Investigating Respiratory Viruses in the Acutely Ill (IVY) Network
The Investigating Respiratory Viruses in the Acutely Ill (IVY) network currently consists of 26 medical centers in 20 U.S. states. Prior to December 2022, the network was known as the “Influenza and Other Viruses in the Acutely Ill network.” IVY was created initially in 2019 to estimate how well influenza (flu) vaccines work to prevent severe flu illness among intensive care unit (ICU) patients. In 2021, IVY expanded to also assess how well flu vaccines work to prevent severe flu and how well COVID-19 vaccines work to prevent severe COVID-19 among hospitalized patients. In 2022, patients hospitalized with respiratory syncytial virus (RSV) were added to begin monitoring severe RSV disease among adults hospitalized in anticipation of forthcoming RSV vaccines.
Flu Vaccine Effectiveness
Measuring vaccine effectiveness (VE) in hospitalized adults is important to better understand how well vaccines protect against hospitalizations and ICU admissions. VE among adults 65 years and older is especially important because this group has the highest flu-associated hospitalization rate compared to other age groups, and also the highest COVID-19-associated hospitalization rate compared to other age groups.
CDC measures VE to evaluate the benefits of flu vaccines and COVID-19 vaccines in different age groups and against different clinical outcomes, such as hospitalization. These efforts help communicate the importance and impact of flu vaccination and COVID-19 vaccination.
Sites and Study Design
Participating sites are located in 20 states:
- Arizona
- California
- Colorado
- Connecticut
- Florida
- Georgia
- Iowa
- Maryland
- Massachusetts
- Michigan
- Minnesota
- Missouri
- New York
- North Carolina
- Ohio
- Oregon
- Tennessee
- Texas
- Utah
- Washington
IVY uses a test-negative observational study design and collects data at participating hospitals.
- More information about how CDC’s VE studies are conducted and how to interpret results is available at How Flu Vaccine Effectiveness and Efficacy are Measured: Questions and Answers for Health Professionals.
- Seasonal Influenza Vaccine Effectiveness Studies show the overall adjusted VE and related references for each season starting in 2004-2005.