New CDC Study Looks at Flu Severity by Virus Type and Subtype

CDC update

September 26, 2023 — ­­­A new CDC study published in The Lancet Microbe found that more than 1 in 6 people hospitalized with flu had severe outcomes such as intensive care unit (ICU) admission, use of invasive mechanical ventilation or extracorporeal membrane oxygenation (MV/ECMO), and death across a subset of people hospitalized with flu over nine seasons in the United States. Despite a higher number of flu A(H3N2) hospitalizations, patients hospitalized with flu A(H1N1)pdm09 or B viruses had a higher likelihood of in-hospital severe outcomes.

Compared to hospitalizations with flu A(H3N2) virus, people hospitalized with flu A(H1N1)pdm09 virus were:

• 42% more likely to be admitted to the ICU.
• 79% more likely to receive MV/ECMO.
• 25% more likely to die.

Compared to hospitalizations with flu A(H3N2) virus, people hospitalized with flu B virus were:

• 6% more likely to be admitted to the ICU.
• 14% more likely to receive MV/ECMO.
• 18% more likely to die.

Similar increases in likelihood of ICU admission and MV/ECMO use were seen for people of all ages hospitalized with A(H1N1)pdm09 viruses, although results were variable by age for influenza B. Before this study, little was known about the relative severity of flu by virus type and subtype in hospitalized people in the United States.

Researchers looked at flu-related hospitalizations from the 2010–2011 through 2018–2019 flu seasons from FluSurv-NET sites in the following 13 states: California, Colorado, Connecticut, Georgia, Maryland, Michigan, Minnesota, Ohio, Oregon, New Mexico, New York, Tennessee, and Utah. FluSurv-NET is a population-based surveillance system that captures laboratory-confirmed flu-related hospitalizations among people of all ages in more than 250 acute-care hospitals in the United States. The FluSurv-NET system represents over 29 million people, or approximately 9% of the U.S. population. Of the of 104,969 flu-related hospitalizations captured by FluSurv-NET system:

• 52% of people were vaccinated against flu.
• 88% of people had at least one comorbid condition.
• 7% of people were hospitalized with flu A(H3N2).
• 6% of people were hospitalized with flu A(H1N1)pdm09.
• 7% of people were hospitalized with flu B.
• ICU admission occurred in 16.7%, MV/ECMO use in 6.5%, and death in 3%.

These findings highlight the importance of annual flu vaccination and early antiviral treatment for patients at increased risk of severe flu, no matter which flu viruses are circulating.