[EVAN-50 study: Effectiveness of polysaccharide pneumococcus vaccine in preventing pneumococcal infections in the over-50 population].

Objective: To assess the effectiveness of 23-valent polysaccharide vaccine in preventing severe pneumococcal infections in adults over 50 years old.

Design: Case-control study.

Setting: Primary Health Care Service, Tarragona, Spain.

Patients: A total of 270 patients >50 with severe pneumococcal disease (invasive pneumococcal disease and non-bacteraemic pneumococcal pneumonia) and 540 control patients randomized from the primary care Centres of the case patients. Case and control patients will be matched for age, sex, family physician, and level of risk for pneumonia.

Main Measurements: Odds ratio (OR) will be used to measure the vaccine effect. Multivariate logistical regression, adjusted for age, sex, and comorbidity, will be conducted. Vaccine effectiveness (VE) will be calculated by the formula, VE = 1-OR. Vaccine effectiveness will be distinguished for the various age groups and at each risk stratum. It will also be estimated by means of indirect cohort analysis, taking as cases the infection caused by vaccine serotypes and as controls infection caused by non-vaccine serotype.

Discussion: The study will give an answer in terms of effectiveness of the vaccine for several age and risk strata. It will contribute to taking a decision regarding the controversial question of the systematic indication, or otherwise, of this vaccine for the elderly.