Respiratory Virus Activity Levels

• Source: National Syndromic Surveillance Program: https://www.cdc.gov/nssp/index.html.
• The acute respiratory illness (ARI) metric categorizes the percentage of emergency department (ED) visits into five activity levels (Very Low, Low, Moderate, High, and Very High), helping people understand the extent of respiratory illness activity in an area. For a list of diagnostic codes for the ARI metric visit: Acute Respiratory Illness Technical Brief.

Methodology for using acute respiratory illness (ARI) as a surveillance metric to describe levels (categories) of emergency department (ED) visits:

Acute respiratory illness (ARI) is a non-specific syndrome that captures a broad range of diagnoses from emergency department visits for respiratory illnesses, ranging from the common cold to infections that can be severe including COVID-19, influenza, and respiratory syncytial virus (RSV). An ARI-related metric classifies the percentage of ED visits due to ARI into 5 levels (categories): Very Low, Low, Moderate, High, and Very High at the regional, state, and local levels.

• For each of the 10 U.S. Department of Health and Human Services Regions separately, we identified the weeks with the lowest activity for respiratory illnesses. This was done using clinical and public health laboratory test positivity data for COVID-19 and RSV from National Respiratory and Enteric Virus Surveillance System (NREVSS), and public health laboratory data from FluView, excluding H5, for recent surveillance years (2022-2023, 2023-2024, and 2024-2025 defined as epidemiologic Week 27 through Week 26 of the following calendar year). The lowest activity weeks serve as the reference periods.
  • Lowest activity for COVID-19 and RSV: identified as weeks in a given year when, for at least two consecutive weeks, <1% of tests for COVID-19 and RSV are positive.
  • Lowest activity for influenza: identified as weeks in a given year when, for at least two consecutive weeks, <2% of tests for influenza in those weeks are positive.
  • Week 27 through Week 26 of the following calendar year was used to define a surveillance year in order to capture summer COVID-19 activity as the start of a given season.
  • Lowest activity weeks for ARI were identified as weeks when COVID-19, influenza, and RSV were all identified as having low activity according to their separate thresholds.

• For each state within a region, the reference periods were used to calculate the values that created the levels (categories) for the 2025-2026 surveillance year.
  

1. First, we calculated the weekly percentage of ED visits attributed to each pathogen and ARI overall within the identified lowest activity periods.
2. Next, we calculated the mean, M, and standard deviation, S, attributed to each pathogen and ARI overall within the identified lowest activity periods. The mean was considered the baseline value for each state and was used as the top value for the Very Low category.
3. Then, we computed the maximum number of standard deviations above the mean for any observed week over all weeks in the three previous seasons (N = maximum number of standard deviations = ( - M) / S).
4. The maximum number of standard deviations (N) was divided by 4 and multiplied by the standard deviation (S = standard deviation). This value was denoted as D, so D = (N / 4) * S.
   

• Using X = percent of ED visits in a week for each pathogen and ARI overall, we classified activity levels as follows:
  • Very Low: X <= M, activity at or below baseline
  • Low: M < X <= M + D
  • Moderate: M + D < X <= M + 2D
  • High: M + 2D < X <= + 3D
  • Very High: M + 3D < X

• Missouri: Due to data quality issues, Missouri's historical data before MMWR Week 10 of 2025 (March 2-8, 2025) could not be used for computing levels, thus leaving insufficient data. To address this, data from all of HHS Region 7 (Missouri, Iowa, Kansas, and Nebraska) were used to compute the 2025-2026 levels for Missouri, excluding Missouri's data prior to 2025 Week 10 in the regional aggregation.

• Hospital Service Area (HSAs): HSAs were created by CDC's National Center for Health Statistics (NCHS) in 1996. An HSA is an area that is relatively self-contained with respect to hospital care. There are 949 HSAs. HSAs can vary in size, encompassing a single city, a county or a larger region within a state. Areas with higher population concentrations may have smaller HSAs to ensure adequate healthcare access, while sparsely populated regions might have larger HSAs that cover a broader geographical area. The National Institutes of Health National Cancer Institute (NCI) modified the HSA designations so that any HSA that crossed state or Surveillance, Epidemiology, and End Results (SEER) Registry boundaries was split so that all counties from one HSA were in one state and/or SEER Registry. The NCI SEER dataset can be accessed here: Health Service Areas (HSA) - SEER Datasets (cancer.gov)

• Display criteria for sub-state levels (categories):
  • Jurisdictions may elect not to display sub-state ED visit data for a variety of reasons.
  • Data from HSAs with fewer than 30% of hospitals participating in NSSP are suppressed to ensure a minimum level of representativeness.
  • Sub-state display of levels (categories) is further limited to ensure that calculations are stable and based on having a sufficient number of total ED visits. For each state, a minimum number of total ED visits (V) is defined as V = 1/D, where D = maximum number of standard deviations above the mean for any observed week divided by 4 (see above). This value of V is determined separately for COVID-19, influenza, RSV, and ARI overall for each state.
  • To assess if an HSA meets the criteria for classification based on sufficient data, data weeks from Week 1 to Week 26 of 2025 were considered. If the total number of ED visits in an HSA during these weeks was equal to or greater than the corresponding V value, it indicated sufficient data. However, if the total number of weeks meeting this minimum count fell below 80% (21 out of 26 weeks), the level (category) was not displayed for that HSA in any week. The percentage of ED visits for COVID-19, influenza, or RSV may still be displayed even if the level (category) is not displayed.

• The map shows the Wastewater Viral Activity Level for states and territories in the United States. This metric indicates whether the amount of virus in the wastewater for specific respiratory illnesses is very low, low, moderate, high, or very high. Wastewater monitoring can detect viruses spreading from one person to another within a community earlier than clinical testing, and before people who are sick go to their doctor or hospital. It can also detect infections without symptoms. If you see increased Wastewater Viral Activity Levels of COVID-19, influenza virus, or RSV, it might indicate that there is a higher risk of infection. For more information, see Data Methods.

• Data representing less than 5% of the population have a hatching pattern on the map, which is a note of caution that the wastewater viral activity level shown may not be representative of the state or territory. States or territories without sufficient data are indicated as "No data". This can be due to no data reported from the state or territory, or all sites not having the minimum weeks of data available to calculate the wastewater viral activity level.

Data Limitations for Influenza A:

• Wastewater data cannot determine the source of influenza A viruses. Detections could come from a human or from an animal (like a bird) or an animal product (like milk from an infected cow).
• Wastewater testing for influenza displayed in these visuals only detects influenza A viruses and does not distinguish between influenza A subtypes (e.g. seasonal influenza A(H1N1) virus, seasonal influenza A(H3N2) virus and avian influenza A(H5) virus).

Data Limitations for RSV:

• Wastewater data will not include waste that may be disposed of outside the wastewater system, such as from people wearing diapers.

For more data limitations, see Data Methods.

• Source: National Syndromic Surveillance Program: https://www.cdc.gov/nssp/index.html
• There are no data available for the following states/territories: Guam, Missouri, and South Dakota. Combined is the sum of COVID-19, influenza, and respiratory syncytial virus (RSV) emergency department visits.
• Additional information available at: Companion Guide: NSSP Emergency Department Data on Respiratory Illness

• Source: The Center for Forecasting and Outbreak Analytics: Current Epidemic Trends (Based on R_t) for States.
• The epidemic trend is based on R_t, the time-varying reproductive number. R_t measures disease transmission. If R_t is above 1, the number of infections is growing because, on average, each infected person is causing more than one new infection; if R_t is below 1, it indicates that infections are declining.
• R_t is estimated using data on Emergency Department visits reported through the National Syndromic Surveillance Program (NSSP).
• The map's color coding reflects whether infections are likely increasing or decreasing:
  • Dark purple: Infections are growing
  • Purple: Infections are likely growing
  • Gray: Infections are likely not changing
  • Green: Infections are likely declining
  • Dark green: Infections are declining
• The accuracy of R_t can be temporarily affected by changes in how many people with infections go to emergency departments (i.e. if a new variant or strain emerges that causes more severe disease) and by changes in testing or reporting practices.
• The data used to estimate R_t are updated frequently, and initially reported counts might later be revised. The data are manually reviewed each week and occasionally implausible outlier values are excluded.
• R_t is not estimated for states in the following cases:
  • Jurisdictions with low emergency department facility participation or where diagnosis information is incomplete;
  • In each of the prior 2 weeks, fewer than 10 (for COVID-19 and Influenza) or 5 (for RSV) emergency department visits were reported;
  • There were detected anomalies in reported values;
  • The model did not pass checks for reliability.
• For additional methods: Behind the Model: CDC's Tools to Assess Epidemic Trends | CFA: Behind the Model | CDC

• Sources: COVID-19 and RSV: National Respiratory and Enteric Virus Surveillance System (NREVSS), a sentinel network of laboratories located through the US, includes clinical, public health and commercial laboratories; additional information available at: https://www.cdc.gov/surveillance/nrevss/index.html. Influenza: Clinical laboratory test results from NREVSS and influenza collaborating laboratories; more details about influenza virologic surveillance are available here: https://www.cdc.gov/flu/weekly/overview.htm.
• Data for recent weeks in gray may be incomplete due to delays in reporting. These data are preliminary and may change as more data become available.
• Data represent laboratory tests performed, not individual people.
• The data are from across the country in all regions.
• The percent of tests positive is calculated by dividing the number of positive tests by the total number of tests administered, then multiplying by 100 [(# of positive tests/total tests) x 100].
• COVID-19: The condition caused by infection with severe acute respiratory syndromic coronavirus type-2 (SARS-CoV-2).
• RSV and COVID-19 are limited to nucleic acid amplification tests (NAATs), also listed as polymerase chain reaction tests (PCR).
• Participating laboratories report weekly to CDC the total number of SARS-CoV-2 and RSV tests performed that week and the number of those tests that were positive.
• COVID-19 laboratory data are available for download here: https://data.cdc.gov/Laboratory-Surveillance/Percent-Positivity-of-COVID-19-Nucleic-Acid-Amplif/gvsb-yw6g
• RSV laboratory data are available for download here: https://data.cdc.gov/Laboratory-Surveillance/Percent-Positivity-of-Respiratory-Syncytial-Virus-/3cxc-4k8q