Seasonal influenza remains a significant contributor to healthcare burden in the United States, causing annual spikes in emergency department visits and hospitalizations. While year-to-year variability is common, broader shifts have occurred in recent seasons. In particular, the aftermath of the COVID-19 pandemic brought about changes in public health behaviors, population immunity, and viral circulation patterns. Amid these changes, early reports from the 2024/2025 flu season suggested more severe presentations and higher demand for acute care services.1 This study was designed to quantify recent trends in flu-related emergency department visits, hospitalizations, and complications, building on more than a decade of data to contextualize the current season within historical patterns.
We examined data from 11 million patients with medically attended influenza infections recorded between August 1, 2013, and April 19, 2025. An influenza infection was identified by a positive lab test or a clinical diagnosis of flu. We then looked for a flu-related hospitalization or emergency department (ED) visit within the following 30 days, which were defined as those with an influenza diagnosis or a secondary condition commonly associated with an influenza infection, including COPD, heart failure, and acute necrotizing hemorrhagic encephalopathy.
Over the past 12 flu seasons, the rate of flu-related ED visits has been the highest in the three most recent seasons. In 2022/2023, 33.1% of patients with influenza visited the ED within 30 days of the infection. This increased slightly to 35.9% in 2023/2024 and remained elevated at 35.0% in 2024/2025. For comparison, just 24.1% of patients visited the ED within 30 days of infection in the 2012/2013 season, highlighting the substantial rise in acute care use in recent years.
Similarly, the proportion of patients hospitalized within 30 days of a flu infection has risen in recent seasons. In the 2022/2023 season, 4.9% of patients were hospitalized within 30 days of a flu infection. This increased to 5.6% in 2023/2024 and further to 7.0% in 2024/2025.
These data come from Cosmos, a dataset created in collaboration with a community of Epic health systems representing more than 299 million patient records from 1,700 hospitals and more than 40,000 clinics from all 50 U.S. states, Lebanon, and Saudi Arabia. This study was completed by two teams that worked independently, each composed of a clinician and research scientists. The two teams came to similar conclusions. Graphics by Brian Olson.
- Preliminary estimated flu disease burden 2024-2025 flu season. U.S. Centers for Disease Control and Prevention. March 28, 2025. https://www.cdc.gov/flu-burden/php/data-vis/2024-2025.html. Accessed April 3, 2025.
