The objective of our study was to evaluate the effectiveness of the 23-valent pneumococcal polysaccharide vaccine (PPV) in preventing hospital admission for communityacquired pneumonia (CAP) in people o65 yrs of age.We conducted a matched case-control study in patients with CAP admitted to five Spanish hospitals. Cases were persons aged o65 yrs admitted to hospital through the emergency department, who presented a clinical and radiological pattern compatible with pneumonia, assessed using established criteria. We matched each case with three control subjects by sex, age (¡5 yrs), date of hospitalisation (¡30 days) and underlying disease. The study period was May 1, 2005 to January 31, 2007. The PPV immunisation status of cases and controls was investigated. Adjusted ORs for vaccination were calculated using logistic regression analysis.A total of 489 cases and 1,467 controls were included in the final analysis. The overall adjusted vaccination effectiveness for all patients was 23.6% (95% CI 0.9-41.0). The adjusted vaccination effectiveness for immunosuppressed patients was 21.0% (95% CI -18.7-47.5).Our results suggest that the PPV may potentially reduce hospitalisations for pneumonia in the elderly and supports vaccination programmes in this age group.KEYWORDS: Case-control study, effectiveness, elderly, pneumococcal polysaccharide vaccine, pneumonia C ommunity-acquired pneumonia (CAP) is an important cause of morbidity and mortality in elderly people and those of any age with underlying diseases [1,2]. In Spain, the overall incidence in adults varies between two and 10 cases per 1,000 persons per yr in all ages and between 14 and 35 per 1,000 persons per yr in persons aged .70 yrs [3,4]. In a Spanish study, the incidence increased dramatically by age in elderly people (9.9/1,000 in people aged 65-74 yrs versus 29.4 in people aged o85 years) [4]. Hospitalisations due to CAP increase with age and may reach 61% for all ages, and 67% in people aged .65 yrs [5,6]. Case-fatality rates may reach 17% in patients aged .75 yrs [5], with higher rates in those with underlying disease [1,3,5]. A substantial proportion of CAP cases requiring hospitalisation are caused by Streptococcus pneumoniae: 30-50% according to most reports [1,[7][8][9][10][11]. Bacteraemic pneumococcal pneumonia, the most severe disease form, accounts for only 10-20% of adult cases of CAP caused by S. pneumoniae, with non-bacteraemic pneumococcal pneumonia being much more frequent [1].The 23-valent pneumococcal polysaccharide vaccine (PPV) has been available in the USA for 25 yrs and is currently licensed in most developed countries. Vaccination is usually recommended for people aged o65 yrs and for high-risk persons aged .2 yrs [1,12,13]. There is a general consensus that observational studies have shown vaccination to be effective in preventing invasive pneumococcal disease [14][15][16]. However, vaccination rates are not high in most countries, partly due to doubts about the vaccine's efficacy and vaccination effectiveness in preventing n...