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 ARI baseline metric to describe levels (categories) of ED visits:

Acute Respiratory Illnesses (ARI) captures a broad range of diagnoses from ED visits for respiratory illness symptoms, and therefore includes severe infections like COVID-19, flu, and RSV disease, and other respiratory illnesses. The percentage of ED visits associated with ARI, are classified into levels (categories): Very Low, Low, Moderate, High, and Very High based on regional and state trends. For additional information on NSSP and ARI methodology, visit Technical Resources | National Syndromic Surveillance Program (NSSP) | CDC

To set a baseline, CDC identified weeks with the lowest respiratory illness activity in each of the 10 Health and Human Services Regions. These "lowest activity" weeks are based on lab test positivity data for COVID-19, flu, and RSV and serve as the reference point for measuring activity levels. Lowest activity was defined as:

• Flu: fewer than 2% of tests are positive
• COVID-19 and RSV: fewer than 1% of tests are positive
• ARI overall: weeks when COVID-19, flu, and RSV all fell below their individual thresholds above.

Each season runs from Week 27 through Week 26 of the following year to capture summer COVID-19 activity. For each of the 10 Health and Human Services Regions separately, we identified the weeks with the lowest activity for respiratory illnesses using clinical and public health laboratory test positivity data for COVID-19, flu, and RSV from NREVSS during recent seasons (2022-2023 and 2023-2024, defined as MMWR Week 27 through Week 26). These lowest activity weeks serve as the reference periods.

For each state within a region, we used the reference periods to calculate the values used to create the levels (categories). First, we calculated the average weekly percentage of ED visits attributed to each pathogen and ARI overall within the identified lowest activity periods.

We found the mean and standard deviation of the weekly percentage of ED visits attributed to each pathogen and ARI overall within the identified lowest activity periods. The mean is considered the baseline value for each state and is the top value for the minimal category. Then, we calculated thresholds above minimal category based on the mean and standard deviation.

Given the mean and standard deviation of the percent of ED visits in each state for each pathogen and ARI overall, we computed the maximum number of standard deviations above the mean for any observed week over all weeks in the two seasons (N = maximum number of standard deviations).

We divided the value this value by 4 and then multiplied by the standard deviation (S = standard deviation), and denoted this value 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:

• minimal: [ X ] <= [ mean ], activity at or below baseline
• low: [ mean ] < [ X ] <= [ mean + D ], activity within 1 standard deviation above baseline
• moderate: [ mean + D ] < [ X ] <= [ mean + 2D ], activity between 1 and 2 standard deviations above baseline
• high: [ mean + 2D ] < [ X ] <= [ mean + 3D ], activity between 2 and 3 standard deviations above baseline
• very high: [ mean + 3D ] < [ X ], activity more than 3 standard deviations above baseline

Hospital service area (HSA)

A Hospital Service Area (HSA) is a geographic region where most residents get their hospital care locally. CDC's National Center for Health Statistics (NCHS) created HSAs in 1996 to provide local health data while protecting patient privacy.

There are about 950 HSAs across the United States. They vary in size — an HSA may cover a single city, a county, or a larger region within a state. Areas with more people tend to have smaller HSAs, while rural areas may have larger ones.

HSA boundaries were later updated by the National Cancer Institute (NCI) to ensure no HSA crossed state lines, so all counties within an HSA fall within a single state. For more information, visit the NCI SEER dataset.

Display criteria for sub-state levels (categories)

Jurisdictions may elect not to display sub-state ED visit data for a variety of reasons. To ensure data accuracy, we do not display results for HSAs where fewer than 30% of hospitals participate in reporting. To ensure reliable results, HSA-level data is only displayed when there are enough total ED visits to support a stable calculation. For each state, a minimum number of total ED visits (M) is defined as M = 1/D, where D = maximum number of standard deviations above the mean for any observed week / 4. This value of M is determined separately for COVID-19, flu, RSV disease, and ARI overall for each state

To determine if an HSA has enough data to display an activity level, CDC examined ED visit counts for Weeks 1 through 26 of 2024. An HSA meets the threshold if its total ED visits equal or exceed the minimum value (M) in at least 80% of those weeks (21 out of 26).

If an HSA does not meet this threshold, the activity level is not displayed. However, the percentage of ED visits associated with COVID-19, flu, or RSV may still be shown even when the activity level is hidden.

Even if the level (category) is not displayed, the percentage of ED visits associated with COVID-19, flu, or RSV disease may still be displayed. With fewer ED visits associated with RSV disease than for COVID-19 or flu, it is possible for categories to display COVID-19 and flu while not showing RSV.

• 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. Flu: Clinical laboratory test results from NREVSS and flu collaborating laboratories; more details about influenza virologic surveillance are available.
  

Data is preliminary and may change as more reports are received. The most recent 3 weeks in gray are more likely to change because of delays in reporting. Data represents laboratory tests performed, not individual people. The data are reported across the country in all regions.

The percent of positive tests is calculated by dividing the number of positive tests by the total number of tests given, then multiplying by 100 [(# of positive tests/total tests) x 100].

RSV and COVID-19 are limited to nucleic acid amplification tests (NAATs), also listed as polymerase chain rection tests (PCR). Participating laboratories report weekly to CDC the total number of COVID-19, flu, and RSV tests performed that week and the number of those tests that were positive.

• COVID-19 laboratory data is available for download.
• RSV laboratory data are available for download.
• Flu laboratory data are available for download.

The map shows the Wastewater Viral Activity Level across US states and territories. Activity levels indicate how much virus is currently detected in the wastewater – ranging from very low to 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 healthcare provider or hospital. It can also detect infections in people without symptoms. For more information, see Data Methods.

Data representing less than 5% of the population has a hatching, or lined, pattern on the map, indicating the wastewater viral activity level may not show the full picture for that state or territory. States or territories without sufficient data are indicated as "No data". This can be because data was not reported from the state or territory, or the monitored sites do not have enough data to calculate the wastewater viral activity level.

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

Data limitations for flu A

Wastewater data cannot determine the source of flu 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 flu displayed in these visuals only detects flu A viruses and does not distinguish between flu A subtypes (seasonal flu A[H1N1] virus, seasonal flu A[H3N2] virus and avian flu A[H5] virus).

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 and South Dakota. for additional information, you can read Companion Guide: NSSP Emergency Department Data on Respiratory Illness

Source: The Center for Forecasting and Outbreak Analytics: Current Epidemic Trends (Based on Rt) 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 ED visits reported through the National Syndromic Surveillance Program (NSSP).

The map's color coding reflects whether infections are likely to increase or decrease:

• 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 EDs, for example 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 extreme or unexpected values that appear unreliable are not included.

R_t is not estimated for states in the following cases:

• Jurisdictions with low ED facility participation or where diagnosis information is incomplete
• Fewer than 10 ED visits for the disease were reported in the prior 2 weeks
• Anomalies were detected 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