Community acquired pneumonia (CAP) is an important health problem with significant morbidity, mortality and cost. The most identified pathogen in CAP is Streptococcus pneumoniae. This was also the causative agent most frequently found in the Ovidius and Triple-P study, two consecutive clinical trials initiated by the St. Antonius Hospital Nieuwegein. Diagnosis of pneumococcal pneumonia can be based on positive blood cultures, sputum cultures, urine antigen testing or a serotype specific antibody response. When pneumococcal pneumonia is diagnosed by a positive culture, a matching serotype specific antibody response is expected. However not all patients in the Ovidius and Triple-P study with a culture proven pneumococcal pneumonia showed an antibody response against the infecting pneumococcal serotype. Patients who survived pneumococcal pneumonia are considered as a high-risk population for pneumococcal disease in the future. Possibly these patients have an impaired immune response against S. pneumoniae. In this study, pneumococcal vaccination of patients with S. pneumoniae CAP in the past enables investigating their immune response after vaccination compared to patients with CAP due another causative agent. Furthermore this study provides information to determine if there is a difference in vaccination response between pneumococcal pneumonia patients who had a culture matching serotype specific antibody response and between pneumococcal pneumonia patients who failed to elicit this response previously. Possibly these latter patients had a temporarily low titre due to the infection but another explanation is that there might be a structurally impaired immune response against S. pneumoniae or certain serotypes.