OBJECTIVES: In adults, the incidence of S. pneumoniaeinfections and the related mortality increase with age and co-morbidities. The Belgian Superior Health Council recommends use of 13-valent pneumococcal conjugate vaccine (PCV13) in adults with elevated risk of pneumococcal diseases. PCV13 has recently proven efficacious in preventing pneumococcal pneumonia and invasive pneumococcal disease (IPD) in the elderly. We investigated the cost-effectiveness of vaccinating the 65-84 y. old cohort (n=862,188) that suffer from co-morbidities or immunosuppression METHODS: A cohort model with a Markov-type process was developed to project the lifetime risks and related costs of IPD and nonbacteremic pneumococcal pneumonia.  Input data came from various sources (literature, existing databases, and observational studies) and were reviewed by a panel of Belgian experts.  PCV13 effectiveness was derived from the recently published CAPiTA clinical trial results for the first 5 years.  Protection gradually declined thereafter to zero by year 16.  Belgian National Health perspective was taken with costs and quality-adjusted life years (QALYs), discounted annually by 3% and 1.5%, respectively.  Sensitivity analyses on key parameters were performed in order to test the robustness of model findings. RESULTS: