FluView Summary ending on December 24, 2022

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

Nationally, the percentage of specimens testing positive for influenza in clinical laboratories declined compared to the previous week. Percent positivity decreased in all regions. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent live attenuated influenza vaccine (LAIV) receipt or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses.

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 public health laboratories according to the 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. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans.

CDC genetically characterized 1,399 influenza viruses collected since May 1, 2022.

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2022-2023 Northern Hemisphere recommended egg-based and cell- or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

• A (H1N1)pdm09: Eighty-nine A(H1N1)pdm09 viruses were antigenically characterized by HI, and 87 (98%) were well recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/588/2019-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines and 87 (98%) were well recognized by ferret antisera to egg-grown A/Victoria/2570/2019-like reference viruses representing the A(H1N1)pdm09 component for the egg-based influenza vaccines.
• A (H3N2): Sixty A(H3N2) viruses were antigenically characterized by HINT; all were well-recognized (reacting at titers that were within 8-fold of the homologous virus titer) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines and 58 (97%) were well-recognized by ferret antisera to egg-grown A/Darwin/9/2021-like reference viruses representing the A(H3N2) component for egg-based influenza vaccines.

Influenza B Viruses

• B/Victoria: Eight influenza B/Victoria-lineage virus were antigenically characterized by HI; all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines and by ferret antisera to egg-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the egg-based influenza vaccines.
• B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with any pathogen that can present with similar symptoms, including influenza, SARS-CoV-2, and RSV. Due to the COVID-19 pandemic, health care-seeking behaviors have changed, and people may be accessing the health care system in alternative settings not captured as a part of ILINet or at a different point in their illness than they might have before the pandemic. Therefore, it is important 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 influenza, SARS-CoV-2, and other respiratory virus activity. CDC is tracking the COVID-19 pandemic in a weekly publication called COVID Data Tracker Weekly Review. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website.

Outpatient Respiratory Illness Visits

Nationwide during week 51, 6.1% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This is above the national baseline of 2.5%. All 10 HHS regions are above their respective baselines. The percent of patient visits for respiratory illness increased in regions 2 and 9 and decreased in all other regions during week 51 compared to week 50. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

* Effective October 3, 2021 (week 40), the ILI definition (fever plus cough or sore throat) no longer includes “without a known cause other than influenza.”

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Outpatient Respiratory Illness Visits by Age Group

More than 70% of ILINet participants provide both the number of patient visits for respiratory illness 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 respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet increased in the 50-64 years and 65+ years age groups, remained stable in the 25-49 years age group, and decreased in the 0-4 years and 5-24 years age groups.

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Outpatient Respiratory Illness 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

Long-term Care Facility (LTCF) Surveillance

LTCFs (e.g., nursing homes/skilled nursing, long-term care for the developmentally disabled, and assisted living facilities) from all 50 states and U.S. territories report data on influenza virus infections among residents through the National Healthcare Safety Network (NHSN) Long-term Care Facility Component. During week 51, 794 (5.6%) of 14,135 reporting LTCFs reported at least one influenza positive test among their residents.

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Additional information about long-term care facility surveillance:
Surveillance Methods | Additional Data

Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 13 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 12,952 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2022, and December 24, 2022. The weekly hospitalization rate observed in week 51 was 2.6 per 100,000 population. The weekly rate observed during week 48 (week ending December 3) is the third highest peak weekly rate observed during all seasons going back to 2010-2011; this follows the 2017-18 season which peaked during week 1 (week ending January 6) and the 2014-15 season which peaked during week 52 (week ending December 27).

The overall cumulative hospitalization rate was 44.3 per 100,000 population. This cumulative hospitalization rate is 4.6 times higher than the highest cumulative in-season hospitalization rate observed in week 51 during previous seasons going back to 2010-2011 (prior season rates ranged from 0.3 per 100,000 to 9.7 per 100,000). However, this in-season cumulative hospitalization rate is still lower than end-of-season hospitalization rates for all but 4 pre-COVID-19-pandemic seasons (2015-16, 2013-14, 2011-12, 2010-11 seasons).

When examining rates by age, the highest rate of hospitalization per 100,000 population was among adults aged 65 and older (123.5). Among adults aged 65 and older, rates were highest among adults aged 85 and older (220). Among persons aged <65 years, hospitalization rates per 100,000 population were highest among children aged 0-4 years (65.1), followed by adults aged 50-64 years (46.5). When examining rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (62), followed by non-Hispanic American Indian or Alaska Native persons (54.1), Hispanic/Latino persons (32.8), non-Hispanic White persons (30.8), and non-Hispanic Asian/Pacific Islander persons (18.3).

Among 12,952 hospitalizations,12,583 (97.2%) were associated with influenza A virus, 202 (1.6%) with influenza B virus, 17 (0.1%) with influenza A virus and influenza B virus co-infection, and 150 (1.2%) with influenza virus for which the type was not determined. Among 2,371 hospitalizations with influenza A subtype information, 1,878 (79.2%) were A(H3N2), and 493 (20.8%) were A(H1N1)pdm09. Based on preliminary data, of the 1,486 laboratory-confirmed influenza-associated hospitalizations with more complete data, 3.1% (95% CI: 2.3%-4.1%) also tested positive for SARS-CoV-2.

Among 1,124 hospitalized adults with information on underlying medical conditions, 96.7% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 356 hospitalized children with information on underlying medical conditions, 67.7% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by obesity, and neurologic disease.

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Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics

HHS Protect Hospitalization Surveillance

Hospitals report to HHS Protect the number of patients admitted with laboratory-confirmed influenza. During week 51, 18,848 patients with laboratory-confirmed influenza were admitted to a hospital.

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Additional HHS Protect hospitalization surveillance information:
Surveillance Methods |  Additional Data

Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on December 29, 2022, 12.1% of the deaths that occurred during the week ending December 24, 2022 (week 51), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 6.8% for this week. Among the 2,117 PIC deaths reported for this week, 866 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 283 listed influenza. While current PIC mortality is due primarily to COVID-19, the proportion due to influenza is increasing. The data presented are preliminary and may change as more data are received and processed.

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Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Influenza-Associated Pediatric Mortality

Fourteen influenza-associated pediatric deaths occurring during the 2022-2023 season were reported to CDC during week 51. Two deaths occurred during week 47 (the week ending November 26, 2022); seven during week 50 (the week ending December 17, 2022), and five during week 51 (the week ending December 24, 2022). All fourteen deaths were associated with influenza A viruses; subtypes were obtained for six of these reports. One was a coinfection with influenza A(H3) and influenza A(H1) viruses, and the remaining five were A(H3) viruses.

A total of 61 influenza-associated pediatric deaths occurring during the 2022-2023 season have 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
FluNet and the Global Epidemiology Reports.

WHO Collaborating Centers for Influenza:
Australia, China, Japan, the United Kingdom, 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 Control.

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

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

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.