Weekly U.S. Influenza Surveillance Report

FluView a weekly influenza surveillance report prepared by the Influenza Division

Note: CDC is tracking the COVID-19 pandemic in a weekly publication called COVID Data Tracker Weekly Review.

2019-2020 Influenza Season Week 33, ending August 15, 2020

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.

Key Updates for Week 16, ending April 24, 2021

Seasonal influenza activity in the United States remains lower than usual for this time of year. 

Viruses

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.

Key Points

  • Flu activity is unusually low at this time.
  • An annual flu vaccine is the best way to protect against flu and its potentially serious complications.
  • There are also flu antiviral drugs that can be used to treat flu illness.

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.

 

results of tests from Clinical Laboratories
Week 16 Data Cumulative
since September 27, 2020
(Week 40)
No. of specimens tested 19,651 959,337
No. of positive specimens (%) 18 (0.1%) 1,792 (0.2%)
Positive specimens by type
Influenza A 10 (55.6%) 655 (36.6%)
Influenza B 8 (44.4%) 1,137 (63.4%)

 

INFLUENZA Virus Isolated
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Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below.  Data from public health laboratories are used to monitor the proportion of circulating viruses that belong to each influenza subtype/lineage.

results of tests from Public Health Laboratories
Week 16 Data Cumulative
since September 27, 2020
(Week 40)
No. of specimens tested 11,097 450,933
No. of positive specimens 2 246
Positive specimens by type/subtype    
         Influenza A 1 (50.0%) 150 (61.0%)
            (H1N1)pdm09 0 16 (42.1%)
             H3N2 0 21 (55.3%)
             H3N2v 0 1 (2.6%)
             Subtyping not performed 1 112
        Influenza B 1 (50.0%) 96 (39.0%)
            Yamagata lineage 0  8 (42.1%)
            Victoria lineage 0 11 (57.9%)
            Lineage not performed 1 77

INFLUENZA Virus Isolated
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Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data

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, health care-seeking behaviors have changed dramatically during the COVID-19 pandemic. Many people are accessing the health care 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 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 16, 1.1% of patient visits reported through ILINet were due to ILI. This percentage is below the national baseline of 2.6%.

national levels of ILI and ARI
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During week 16, compared with week 15, the percentage of visits for ILI increased for one region (Region 10) and remained stable (change of ≤ 0.1%) for 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 16 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 15. There has been an increasing trend among persons aged 0-4 years, while the remaining age groups have experienced a stable or slightly increasing trend.

national levels of ILI and ARI by age group
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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).

ILI Activity by State/Jurisdiction and Core Based Statistical Area
Activity Level Number of Jurisdictions Number of CBSAs
Week 16

(Week ending
Apr. 24, 2021)

Week 15

(Week ending 
Apr. 17, 2021)

Week 16

(Week ending
Apr. 24, 2021)

Week 15

(Week ending
Apr. 17, 2021)

Very High 0 0 0 0
High 0 0 0 0
Moderate 0 0 2 4
Low 0 0 32 24
Minimal 54 54 568 597
Insufficient Data 1 1 327 304

 

*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 24, 2021, FluSurv-NET sites in 14 states reported 224 laboratory confirmed influenza hospitalizations for an overall cumulative hospitalization rate of 0.8 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-tenth the rate at this time during the 2011-12 season.


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 29, 2021, 11.8% of the deaths that occurred during the week ending April 24, 2021 (week 16), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 6.7% for week 16. Among the 2,429 PIC deaths reported for this week (week 16), 1,630 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and three 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.

INFLUENZA Virus Isolated
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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 16.

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

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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.