NNDSS

The National Notifiable Diseases Surveillance System (NNDSS) is a nationwide public health surveillance collaboration managed by the Centers for Disease Control and Prevention (CDC) in partnership with the Council of State and Territorial Epidemiologists (CSTE). NNDSS coordinates the voluntary transmission of standardized case notifications for nationally notifiable conditions from 50 state health departments, territories, and freely associated jurisdictions to the federal level. In epidemiology and population health research, NNDSS provides the foundational data infrastructure used to monitor disease incidence trends, detect multi-jurisdictional outbreaks, and publish weekly provisional surveillance counts.

What It Is and Who Runs It

NNDSS operates under the auspices of the CDC’s National Center for Emerging and Zoonotic Infectious Diseases (NCEZID) and the Center for Surveillance, Epidemiology, and Laboratory Services. It is not an autonomous federal data collection program that bypasses local authorities; rather, it is a shared federal-state data exchange system that integrates decentralized public health reporting into a unified national picture.

The operational framework of NNDSS relies on a close constitutional partnership between state jurisdictions and the federal government. While states hold sovereign police powers under the Tenth Amendment to mandate disease reporting within their borders, participation in federal NNDSS notification is voluntary. The Council of State and Territorial Epidemiologists (CSTE)—a professional body representing state public health epidemiologists—works with CDC scientists to evaluate emerging pathogens and establish consensus on which conditions warrant national monitoring.

Jurisdictional Eligibility and Data Access Governance

Participation in NNDSS and access to its data products follow strict jurisdictional eligibility standards and governance protocols designed to protect patient privacy and support scientific investigation:

• Reporting Jurisdictions: Primary data providers are the 50 state public health agencies, the District of Columbia, five U.S. territories (American Samoa, Guam, Northern Mariana Islands, Puerto Rico, U.S. Virgin Islands), and three freely associated states (Federated States of Micronesia, Republic of the Marshall Islands, Republic of Palau).
• Public Research Access: De-identified, aggregated case counts are publicly accessible through open data channels. Researchers can download provisional weekly tables via the CDC Open Data portal (catalog ID x9gk-5huc) and query finalized annual surveillance summaries through CDC WONDER (Wide-ranging ONline Data for Epidemiologic Research).
• Restricted Research Data: Investigators requiring microdata with granular geographic, demographic, or clinical variables must submit formal research protocols and Data Use Agreements (DUAs) through CDC program offices, subject to jurisdictional approval and Institutional Review Board (IRB) review.

Surveillance Scope and Case Definitions

NNDSS tracks approximately 120 infectious diseases and non-infectious conditions designated as nationally notifiable. These conditions include acute infections (such as measles, meningococcal disease, and pertussis), foodborne illnesses (salmonellosis, shiga toxin-producing Escherichia coli), vector-borne pathogens (Lyme disease, West Nile virus), viral hepatitis, sexually transmitted infections, and occupational conditions (such as elevated blood lead levels and silicosis).

To ensure epidemiological validity across disparate legal jurisdictions, every notifiable condition requires a standardized surveillance case definition. Surveillance case definitions are not diagnostic tools intended for clinical decision-making or individual patient care; rather, they provide uniform criteria that enable public health officials to classify and enumerate cases consistently across boundaries. Updated annually through CSTE Position Statements, case definitions establish three primary classification tiers:

• Confirmed Case: Meets specific laboratory criteria (e.g., pathogen culture, molecular detection by PCR, or diagnostic genomic sequencing) or combined clinical and epidemiological linkage criteria.
• Probable Case: Meets presumptive laboratory evidence and supportive clinical criteria without definitive diagnostic confirmation.
• Suspect Case: Meets initial clinical presentation criteria suggestive of infection during active outbreak investigations or pending laboratory verification.

Data Notification Architecture and Transmission Steps

Case data move from local clinical encounters to federal surveillance repositories through structured sequential stages:

1. Local Clinical and Laboratory Reporting: Under state statutory authority, healthcare providers, diagnostic laboratories, and hospital infection preventionists are legally required to report designated reportable conditions to their local or state health department. Modern hospitals increasingly utilize automated electronic Case Reporting (eCR) systems embedded within electronic health records (EHRs).
2. Jurisdictional Case Verification: State and local epidemiologists review clinical records, contact tracing data, and laboratory confirmations to verify that incoming reports meet CSTE surveillance case definitions.
3. Standardized HL7 Message Encoding: Verified cases are extracted from state surveillance databases and encoded according to Health Level Seven International (HL7) standards. Under the NNDSS Modernization Initiative (NMI), jurisdictions transition legacy flat files to HL7 Message Mapping Guides (MMGs), which capture rich clinical, epidemiological, and vaccination details.
4. Federal Ingestion and Quality Validation: Encrypted HL7 messages are transmitted securely to CDC through the National Electronic Disease Surveillance System (NEDSS) base system or equivalent jurisdictional platforms. CDC automated pipelines perform syntax verification, deduplication, and anomaly detection.
5. Provisional and Final Dissemination: CDC publishes weekly provisional tables structured by MMWR weeks on the CDC Open Data portal. At the close of each epidemiological reporting year, state health departments reconcile data discrepancies, allowing CDC to publish definitive, reconciled annual summaries.

Typical Timeline Phases

Surveillance data flow through predictable operational cycles that research teams must account for when designing studies in epidemiology and public health:

• Immediate / Daily Phase (Acute Outbreaks): For high-priority conditions (such as botulism, anthrax, or novel influenza), local jurisdictions notify CDC within 24 hours of identification to coordinate immediate epidemic response.
• Weekly Cycle: Jurisdictions transmit case batches weekly to close out the preceding epidemiological week. CDC aggregates notifications and refreshes provisional data tables every Friday.
• Annual Reconciliation (12–18 Month Lag): Following the end of the calendar year, state epidemiologists audit provisional counts, resolve reporting delays, and cross-reference laboratory registries. Finalized annual surveillance summaries typically appear 12 to 18 months following the close of the surveillance year.

Common Mistakes and Data Interpretation Pitfalls

Epidemiologists and biostatisticians analyzing NNDSS records frequently encounter analytical traps that can bias population inferences:

• Confusing Reportable with Notifiable: A “reportable” condition is legally mandated by state law for local clinicians and labs, whereas a “notifiable” condition is voluntarily shared by states with the CDC. Not every state-reportable disease is nationally notifiable, and state reporting lists vary substantially.
• Ignoring Reporting Delays: Weekly provisional counts are incomplete due to testing backlogs and local investigation queues. Researchers must never interpret a drop in the latest 2–4 MMWR weeks as a genuine epidemiological decline.
• Equating Surveillance Definitions with Clinical Diagnosis: Because surveillance definitions prioritize specificity to ensure consistent tracking across jurisdictions, patients who do not meet strict laboratory thresholds for surveillance may still have active clinical disease requiring medical treatment.
• Failing to Account for Definition Changes: When CSTE updates a case definition (for example, integrating rapid antigen tests alongside PCR), apparent sudden jumps or drops in disease incidence often reflect altered classification criteria rather than biological shifts. Methodologists trained in biostatistics and epidemiology evaluate interrupted time-series models to control for these artifactual shifts.

Related Federal Surveillance Programs

NNDSS operates alongside several specialized surveillance systems managed by the federal government, each tailored to distinct epidemiological objectives:

Surveillance Program	Managing Federal Body	Data Collection Model	Primary Research Strength
NNDSS	CDC / CSTE	Case notifications from 50 states and territories using HL7 standards	National incidence trends, pathogen-specific case definitions, MMWR weekly tables
NSSP (National Syndromic Surveillance Program)	CDC	Real-time emergency department chief complaint and diagnostic logs	Early-warning outbreak signals before formal laboratory confirmation
NHSN (National Healthcare Safety Network)	CDC	Healthcare facility tracking of healthcare-associated infections (HAIs)	Hospital infection control, device-associated infections, antimicrobial use
NVSS (National Vital Statistics System)	CDC / NCHS	Death certificates and birth records from state vital registries	Final cause-specific mortality counts, excess death modeling, life expectancy

Frequently Asked Questions

What is the difference between reportable diseases and nationally notifiable diseases?

Reportable diseases are established by individual state laws and regulations, requiring healthcare providers, hospitals, and laboratories to report diagnosed cases to local public health authorities. Nationally notifiable diseases are conditions that states voluntarily agree to report to the CDC through NNDSS for nationwide tracking. The list of reportable diseases varies by state, whereas the nationally notifiable list is recommended collaboratively by CSTE.

How often are NNDSS surveillance case definitions updated?

Surveillance case definitions are reviewed and updated annually through formal Position Statements voted on by the Council of State and Territorial Epidemiologists (CSTE). Updates incorporate new diagnostic technologies, changing pathogen epidemiology, and revised public health priorities.

Can researchers download raw NNDSS patient records?

No. Publicly accessible NNDSS files on CDC Open Data and CDC WONDER provide aggregated counts and summary rates to prevent individual patient re-identification. Qualified researchers requiring detailed microdata must apply for restricted data access through CDC program offices and enter into formal Data Use Agreements.

How does NNDSS handle reporting delays and data revisions?

NNDSS publishes provisional data weekly on data.cdc.gov to provide rapid situational awareness. Because case investigations and laboratory confirmatory testing require time, provisional numbers are revised continuously throughout the year. Definitive counts are published only after a comprehensive annual reconciliation process between CDC and reporting states.

Sources

• CDC — Surveillance Case Definitions for Current and Historical Conditions — Official case definitions portal, CSTE position statement updates, and criteria guidelines. Accessed 2026-11-07.
• CDC Open Data Portal — NNDSS Weekly Data Catalog — Dataset specification (catalog ID x9gk-5huc), reporting areas, MMWR weekly schema, and provisional metrics. Accessed 2026-11-07.
• CDC WONDER — Wide-ranging ONline Data for Epidemiologic Research — Finalized annual NNDSS tables, public health query architecture, and population rate parameters. Accessed 2026-11-07.
• Council of State and Territorial Epidemiologists (CSTE) — Nationally Notifiable Conditions governance, annual committee voting procedures, and position statements. Accessed 2026-11-07.
• Morbidity and Mortality Weekly Report (MMWR) — CDC epidemiological dissemination channel, surveillance summaries, and case definition notices. Accessed 2026-11-07.