The 2024–25 Flu Season: The Highest Hospitalization Rate Since 2010–11

This page summarizes a report in CDC's MMWR by CDC and the state and academic partners of the Influenza Hospitalization Surveillance Network (FluSurv-NET), using preliminary data for October 2024–April 2025.

CDC classed the 2024–25 U.S. flu season as high severity for all age groups, the first such season since 2017–18. Influenza A(H1N1)pdm09 and A(H3N2) viruses circulated together.

How it was measured

FluSurv-NET tracks laboratory-confirmed flu hospitalizations of all ages in about 300 hospitals in 14 states, covering 9% of the U.S. population. A case is a resident of the area admitted between October 1 and April 30 with a positive flu test in the 14 days before or during the stay. Clinical details came from medical records for a random sample of patients, and seasons back to 2010–11 (except 2020–21, when there were too few cases) were compared.

Record hospitalization rates

• FluSurv-NET counted 38,960 flu hospitalizations.
• The season's cumulative rate, 127.1 per 100,000, was higher than any end-of-season rate since 2010–11 (median 62.0; range 8.7 in 2011–12 to 102.9 in 2017–18).
• The weekly rate peaked at 13.5 per 100,000 in early February, the highest since 2010–11 (earlier peaks ranged from 1.1 to 10.2), and higher than any weekly COVID-19 hospitalization peak since Omicron emerged in January 2022. The season's cumulative flu rate was also higher than that season's COVID-19 or RSV rates.
• In every age group, rates were 1.8 to 2.8 times the historical median.

For every age group under 75, rates were higher than in any season since 2010–11.

Flu types. Influenza A drove most hospitalizations (122.0 per 100,000, against 4.8 for influenza B), with A(H1N1)pdm09 at 72.2 and A(H3N2) at 49.5. Among people 75 and older, H1N1 (339.5) exceeded H3N2 (244.8), the reverse of 2017–18 (H3N2 501.3, H1N1 37.6).

Why weren't the oldest adults hit harder? People 75 and older keep immunity against A(H1) viruses from early childhood exposures and historically suffer most in H3N2-dominated seasons. In 2017–18, 84% of circulating influenza A was H3N2; in 2024–25 the two subtypes circulated equally. Lower flu vaccination coverage in recent seasons, along with virus characteristics, may help explain the high rates overall.

Who was hospitalized and how they fared

Among 10,269 sampled patients:

• Underlying conditions: 89.1% had at least one. The most common were asthma in children (14.0% at 0–4, 35.9% at 5–17), obesity at 18–49 (43.9%), chronic metabolic disease such as diabetes at 50–64 (45.6%), and cardiovascular disease at 65–74 (57.0%) and 75 and older (69.3%). Nearly 11% had none, a reminder that healthy people can also be hospitalized.
• Complications: pneumonia (30.0%), sepsis (18.5%) and acute renal failure (18.1%) were most common, as in earlier seasons.
• Antivirals: children 5–17 were least likely to be treated (61.6%) and adults 75 and older most likely (88.3%).

With severity measures similar to past seasons, the season's toll likely came from more infections, not unusually severe illness, meaning more patients needing ICU beds and ventilators in absolute terms.

What CDC recommends

• An annual flu vaccine for everyone 6 months and older without contraindications.
• Prompt antiviral treatment for every patient hospitalized with suspected or confirmed flu. Antivirals improve outcomes, including in-hospital survival, but treatment remains too low, especially for children and adolescents.
• Handwashing and other nonpharmacologic measures may also reduce spread.

Limitations

• Testing depends on clinicians, so hospitalizations may be undercounted.
• Influenza A subtype was missing for a median 56% of patients and was imputed, so subtype rates should be read cautiously, though imputed patterns matched public health laboratory data.
• Changes in admission practices could not be measured, though the share of patients likely admitted for flu-like illness rose from 79% in 2021–22 to 87% in 2024–25.
• Vaccination status was missing for 28.5% of patients this season.
• The network covers 9% of the population and may not represent the whole country.

Sources

• O'Halloran A, Whitmill Habeck J, Gilmer M, et al. "Influenza-Associated Hospitalizations During a High Severity Season — Influenza Hospitalization Surveillance Network, United States, 2024–25 Influenza Season." MMWR 74(34). https://www.cdc.gov/mmwr/volumes/74/wr/mm7434a1.htm
• Supplementary materials: https://stacks.cdc.gov/view/cdc/232060
• The report's figures are not reproduced, because the report was prepared jointly with state health departments and universities.