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:

IVY uses a test-negative observational study design and collects data at participating hospitals.