2023-2024 Influenza Season Summary: Influenza Severity Assessment, Burden and Burden Prevented

Overview

Influenza Severity Assessment and Estimated Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths that Occurred and those that were Prevented by Vaccination in the United States – 2023-2024 Influenza Season

This summary is an addendum to Influenza Activity in the United States during the 2023–2024 Season and Composition of the 2024–2025 Influenza Vaccine | Influenza (Flu) | CDC. CDC uses mathematical models to estimate the numbers of influenza illnesses, medical visits, hospitalizations, and deaths that occur and are prevented by vaccination each year12. The estimates of the burden and prevented burden of influenza in the population in the United States are used to inform policy and communications related to influenza prevention and control, including promoting vaccination as the best way to prevent influenza.

2023-2024 Severity Assessment

During the 2023-2024 season, the severity of the influenza season was classified as moderate, and influenza activity returned to levels seen during seasons before the COVID-19 pandemic for both the timing and intensity of virus activity3. Seasonal influenza severity was assessed for three age groups including pediatric (0-17 years), adult (18-64 years), and older adult (≥65 years) and using data for three indicators including the percentage of influenza-like illness visits out of all outpatient visits, the rate of influenza-related hospitalizations, and the percentage of influenza deaths out of all-cause deaths4. The 2023-2024 season was classified as moderately severe across all ages and for each age group5.

2023-2024 Disease Burden Estimates

Previous estimates of disease burden using preliminary hospitalization case and death counts for the 2023-2024 season were reported in the technical report describing the 2023-2024 influenza season3. Here updated influenza hospitalizations and deaths along with other updated data sources are used to present the end of season disease burden estimates.

Using information on influenza-confirmed hospitalizations and deaths from October 1, 2023 through April 30, 2024, CDC estimates that 40 million people were sick with influenza, 18 million people visited a healthcare provider for influenza, 470,000 had influenza-related hospitalizations, and 28,000 influenza-related deaths occurred during the 2023-2024 season6. While estimates of illnesses related to and medical visits for influenza were similar to those observed during the 2017-2018 season, influenza-related hospitalizations were similar to what was observed during the 2016-2017 season and influenza-related deaths were similar to the 2018-2019 season.

CDC's estimates of hospitalizations and deaths associated with influenza during the 2023–2024 season are a reminder that influenza viruses can cause severe disease and death. Older adults aged ≥65 years accounted for only 8% of medical visits for influenza but 68% of deaths and 50% of hospitalizations, which is similar to past seasons. Adults aged 50-64 years accounted for 20% of influenza-related hospitalizations and 20% of influenza-related deaths, and those aged 18-49 years accounted for an additional 20% of influenza-related hospitalizations These findings continue to highlight that older adults are particularly vulnerable to severe disease with influenza virus infection and that influenza prevention measures such as vaccination are important to reducing the impact of seasonal epidemics on the population and healthcare system.

Deaths in children with laboratory-confirmed influenza virus infection have been a reportable disease in the United States since 2004; 205 deaths were reported for the 2023-2024 season as of November 15, 20245. However, influenza -related pediatric deaths are likely under-reported, as not all children whose death was related to an influenza virus infection may have been tested for influenza. We estimate that an additional 521 deaths associated with influenza occurred during the 2023-2024 season among children aged <18 years resulting in an estimated 724 pediatric deaths.

2023-2024 Burden Prevented by Vaccination

To estimate the burden prevented by vaccination for the 2023-2024 season, estimates of disease burden, vaccine coverage, and two measures of vaccine effectiveness (VE) were used including VE against outpatient medical visits and VE against hospitalization associated with influenza. Vaccine coverage for influenza vaccination during the 2023-2024 season was lowest (32%) among those aged 18-49 years and highest (69%) among those aged ≥65 years in the United States7. Pooled vaccine effectiveness against influenza-related outpatient medical visits from multiple systems varied by age group and was lowest (37%) among those aged ≥65 years and highest (58%) among those aged 6 months to 4 years. Pooled vaccine effectiveness against influenza-related hospitalization from multiple systems was lowest (35%) among those aged ≥65 years and highest (72%) among those aged 6 months to 4 years8. During the 2023-2024 season, CDC estimates that influenza vaccination prevented 9.8 million influenza-related illnesses, 4.8 million medical visits, 120,000 influenza-related hospitalizations, and 7,900 influenza-related deaths.

The number and proportion of influenza-related illnesses, medically attended illnesses, hospitalizations, and deaths prevented by vaccination during 2023–2024 season varied by age group, due to age-specific differences in influenza disease burden, vaccine coverage, and vaccine effectiveness. Influenza vaccination prevented the fewest illnesses among adults 65 years and older (8% of illnesses prevented). Despite a relatively high vaccine coverage (69%) in this age group, the vaccine effectiveness for outpatient medical visits (37%) and vaccine effectiveness against hospitalizations (35%) was lowest. Influenza vaccination prevented the highest number of community illnesses (31%) and medically attended illnesses (34%) among children aged 5-17 years, an age group in which vaccine uptake was moderate (53%), and vaccine effectiveness was also moderate (52% against outpatient medical visits, 51% against hospitalization). Conversely, influenza vaccination prevented the highest number of hospitalizations and deaths among older adults aged ≥65 years.

Conclusion

During the 2023-2024 influenza season, CDC estimates that influenza was associated with 40 million illnesses, 18 million medical visits, 470,000 hospitalizations, and 28,000 deaths. In addition, influenza vaccination prevented 9.8 million influenza-related illnesses, 4.8 million influenza-related medical visits, 120,000 influenza-related hospitalizations, and 7,900 influenza-related deaths.

Disclaimer:

Disease burden estimates for the 2023-2024 influenza season are preliminary and will be updated at least two additional times; however, this report will not be updated. To access the most up to date burden estimates, please visit the past season's burden page9.