this boundary requires crosswalk tables.
• Race/ethnicity misclassification: Death certificates rely on funeral director observation, leading to undercounting for American Indian/Alaska Native and Asian populations.

FAQ

Is CDC WONDER free to use? Completely free, no registration required. The data is public domain.

How current is the mortality data? Typically 12-18 months behind. The 2024 data usually becomes available in mid-2026. Provisional data is available faster for COVID-19 and drug overdose deaths.

Can I get individual-level records? No. WONDER only provides aggregate counts and rates. Individual death certificate data requires a research agreement with NCHS.

What is the difference between WONDER and data.cdc.gov? WONDER is the older query system with more historical data. data.cdc.gov is the newer open data portal with API access. Some datasets appear in both, but the collections are not identical.

How do I cite WONDER data? The CDC provides a suggested citation on each query result page. Include the dataset name, query parameters, and date accessed.