Jonas Salk began his career in microbiology working with the inactivated influenza vaccine. He would later use the formalin inactivation process to create the poliomyelitis vaccine. Using a newly created modern cell culture technique, Salk was able to mass produce vaccines for the large-scale polio vaccine field trial. Under the direction of Thomas Francis, Jr., the field trial utilized both a double-blinded placebo-controlled trial and an observational study, and included almost 2 million American schoolchildren as subjects. The vaccine proved to be both safe and effective. Although later replaced by the Sabin oral vaccine, the Salk inactivated vaccine played a large part in the eventual eradication of polio from the Western Hemisphere. After his work on the polio vaccine, he created the Salk Institute, a private institution dedicated to research. Subsequently, until his death in 1995, Salk conducted research on an HIV vaccine.