Dr. Von Niemeyer, Professor of Medicine at Tübingen, 1878.

The remarkable fact that, even at the present day, extensive epidemics of small-pox occur in spite of most persons being vaccinated or even revaccinated, once or oftener, has induced me to make some very careful observations regarding the duration of the protective power of vaccination; I have not yet completed this work, but have gone far enough to satisfy myself that the protection afforded against variola by vaccination, is for a far shorter period than is generally supposed.