Letter from Arie C. Van Ravenswaay, United States Army Air Forces to Michael Heidelberger

During World War II Heidelberger developed a vaccine against pneumococcal pneumonia that was tested among 20,000 trainees at an air base in Sioux Falls, South Dakota. The test, in which half the trainees received the vaccine while the other half did not, proved that a vaccine made from a mixture of purified capsular polysaccharides from four different types of pneumococcus was effective against the disease.