Clinical effectiveness of influenza vaccination in persons younger than 65 years with high-risk medical conditions - The PRISMA Study

Background: Influenza vaccination has consistently been shown to prevent all-cause death and hospitalizations during influenza epidemics among seniors. However, such benefits have not yet been demonstrated among younger individuals with high-risk medical conditions. In the present study, we evaluated the effectiveness of influenza vaccine in persons recommended for vaccination of any age during an epidemic. Methods: We conducted a case-control study during the 1999-2000 influenza A epidemic nested in a cohort of 75 227 primary care patients. End points were all-cause mortality and episodes of hospitalizations or general practitioner (GP) visits for influenza, pneumonia, other acute respiratory disease, acute otitis media, myocardial infarction, heart failure, and stroke. The effectiveness of vaccination was evaluated by means of logistic regression analysis with adjustments for age, sex, prior health care use, medication use, and comorbid conditions. Results: Among high-risk children and adolescents younger than 18 years (n = 5933; 8% of the study population), 1 death, 3 hospitalizations for pneumonia, and 160 GP visits occurred. After adjustments, 43% (95% confidence interval [CI] 10%-64%) of visits were prevented. Among high-risk adults aged between 18 and 64 years (n = 24 928; 33% of the study population), 47 deaths, 23 hospitalizations, and 363 GP visits occurred. After adjustments, vaccination prevented 78% of deaths (95% CI, 39%-92%), 87% of hospitalizations (95% CI, 39%-97%), and 26% of GP visits (95% CI, 7%-47%). Among elderly persons (n=44366; 59% of the study population), 272 deaths and 166 hospitalizations occurred, and after adjustments the vaccine prevented these end points by 50% (95% CI, 23%-68%) and 48% (95% CI, 7%-71%), respectively. Conclusion: Persons with high-risk medical conditions of any age can substantially benefit from annual influenza vaccination during an epidemic.