Effectiveness of the 2010–2011 seasonal influenza vaccine in preventing confirmed influenza hospitalizations in adults: A case–case comparison, case-control study

Highlights ► We perform a case–case comparison as an improvement of the test-negative design. ► We report IVE estimates with a low probability of bias. ► Influenza vaccination halved the risk of confirmed influenza hospitalization. ► This effect was consistent regardless of age over 60. ► The measured effect was specific for confirmed influenza hospitalizations.