FluView Summary ending on March 4, 2023

Viruses

Illness

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

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

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 remained stable (change of <0.5 percentage points). 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 2,320 influenza viruses collected since October 2, 2022. To reflect the genetic diversity of the HA genes in recent A(H1N1)pdm09 and A(H3N2) viruses, new subclades have been designated. The vast majority of A(H1N1)pdm09 viruses collected in the U.S. this season express HA genes belonging to 6B.1A.5a.2 but this clade has been split into the 2a and 2a.1 subclades. All A(H3N2) viruses collected thus far in the U.S. this season express HA genes belonging to clade 3C.2a1b.2a.2 and this clade has been split into multiple subclades. The new subclade designations improve the ability to track the evolution and co-circulation of multiple groups of influenza viruses.

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 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-five A(H1N1)pdm09 viruses were antigenically characterized by HI, and 84 (99.9%) 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.
• A (H3N2): One hundred and seventy-nine A(H3N2) viruses were antigenically characterized by HINT, and 167 (93%) 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.

Influenza B Viruses

• B/Victoria: Thirteen influenza B/Victoria-lineage virus were antigenically characterized by HI, and thirteen (100%) 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.
• B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

2023-2024 Influenza Season – U.S. Influenza Vaccine Composition:

The World Health Organization (WHO) has recommended the Northern Hemisphere 2023-2024 influenza vaccine composition, and the Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) subsequently made the influenza vaccine composition recommendation for the United States. Both agencies recommend that influenza vaccines contain the following:

Egg-based vaccines

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus
• an A/Darwin/9/2021 (H3N2)-like virus
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
• a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus

Cell- or recombinant-based vaccines

• an A/Wisconsin/67/2022 (H1N1)pdm09-like virus
• an A/Darwin/6/2021 (H3N2)-like virus
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus
• a B/Phuket/3073/2013 (B/Yamagata lineage)-like virus

The A(H1N1)pdm09 recommendation represents an update to the 2022-2023 Northern Hemisphere vaccines. These vaccine recommendations were based on several factors, including global influenza virologic and epidemiologic surveillance, genetic characterization, antigenic characterization, and the candidate vaccine viruses that are available for production.

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 9, 2.4% 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 has decreased compared to week 8 and is below the national baseline of 2.5%. Eight of the 10 HHS regions are below their respective baselines; regions 2 and 9 are above their respective baselines. 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 remained stable (change of ≤ 0.1 percentage points) for four age groups (5-24 years, 25-49 years, 50-64 years, 65+ years) and decreased in the 0-4 years age group in week 9 compared to week 8.

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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 9, 67 (0.5%) of 14,305 reporting facilities reported at least one influenza positive test among their residents. This percentage remains stable compared to week 8.

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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 14 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 17,565 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2022, and March 4, 2023. The weekly hospitalization rate observed in week 9 was 0.1 per 100,000 population. The weekly rate observed during week 48 (week ending December 3, 2022) is the third highest peak weekly rate observed during all seasons going back to 2010-2011; this follows the 2017-2018 season, which peaked during week 1 (week ending January 6, 2018), and the 2014-2015 season which peaked during week 52 (week ending December 27, 2014).

The overall cumulative hospitalization rate was 60.0 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed in week 9 during previous seasons going back to 2010-2011, following the 2017-2018 season. However, this in-season cumulative hospitalization rate is similar to the end-of-season hospitalization rates for 4 seasons (2014-2015, 2016-2017, 2018-2019, and 2019-2020 seasons) and lower than the end-of-season hospitalization rate for the 2017-2018 season, going back to 2010-2011.

When examining rates by age, the highest rate of hospitalization per 100,000 population was among adults aged 65 and older (178.6). Among adults aged 65 and older, rates were highest among adults aged 85 and older (326.8). Among persons aged <65 years, hospitalization rates per 100,000 population were highest among children aged 0-4 years (78.4), followed by adults aged 50-64 years (64.6). When examining rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (85.1), followed by non-Hispanic American Indian or Alaska Native persons (69.4), non-Hispanic White persons (50.6), Hispanic/Latino persons (46.1), and non-Hispanic Asian/Pacific Islander persons (25.8).

Among 17,565 hospitalizations, 17,007 (96.8%) were associated with influenza A virus, 380 (2.2%) with influenza B virus, 26 (0.1%) with influenza A virus and influenza B virus co-infection, and 152 (0.9%) with influenza virus for which the type was not determined. Among 4,076 hospitalizations with influenza A subtype information, 3,081 (75.6%) were A(H3N2), and 995 (24.4%) were A(H1N1)pdm09. Based on preliminary data, of the 12,746 laboratory-confirmed influenza-associated hospitalizations with more complete data, 3.9% (95% CI: 3.1%-4.7%) also tested positive for SARS-CoV-2.

Among 2,912 hospitalized adults with information on underlying medical conditions, 96.9% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 786 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 37.8% were pregnant. Among 1,034 hospitalized children with information on underlying medical conditions, 63.7% 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 | RESP-NET Interactive

HHS Protect Hospitalization Surveillance

Hospitals report to HHS Protect the number of patients admitted with laboratory-confirmed influenza. During week 9, 1,418 patients with laboratory-confirmed influenza were admitted to a hospital. This was a decrease of > 5% compared to week 8.

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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 March 9, 2023, 8.9% of the deaths that occurred during the week ending March 4, 2023 (week 9), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage remained stable (< 0.3 percentage point change) compared to week 8 and is above the epidemic threshold of 7.3% for this week. Among the 2,172 PIC deaths reported for this week, 876 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 22 listed influenza. While current PIC mortality is due primarily to COVID-19, the proportion due to influenza increased from October through mid-December, decreased for seven weeks, and has been stable at low levels for the past five weeks. 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

Eight influenza-associated pediatric deaths occurring during the 2022-2023 season were reported to CDC during week 9. The deaths occurred between week 43 of 2022 (the week ending October 29, 2022) and week 8 of 2023 (the week ending February 25, 2023). All eight deaths were associated with influenza A viruses. Four of the influenza A viruses had subtyping performed; all four were A(H3) viruses.

A total of 125 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

Trend Indicators

Increasing:
Decreasing:
Stable:

Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
Long-term Care Facilities: Up or down arrows indicate change of greater than or equal to 5% of the percent of facilities reporting at least one influenza positive test among their residents compared to the previous week.
HHS Protect Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.3 percentage points of the percent of deaths due to PIC compared to the previous week.

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.