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.
| Week 50 | Data Cumulative since September 29, 2019 (week 40) |
|
|---|---|---|
| No. of specimens tested | 33,918 | 320,906 |
| No. of positive specimens (%) | 5,515 (16.3%) |
23,386 (7.3%) |
| Positive specimens by type | ||
| Influenza A | 1,735 (31.5%) | 6,891 (29.5%) |
| Influenza B | 3,780 (68.5%) | 16,495 (70.5%) |
View Chart Data | View Full Screen
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.
| Week 50 | Data Cumulative since September 29, 2019 (week 40) |
|
|---|---|---|
| No. of specimens tested | 1,413 | 17,865 |
| No. of positive specimens | 733 | 6,045 |
| Positive specimens by type/subtype | ||
| Influenza A | 282 (38.5%) | 2,487 (41.1%) |
| (H1N1)pdm09 | 231 (87.2%) | 1,617 (69.3%) |
| H3N2 | 34 (12.8%) | 718 (30.7%) |
| Subtyping not performed | 17 | 152 |
| Influenza B | 451 (61.5%) | 3,558 (58.9%) |
| Yamagata lineage | 7 (2.2%) | 72 (2.7%) |
| Victoria lineage | 310 (97.8%) | 2,562 (97.3%) |
| Lineage not performed | 134 | 924 |
Nationally influenza B/Victoria viruses have been reported more frequently than other influenza viruses this season followed by A(H1N1)pdm09. The predominant virus varies by region. Regional and state level data about circulating influenza viruses can be found on FluView Interactive. The predominant virus also varies by age group. Nationally, influenza B/Victoria viruses are the most commonly reported influenza viruses among children age 0-4 years (49% of reported viruses) and 5-24 years (59% of reported viruses), while A(H3N2) viruses are the most commonly reported influenza viruses among persons 65 years of age and older (42% of reported viruses). Among adults aged 25-64 years, approximately equal proportions of influenza A(H1N1)pdm09 and B/Victoria viruses (39% and 35%, respectively) have been reported. Additional age data can be found on FluView Interactive.
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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 used for developing new influenza vaccines and to monitor evolutionary changes that continually occur in circulating influenza. CDC also tests susceptibility of influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.
CDC genetically characterized 501 influenza viruses collected in the U.S. from September 29, 2019 to December 14, 2019.
| Virus Subtype or Lineage | Genetic Characterization | ||||
|---|---|---|---|---|---|
| Total No. of Subtype/Lineage Tested | Clade | Number (% of subtype/lineage tested) | Subclade | Number (% of subtype/lineage tested) | |
| A/H1 | 123 | ||||
| 6B.1A | 123 (100%) | ||||
| A/H3 | 162 | ||||
| 3C.2a | 162 (100%) | 2a1 | 162 (100%) | ||
| 2a2 | 0 | ||||
| 2a3 | 0 | ||||
| 2a4 | 0 | ||||
| 3C.3a | 0 | 3a | 0 | ||
| B/Victoria | 197 | ||||
| V1A | 197 (100%) | V1A | 0 | ||
| V1A.1 | 23 (11.7%) | ||||
| V1A.3 | 174 (88.3%) | ||||
| B/Yamagata | 19 | ||||
| Y3 | 19 (100%) | ||||
CDC antigenically characterizes a subset of influenza viruses by hemagglutination inhibition (HI) or neutralization based Focus Reduction assays (FRA). Antigenic drift is evaluated by comparing antigenic properties of cell-propagated reference viruses representing currently recommended vaccine components with those of cell-propagated circulating viruses. CDC antigenically characterized 94 influenza viruses collected in the United States from September 29, 2019, to December 14, 2019.
Influenza A Viruses
- A (H1N1)pdm09: 42 A(H1N1)pdm09 viruses were antigenically characterized by HI with ferret antisera, and all were antigenically similar (reacting at titers that were within 4-fold of the homologous virus titer) to cell-propagated A/Brisbane/02/2018-like reference viruses representing the A(H1N1)pdm09 component for the 2019-20 Northern Hemisphere influenza vaccines.
- A (H3N2): 17 A(H3N2) viruses were antigenically characterized by FRA with ferret antisera, and 12 (70.6%) were antigenically similar to cell-propagated A/Kansas/14/2017-like reference viruses representing the A(H3N2) component for the 2019-20 Northern Hemisphere influenza vaccines.
Influenza B Viruses
- B/Victoria: 25 B/Victoria lineage viruses, including viruses from both co-circulating sub-clades, were antigenically characterized by HI with ferret antisera, and 19 (76%) were antigenically similar to cell-propagated B/Colorado/06/2017-like reference viruses representing the B/Victoria component for the 2019-20 Northern Hemisphere influenza vaccines.
- B/Yamagata: 10 B/Yamagata lineage viruses were antigenically characterized by HI with ferret antisera, and all 10 (100%) were antigenically similar to cell-propagated B/Phuket/3073/2013-like reference viruses representing the B/Yamagata component for the 2019-20 Northern Hemisphere influenza vaccines.
CDC assesses susceptibility of influenza viruses to the antiviral medications oseltamivir, zanamivir, peramivir, and baloxavir using next generation sequence analysis supplemented by laboratory assays. Viruses collected in the United States since September 29, 2019, were tested for antiviral susceptibility as follows:
| Antiviral Medication | Total Viruses | A/H1 | A/H3 | B/Victoria | B/Yamagata | ||
|---|---|---|---|---|---|---|---|
| Neuraminidase Inhibitors | |||||||
| Oseltamivir | Viruses Tested | 462 | 103 | 152 | 190 | 17 | |
| Reduced Inhibition | (0.0%) | (0.0%) | (0.0%) | (0.0%) | (0.0%) | ||
| Highly Reduced Inhibition | 1 (0.2%) | 1 (1.0%) | (0.0%) | (0.0%) | (0.0%) | ||
| Peramivir | Viruses Tested | 462 | 103 | 152 | 190 | 17 | |
| Reduced Inhibition | (0.0%) | (0.0%) | (0.0%) | (0.0%) | (0.0%) | ||
| Highly Reduced Inhibition | 1 (0.2%) | 1 (1.0%) | (0.0%) | (0.0%) | (0.0%) | ||
| Zanamivir | Viruses Tested | 462 | 103 | 152 | 190 | 17 | |
| Reduced Inhibition | 1 (0.2%) | (0.0%) | (0.0%) | 1 (0.5%) | (0.0%) | ||
| Highly Reduced Inhibition | (0.0%) | (0.0%) | (0.0%) | (0.0%) | (0.0%) | ||
| PA Endonuclease Inhibitor | |||||||
| Baloxavir | Viruses Tested | 423 | 92 | 149 | 167 | 15 | |
| Reduced Susceptibility | (0.0%) | (0.0%) | (0.0%) | (0.0%) | (0.0%) | ||
Outpatient Illness Surveillance
ILINet
Nationwide during week 50, 3.9% of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) were due to influenza-like illness (ILI). This percentage is above the national baseline of 2.4%.
View Chart Data | View Full Screen
On a regional level, the percentage of outpatient visits for ILI ranged from 1.9% to 7.8% during week 50. All regions reported a percentage of outpatient visits for ILI which is equal to or above their region-specific baselines.
ILI Activity Map
Data collected in ILINet are used to produce a measure of ILI activity* by state.
During week 50, the following ILI activity levels were experienced:
- High – Puerto Rico, New York City, and 19 states (Alabama, Arizona, Arkansas, Colorado, Florida, Georgia, Kansas, Kentucky, Maryland, Mississippi, Nebraska, New Mexico, Oklahoma, Oregon, South Carolina, Tennessee, Texas, Virginia, and Washington)
- Moderate – the District of Columbia and six states (Connecticut, Illinois, Minnesota, New Jersey, North Carolina, and North Dakota)
- Low – 10 states (California, Hawaii, Indiana, Massachusetts, Nevada, New York, Pennsylvania, Utah, Wisconsin, and Wyoming)
- Minimal – 14 states (Alaska, Delaware, Idaho, Iowa, Maine, Michigan, Missouri, Montana, New Hampshire, Ohio, Rhode Island, South Dakota, Vermont, and West Virginia)
- Data were insufficient to calculate an ILI activity level from the U.S. Virgin Islands and Louisiana.
*Data collected in ILINet may disproportionally represent certain populations within a state, and therefore, may not accurately depict the full picture of influenza activity for the whole state. Differences in the data presented here by CDC and independently by some state health departments likely represent differing levels of data completeness with data presented by the state 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 influenza activity reported by state and territorial epidemiologists indicates geographic spread of influenza viruses but does not measure the severity of influenza activity.
During week 50 the following influenza activity was reported:
- Widespread – Puerto Rico and 30 states (Alabama, Alaska, Arizona, California, Connecticut, Delaware, Florida, Georgia, Idaho, Indiana, Kentucky, Louisiana, Maryland, Massachusetts, Minnesota, Mississippi, Montana, Nebraska, Nevada, New Mexico, New York, North Carolina, Ohio, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, and Washington)
- Regional – 17 states (Arkansas, Colorado, Illinois, Iowa, Kansas, Maine, Michigan, Missouri, New Hampshire, New Jersey, North Dakota, Oklahoma, Rhode Island, South Dakota, Utah, Wisconsin, and Wyoming)
- Local – the District of Columbia and two states (Vermont and West Virginia)
- Sporadic – the U.S. Virgin Islands and one state (Hawaii)
- Guam did not report.
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 the Emerging Infections Program (EIP) states and Influenza Hospitalization Surveillance Project (IHSP) states.
A total of 1,593 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2019, and December 14, 2019. The overall hospitalization rate was 5.5 per 100,000 population. The highest rate of hospitalization was among adults aged ≥65 (12.7 per 100,000 population), followed by children aged 0-4 (10.9 per 100,000 population) and adults aged 50-64 (5.5 per 100,000 population). Among 1,593 hospitalizations, 855 (53.7%) were associated with influenza A virus, 722 (45.3%) with influenza B virus, 9 (0.6%) with influenza A virus and influenza B virus co-infection, and 7 (0.4%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 137 (73.3%) were A(H1N1)pdm09 virus and 50 (26.7%) were A(H3N2).
Additional hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods | FluView Interactive
Pneumonia and Influenza (P&I) Mortality Surveillance
Based on National Center for Health Statistics (NCHS) mortality surveillance data available on December 19, 2019, 5.5% of the deaths occurring during the week ending December 7, 2019 (week 49) were due to P&I. This percentage is below the epidemic threshold of 6.6% for week 49.
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Additional pneumonia and influenza mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive
Influenza-Associated Pediatric Mortality
Nine influenza-associated pediatric deaths occurring between week 44 (the week ending November 2, 2019) and week 50 (the week ending December 14, 2019) were reported to CDC during week 50. Two deaths were associated with influenza A(H1N1)pdm09 virus infection and seven were associated with influenza B viruses. Two of the influenza B viruses had lineage determined and both were B/Victoria viruses.
A total of 19 influenza-associated pediatric deaths occurring during the 2019-2020 season have been reported to CDC.
- Thirteen deaths were associated with influenza B viruses. Five of these had the lineage determined and all were B/Victoria viruses.
- Six deaths were associated with influenza A viruses. Four of these had subtyping performed and all were A(H1N1)pdm09 viruses.
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. To access these tools, visit http://www.cdc.gov/flu/weekly/fluviewinteractive.htm
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 at https://www.cdc.gov/niosh/topics/absences/default.html
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 FluNet and the Global Epidemiology Reports.
WHO Collaborating Centers for Influenza located in Australia, China, Japan, the United Kingdom, and the United States (CDC in Atlanta, Georgia).
Europe: For the most recent influenza surveillance information from Europe, please see WHO/Europe and the European Centre for Disease Prevention and Control at http://www.flunewseurope.org/.
Public Health Agency of Canada: The most up-to-date influenza information from Canada is available at http://www.phac-aspc.gc.ca/fluwatch/
Public Health England: The most up-to-date influenza information from the United Kingdom is available at https://www.gov.uk/government/statistics/weekly-national-flu-reports
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.
An overview of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component, is available at: http://www.cdc.gov/flu/weekly/overview.htm.
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