In 1947, Salk accepted an appointment to the University of Pittsburgh School of Medicine. In 1948, he undertook a project funded by the National Foundation for Infantile Paralysis to determine the number of different types of polio virus. Salk saw an opportunity to extend this project towards developing a vaccine against polio, and, together with the skilled research team he assembled, devoted himself to this work for the next seven years. The field trial set up to test the Salk vaccine was, according to O'Neill, "the most elaborate program of its kind in history, involving 20,000 physicians and public health officers, 64,000 school personnel, and 220,000 volunteers." Over 1,800,000 school children took part in the trial.[4] When news of the vaccine's success was made public on April 12, 1955, Salk was hailed as a "miracle worker," and the day "almost became a national holiday." His sole focus had been to develop a safe and effective vaccine as rapidly as possible, with no interest in personal profit. When he was asked in a televised interview who owned the patent to the vaccine, Salk replied: "There is no patent. Could you patent the sun?"[5]