FluView Summary ending on April 3, 2021

Viruses

Illness

Severe Disease

All data are preliminary and may change as more reports are received.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

U.S. Virologic Surveillance:

Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza) are used to monitor whether influenza activity is increasing or decreasing.

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Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local health laboratories using Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines and to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of influenza viruses to antiviral medications, including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

Virus characterization data will be updated later this season when a sufficient number of specimens have been tested.

Outpatient Illness Surveillance

Please note, the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for influenza-like illness (ILI), not laboratory-confirmed influenza, and will capture visits due to other respiratory pathogens, such as SARS-CoV-2, that present with similar symptoms. In addition, healthcare-seeking behaviors have changed dramatically during the COVID-19 pandemic. Many people are accessing the healthcare system in alternative settings, which may or may not be captured as a part of ILINet. Therefore, ILI data, including ILI activity levels, should be interpreted with extreme caution. It is particularly important at this time to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a complete and accurate picture of both influenza and COVID-19 activity. CDC is tracking the COVID-19 pandemic in a weekly publication called COVID Data Tracker Weekly Review.

ILINet

Nationwide during week 13, 1.0% of patient visits reported through ILINet were due to ILI. This percentage is below the national baseline of 2.6%.

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During week 13, compared with week 12, the percentage of visits for ILI increased for one region (Region 8) and remained stable (change of ≤ 0.1%) in the remaining nine regions. All regions reported percentages of outpatient visits for ILI below their region-specific baselines.

ILI Visits by Age Group

About 65% of ILINet providers provide both the number of patient visits for ILI and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for ILI by age group. The percentages of visits for ILI reported in ILINet in week 13 increased for one age group (0–4 years) and remained stable (change of ≤0.1%) for the remaining age groups (5–24 years, 25–49 years, 50–64 years, and 65 years and older) compared with week 12. There has been an increasing trend among persons aged 0-4 years, while the remaining age groups (5–24 years, 25–49 years, 50–64 years, and 65 years and older) have experienced a stable or decreasing trend.

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ILI Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map

Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists

The geographic spread of influenza as reported by state and territorial epidemiologists indicates geographic spread of influenza viruses but does not measure the severity of influenza activity. Due to the impact of COVID-19 on ILI surveillance, and the fact that the state and territorial epidemiologists report relies heavily on ILI activity, reporting for this system will be suspended for the 2020-21 influenza season. Data from previous seasons is available on FluView Interactive.

Additional geographic spread surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Influenza-Associated Hospitalizations:

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population.

Between October 1, 2020, and April 3, 2021, FluSurv-NET sites in 14 states reported 215 laboratory confirmed influenza hospitalizations for an overall cumulative hospitalization rate of 0.7 per 100,000 population. This is much lower than average for this point in the season and lower than rates for any season since routine data collection began in 2005, including the low severity 2011-12 season. The current rate is one-eighth the rate at this time during the 2011-12 season. Hospitalization rates stratified by age will be presented if case counts increase to a level that produces stable rates by age.

Additional hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods | FluView Interactive: Rates by Age or Patient Characteristics

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on April 8, 2021, 11.4% of the deaths that occurred during the week ending April 3, 2021 (week 13), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 7.0% for week 13. Among the 2,141 PIC deaths reported for this week (week 13), 1,399 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and six listed influenza, indicating that current PIC mortality is due primarily to COVID-19 and not influenza.

The data presented are preliminary and may change as more data are received and processed.

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Additional pneumonia and influenza mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Influenza-Associated Pediatric Mortality

No influenza-associated pediatric deaths were reported to CDC during week 13.

One influenza-associated pediatric death occurring during the 2020-2021 season has been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.

World Health Organization:
Additional influenza surveillance information from participating WHO member nations is available through
FluNetexternal icon and the Global Epidemiology Reports.external icon

WHO Collaborating Centers for Influenza:
Australiaexternal icon, Chinaexternal icon, Japanexternal icon, the United Kingdomexternal icon, and the United States (CDC in Atlanta, Georgia)

Europe:
The most up-to-date influenza information from Europe is available from WHO/Europe and the European Centre for Disease Prevention and Controlexternal icon.

Public Health Agency of Canada:
The most up-to-date influenza information from Canada is available in Canada’s weekly FluWatch reportexternal icon.

Public Health England:
The most up-to-date influenza information from the United Kingdom is available from Public Health Englandexternal icon.

Any links provided to non-Federal organizations are provided solely as a service to our users. These links do not constitute an endorsement of these organizations or their programs by CDC or the Federal Government, and none should be inferred. CDC is not responsible for the content of the individual organization web pages found at these links.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.