1. The Influenza Hospitalization Surveillance Network (FluSurv-NET) data are preliminary and subject to change as more data become available. In particular, case counts and rates for recent hospital admissions are subject to reporting delays. As data are received each week, prior case counts and rates are updated accordingly. FluSurv-NET conducts population-based surveillance for laboratory-confirmed influenza-associated hospitalizations in children <18 years of age and adults. The FluSurv-NET catchment area includes more than 90 selected counties or county equivalents in California, Colorado, Connecticut, Georgia, Maryland, Michigan, Minnesota, New Mexico, New York, North Carolina, Ohio, Oregon, Tennessee, and Utah. Additional counties in Idaho (2010 to 2011), Iowa (2012 to 2013, 2020-2022), Oklahoma (2010 to 2011), and Rhode Island (2010 to 2013) have also contributed to the network. Incidence rates (per 100,000 population) are calculated using either the National Center for Health Statistics' (NCHS) vintage bridged-race postcensal population estimates (for years prior to 2020) or U.S. Census vintage unbridged-race postcensal population estimates (for 2020 through present) for the counties or county equivalents included in the surveillance catchment area. The rates provided are likely to be underestimated as influenza-associated hospitalizations might be missed due to test availability and provider or facility testing practices.

2. In most influenza seasons, case counts reflect cases hospitalized during October 1 – April 30 of each influenza season. Cases hospitalized during Week 39 are included in the totals for Week 40, while cases hospitalized during Week 18 are included in the totals for Week 17. In 2009-10, case counts reflect cases hospitalized during Sept 1, 2009 through April 30, 2010 (Week 35 – Week 17). In the 2021-22, rates reflect cases hospitalized beginning October 1, 2021 (Week 39), but given late influenza season activity, the season was extended to June 11, 2022 (Week 23). In 2023-2024, surveillance was extended to September 28, 2024 (Week 39) for situational awareness during the ongoing outbreak of highly pathogenic avian influenza (HPAI) A(H5N1) virus among birds, poultry, dairy cattle, and other animals in the United States (H5N1 Bird Flu: Current Situation Summary | Avian Influenza (Flu) (cdc.gov)).

1. The Influenza Hospitalization Surveillance Network (FluSurv-NET) data are preliminary and subject to change as more data become available.

2. The denominator for characteristic data (except for intensive care unit, mechanical ventilation, in-hospital mortality, and discharge diagnoses) is the total number of patients with non-missing data for that characteristic. The denominator for intensive care unit, mechanical ventilation, in-hospital mortality, and pneumonia diagnosis is restricted to cases who are no longer hospitalized and who have complete medical chart reviews. These data will be updated each week as additional chart reviews are completed.

3. White, Black, Asian/Pacific Islander and American Indian/Alaska Native all represent non-Hispanic ethnicity groups; Other includes persons in multiple race categories and persons for whom race is unknown.

4. Starting with the 2017-18 season, due to the large numbers of identified cases, a sampling strategy was implemented in certain seasons and for certain age groups for medical chart abstractions. Due to the sampling methodology, weighted proportions are presented for seasons where sampling was implemented. More detailed information on sampling is described at: Influenza Hospitalization Surveillance Network.

5. Due to the emergence of the COVID-19 pandemic, data are insufficient for select variables during the 2020-21 influenza seasons.

6. Data on Mechanical Ventilation, Pneumonia Diagnosis, and Antiviral Treatment not available for IA during 2021-2022. Data on Pneumonia Diagnosis and Antiviral Treatment not available for NC during the 2023-24 and 2024-25 seasons. Data on ICU, Mechanical Ventilation, Pneumonia Diagnosis and Antiviral Treatment not available for OH during 2024-25.

1. The Influenza Hospitalization Surveillance Network (FluSurv-NET) data are preliminary and subject to change as more data become available.

2. The denominator for each underlying medical condition is the total number of patients with non-missing data for that condition. Pregnancy percentage is calculated using the number of female cases of childbearing age (15 to 44 years) as the denominator. Starting with the 2022-23 season, the childbearing age range is 15-49 years.

3. Collection of data on specific underlying medical conditions has changed over time; data are not displayed for conditions that were not collected in particular seasons.

4. Starting with the 2019-20 season, in order to better standardize data collection methods, substantial changes were made to how underlying medical conditions are collected including the expansion of several underlying condition categories and sub-categories. Observed changes in prevalence of underlying medical conditions beginning in 2019-20 may be partly due to the changes in methodology.

5. Starting with the 2017-18 season, due to the large numbers of identified cases, a sampling strategy was implemented in certain seasons and for certain age groups for medical chart abstractions Due to the sampling methodology, weighted proportions are presented for seasons where sampling was implemented. More detailed information on sampling is described at: Influenza Hospitalization Surveillance Network.

6. Data are insufficient for underlying medical conditions during the 2020-21 influenza season.

7. Data on underlying medical conditions not available for IA in 2021-22 or NC in 2023-24 and 2024-25 or OH in 2024-2025.