Influenza vaccine effectiveness in preventing outpatient, inpatient, and severe cases of laboratory-confirmed influenza.

Background. In most seasons, the influenza vaccine is effective in preventing influenza, but it is not clear whether it is equally effective in preventing mild and severe cases. We designed a case-control study to compare the effectiveness of the influenza vaccine in preventing outpatient, inpatient, and severe or fatal cases of laboratory-confirmed influenza. Methods. Hospitalized patients (n = 691) with laboratory-confirmed influenza in the 2010–2011 season recruited in 29 Spanish hospitals were individually matched by age, admission/visit date, and province with an outpatient with laboratory-confirmed influenza and an outpatient control. Severe cases were considered those patients admitt…