Acute respiratory infections, often due to Streptococcus pneumoniae (pneumococcus), are a primary cause of death in young children in developing countries. A new vaccine effectively prevents the most serious form of pneumococcal disease and also reduces nasopharyngeal colonization with pneumococci. Because only some people who are infected become ill, researchers must study tens of thousands of vaccinated individuals over a long period of time to determine whether the vaccine guards against disease. Dr. Käyhty is leading an international consortium of investigators whose goal is to establish a quick and inexpensive method of determining the efficacy and expected effectiveness of the pneumonia vaccine.