Influenza and Other Viruses in the Acutely Ill (IVY)

The Influenza and Other Viruses in the Acutely Ill (IVY) network consists of 21 large, adult hospitals in 20 U.S. cities. IVY was initially created in 2019 to estimate how well influenza (flu) vaccines work at preventing severe flu illness among intensive care unit (ICU) patients. As of April 1, 2021, IVY has expanded to also enroll all patients hospitalized with COVID-19. During the 2021-2022 flu season, the network will also enroll patients hospitalized with flu.

Flu Vaccine Effectiveness

Measuring flu vaccine effectiveness (VE) in hospitalized adults is important to better understand how well vaccines protect against flu hospitalizations and ICU admissions. VE in adults 65 years and older is especially important because this group has the highest flu hospitalization rates compared to other age groups. Also, adults who are hospitalized may have more underlying conditions than adults receiving outpatient care(learn more about the US Flu VE Network).

CDC measures VE each year to evaluate the benefits of flu vaccines in different age groups and against different clinical outcomes, like hospitalization. These efforts inform U.S. vaccine recommendations and policy decisions aimed at providing the best protection from flu and its potentially serious complications.

Sites and Study Design

Participating sites are located in 18 states:

  • California
  • Colorado
  • Florida
  • Georgia
  • Iowa
  • Maryland
  • Massachusetts
  • Michigan
  • Minnesota
  • Missouri
  • New York
  • North Carolina
  • Ohio
  • Oregon
  • Tennessee
  • Texas
  • Utah
  • Washington
IVY sites

IVY sites

IVY uses a test-negative observational study design similar to the US Flu VE Network and collects data at participating hospitals.

Other Information on CDC Vaccine Effectiveness (VE) Estimates