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Respiratory Virus Activity Levels

About

Provides an update on how COVID-19, influenza, and RSV may be spreading nationally and in your state.

Summary

Level of Respiratory Illness Activity

Respiratory illness activity is monitored using the acute respiratory illness (ARI) metric. ARI captures a broad range of diagnoses from emergency department visits for respiratory illnesses, from the common cold to severe infections like influenza, RSV and COVID-19. It captures illnesses that may not present with fever, offering a more complete picture than the previous influenza-like illness (ILI) metric. Refer to data notes for more details.

Wastewater Map

Wastewater (sewage) can be tested to detect traces of infectious diseases circulating in a community, even if people don't have symptoms. You can use these data as an early warning that levels of infections may be increasing or decreasing in your community. COVID-19 wastewater trends may differ from some health outcome findings, such as hospitalization trends, as COVID-19 is causing severe disease less frequently than earlier in the pandemic.

Influenza levels are for Influenza A only, which includes the avian influenza A(H5). Wastewater data cannot determine the source of viruses (from humans, animals, or animal products). Refer to data notes for more details, including important considerations for interpreting wastewater findings.

* Level based on a small segment (less than 5%) of the population and may not be representative of the state/territory. Read more »

Emergency Department Visits for Viral Respiratory Illness

Weekly percent of total emergency department visits associated with COVID-19, influenza, and RSV. Refer to data notes for more details.

Epidemic Trends

CDC uses data from emergency department visits to model epidemic trends. This model helps tell whether the number of new respiratory infections is growing or declining in your state. While this model tells us the trend, it does not tell us the actual number of current infections of SARS-CoV2, the virus that causes COVID-19 or influenza virus. Refer to data notes for more details.

Percent of Tests Positive for Respiratory Viruses

Weekly percent of tests positive for the viruses that cause COVID-19, influenza, and RSV at the national level. Preliminary data are shaded in gray. Refer to data notes for more details.

Data Notes

  • 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 (Minimal, 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:

ARI captures a broad range of diagnoses from emergency department visits for respiratory illnesses, from the common cold to severe infections like influenza, RSV and COVID-19. The ARI baseline metric classifies the percentage of ED visits due to ARI, influenza, RSV, and COVID-19 into levels (categories): Minimal, Low, Moderate, High, and Very High based on regional, state, and local trends.

  • 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, influenza, 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.
    • Lowest activity for influenza: identified as weeks with <2% of attributed percentage of positive tests.
    • Lowest activity for COVID-19 and RSV: identified as weeks with <1% of attributed percentage of positive tests.
    • Week 27 through Week 26 was used to define a season in order to capture summer COVID-19 activity as the start of a given season.
    • Lowest activity weeks for ARI were identified as weeks where all of COVID-19, influenza, and RSV were identified as having a low activity according to their separate thresholds.
  • 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 (HSAs): HSAs were created by the National Center for Health Statistics (NCHS) in CDC in 1996. An HSA is an area that is relatively self-contained with respect to hospital care as a way to balance providing local data while preserving privacy. There are approximately 950 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 to cover a broader geographical area. The National Institutes of Health National Cancer Institute (NCI) Modified SEER Registry updated the HSA designations so that any HSA that crossed state or 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.
    • 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 (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/influenza, RSV, and ARI overall for each state.
    • To assess if an HSA meets the criteria for classification based on sufficient data, the weeks within Weeks 1 through 26 of Q1-Q2 2024 were considered. If the total number of ED visits in an HSA during these weeks is equal to or greater than the corresponding M value, it indicates sufficient data. However, if the total number of weeks meeting this minimum count falls below 80% (21 out of 26 weeks), the level (category) was not displayed for that HSA. Even if the level (category) is not displayed, the percentage of ED visits for COVID-19, flu, or RSV may still be displayed.
    • With fewer ED visits for RSV than for COVID-19 or flu, it is possible for sub-state levels (categories) to be displayed for COVID-19 and flu while being suppressed for RSV.

  • 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 minimal, 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: The Center for Forecasting and Outbreak Analytics: Current Epidemic Trends (Based on Rt) for States.
  • The epidemic trend is based on Rt, the time-varying reproductive number. Rt measures disease transmission. If Rt is above 1, the number of infections is growing because, on average, each infected person is causing more than one new infection; if Rt is below 1, it indicates that infections are declining.
  • Rt 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 Rt 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 Rt 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.
  • Rt is not estimated for states in the following cases:
    • Jurisdictions with low emergency department facility participation or where diagnosis information is incomplete;
    • Fewer than 10 emergency department visits for the disease were reported in either of the prior 2 weeks;
    • 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 U.S. World Health Organization 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 preliminary 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 RSV tests performed that week and the number of those tests that were positive. The RSV trend graphs display the national average of the weekly % test positivity for the current, previous, and following weeks in accordance with the recommendations for assessing RSV trends by percent ( https://academic.oup.com/jid/article/216/3/345/3860464).
  • 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

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Content Source:
National Center for Immunization and Respiratory Diseases (NCIRD)