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The PRISMA Study

Eelko Hak, MSc, PhD; Erik Buskens, MD, PhD; Gerrit A. van Essen, MD, PhD; Dinny H. de Bakker, PhD; Diederick E. Grobbee, MD, PhD; Margot A. J. B. Tacken, MSc; Ben A. van Hout, PhD; Theo J. M. Verheij, MD, PhD

Arch Intern Med. 2005;165:274-280.

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 = 44 366; 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.

Author Affiliations: Julius Center for Health Sciences and Primary Health Care, University Medical Center Utrecht, Utrecht, the Netherlands (Drs Hak, Buskens, van Essen, Grobbee, van Hout, and Verheij); National Institute for Primary Health Care, Utrecht (Dr de Bakker); and Centre for Quality of Care Research, University Medical Center Nijmegen, Nijmegen, the Netherlands (Ms Tacken)

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