In the 1881 Pasteur anthrax experiment, what protected vaccinated sheep from the disease?

In the 1881 Pasteur anthrax experiment, an anthrax vaccine protected vaccinated sheep from the disease.

Louis Pasteur and his collaborators tested a weakened preparation of the anthrax bacterium on farm animals at Pouilly-le-Fort in France. Twenty-five sheep received the vaccine in two doses, while an equal group remained unvaccinated. Later, both groups were exposed to a virulent anthrax culture.

The vaccinated sheep survived, while the unvaccinated sheep died or became gravely ill. The public demonstration helped establish the practical value of laboratory-made vaccines and strengthened Pasteur’s germ theory of disease.

Pasteur’s anthrax vaccine was not the same as a modern purified vaccine. It used attenuated bacteria, and the precise method of weakening the organism was not fully disclosed at the time. The experiment also differed from Pasteur’s earlier work on chicken cholera and later rabies research. Its central result was specific: prior exposure to a weakened anthrax preparation could protect animals against virulent anthrax.