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.[1] 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.