A pioneering scientist who developed a polio vaccine used for two years before Jonas Salk’s injectable version has died. Dr. Hilary Koprowski was 96.

Koprowski developed an oral vaccine using the live polio virus that was first used on humans in 1950.

Koprowski’s son Christopher says his father’s vaccine was the first to show clinical success. Salk famously developed an injectable version later while Dr. Albert Sabin was the first to have an oral vaccine licensed in the U.S.

But Christopher Koprowski says his father was happy with the scientific recognition he received without the celebrity of his better-known fellow researchers.

Hilary Koprowski went on to be the director of The Wistar Institute in Philadelphia from 1957 to 1991.