Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN)

Note: The Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) ended enrollment on July 31, 2021, and is no longer being used to measure flu vaccine effectiveness. The information provided on this page is for historical background and reference. CDC continues to use its other flu vaccine effectiveness (VE) networks, including the Influenza and Other Viruses in the Acutely Ill (IVY) network, to collect information on adults hospitalized with flu.

The Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) was created to estimate how well the flu vaccine works at preventing severe flu illness, especially among older adults.

Flu Vaccine Effectiveness

Measuring vaccine effectiveness (VE) against flu hospitalizations in older adults is important because this group has the highest hospitalization rate compared to other age groups. Also, adults who are hospitalized are more likely to have one or more underlying conditions than adults receiving outpatient care (learn more: U.S. 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 help communicate the importance and impact of flu vaccination.

Other HAIVEN Studies

In addition to vaccine effectiveness, HAIVEN also described severe flu outcomes in hospitalized adults, such as length of hospital stay, need for intensive care, and use of ventilation. Describing these outcomes helps us better understand the patients impacted by severe flu.

HAIVEN also studied respiratory infections in hospitalized older adults, including COVID-19, RSV, and human metapneumovirus.

Sites and Study Design

Participating sites were located in four states:

  • Michigan
  • Pennsylvania
  • Tennessee
  • Texas

HAIVEN used a test-negative observational study design similar to the U.S. Flu VE Network and collected data at participating hospitals.

Other Information on CDC Vaccine Effectiveness Estimates