War II the Koprowskis settled in Pearl River, New York, where Hilary was hired as a researcher for Lederle Laboratories, the pharmaceutical division of American Cyanamid. Here he began his polio experiments, which ultimately led to the creation of the first oral polio vaccine. Koprowski served as director of the Wistar Institute, 1957–91, during which period Wistar achieved international recognition for its vaccine research and became a National Cancer Institute Cancer Center. Koprowski died on April 11, 2013, aged 96, in Wynnewood, near Philadelphia, Pennsylvania, of pneumonia. He and his wife are buried at West Laurel Hill Cemetery, Southlawn Section, Lot 782, Bala Cynwyd, Pennsylvania. Hilary Koprowski and his late wife had two sons. Their first child Claude (born in Paris, 1940), who died in 2020, was a retired physician. Their second son, Christopher (born 1951) is a retired physician certified in two specialties, neurology and radio-oncology. He is also the former chair of the department of radiation oncology at Christiana Hospital in Delaware. Polio vaccine While at Lederle Laboratories, Koprowski created an early polio vaccine, based on an orally administered attenuated polio virus. In researching a potential polio vaccine, he had focused on live viruses that were attenuated (rendered non-virulent) rather than on killed viruses (the latter became the basis for the injected vaccine subsequently developed by Jonas Salk). Koprowski viewed the live vaccine as more powerful, since it entered the intestinal tract directly and could provide lifelong immunity, whereas the Salk vaccine required booster shots. Also, administering a vaccine by mouth is easy, whereas an injection requires medical facilities and is more expensive. Koprowski developed his polio vaccine by attenuating the virus in brain cells of a cotton rat, Sigmodon hispidus, a New World species that is susceptible to polio. He administered the vaccine to himself in January 1948 and, on 27 February 1950, to 20 children at Letchworth Village, a home for disabled persons in Rockland County, New York. Seventeen of the 20 children developed antibodies to polio virus — the other three apparently already had antibodies — and none of the children developed complications. Within 10 years, the vaccine was being used on four continents. Albert Sabin's early work with attenuated-live-virus polio vaccine was developed from attenuated polio virus that Sabin had received from Koprowski. Rabies vaccine In addition to his work on the polio vaccine, Koprowski (along with Stanley Plotkin and Tadeusz Wiktor) did significant