Eruptions are probably usually due to a chemical irritation produced by the development of the vaccinia; they are analogous to the eruptions following the injection of antitoxine and the ingestion of various drugs. They vary in appearance, sometimes resemble the eruption of measles or of scarlet fever, and again are urticarial; they are macular, papular, and vesicular.
When a moist eczema is present there may be auto-inoculation of the pock on the affected area and a general confluent vaccine eruption appear.
Immunity.—The immunity against smallpox, or vaccinia, produced by vaccination is of gradual growth, and is not complete until the period of suppuration, about the beginning of the second week. Natural immunity is said to exist, and is probable, but it is exceedingly rare. Vaccination of a pregnant woman rarely, if ever, confers immunity on the fœtus.
Duration of Immunity.—Sometimes a single vaccination gives immunity for life. Usually, however, susceptibility returns at latest seven to ten years after vaccination, and the second vaccination may give immunity for the rest of the lifetime, or susceptibility may return again and again. Failure of active, properly inserted virus shows only that the person so vaccinated is at that time immune, but conveys absolutely no information about the condition a few months later. The appearance of the scar is not a trustworthy guide as to immunity. Susceptibility to vaccination returns frequently within one year, and has returned in three months from the time of a successful vaccination. Susceptibility to smallpox probably returns, as a rule, later than susceptibility to vaccinia. It is rare that a case even of varioloid occurs within five years of a successful vaccination.
Conclusions.—Every child should be vaccinated at the time of election during the first year of life, and should be revaccinated before beginning school-life with its possibility of exposure. Every person, no matter at what age, should be vaccinated at a time of possible exposure to smallpox unless he has been successfully vaccinated within three months.
VARIOLA ERYTHEMATOSA.
(First day of eruption).
VARIOLA PAPULOSA.
(Second day).