Vaccination (Lat. Vacca, A Cow), inoculation for cowpox as a protection against smallpox, first practised by Dr. Edward Jenner in 1796. (See Jexner.) On the 2d or 3d day after virus taken from a perfect vaccine vesicle, whether from the cow or the human subject, is placed in contact with the denuded dermis or true skin, the puncture is observed to be slightly inflamed. On the 4th or 5th day a vesicle is observed surrounded by a slight blush of inflammation, and containing a little colorless, transparent fluid. This increases until the 8th day, when it should be from a quarter to half an inch in diameter, the blush of inflammation surrounding it at the same time having become more marked. The vesicle is umbilicated, that is, its centre is depressed below the level of the circumference, in this respect resembling the pustules of smallpox. It is compound, being made of 10 to 14 distinct cells; one of these, carefully punctured, discharges a minute drop of fluid, leaving the other cells still distended. On the 8th day the blush of surrounding inflammation, heretofore very slight, begins to extend, forming what is termed the areola; it attains its greatest diameter by the 11th day, after which it gradually fades and disappears.

With the appearance of the areola the vesicle becomes darker and dryer, and gradually concretes into a brown translucent crust, which falls off about the 20th day, leaving a circular depressed cicatrix. About the 8th or 9th day there is usually some slight febrile disturbance, though it is often scarcely noticeable. Such is the course of the true vaccine vesicle when uninterfered with, either by the presence of constitutional disease or by the accidental occurrence of inflammation. - When vaccination was first introduced, it was believed that it would afford in all cases complete and permanent protection from smallpox. But it was discovered that those who had been well and thoroughly vaccinated were still liable to some extent to attacks of smallpox; and though in general the disease was modified (varioloid) and rendered shorter and milder, still it occasionally resulted in death. The degree of protection afforded by vaccination thus became a question of great interest. Its extreme value was easily demonstrated by statistical researches. In England, in the last half of the 18th century, out of every 1,000 deaths, 96 occurred from smallpox; in the first half of the present century, out of every 1,000 deaths, but 35 were caused by that disease.

The amount of mortality in a country by smallpox seems to bear a fixed relation to the extent to which vaccination is carried out. In all England and Wales, for some years previous to 1853, the proportional mortality by smallpox was 21.9 to 1,000 deaths from all causes; in London it was but 16 to 1,000; in Ireland, where vaccination was much less general, it was 49 to 1,000, while in Connaught it was 60 to 1,000. On the other hand, in a number of European countries where vaccination was more or less compulsory, the proportionate number of deaths from smallpox about the same time varied from 2 per 1,000 of all causes in Bohemia, Lombardy, Venice, and Sweden, to 8-33 per 1,000 in Saxony. Although in many instances persons. who had been vaccinated were attacked with smallpox in a more or less modified form, it was noticed that the persons so attacked had been commonly vaccinated many years previously. The mere lapse of time in many cases seems sufficient to destroy the protective influence of vaccination. The duration of the protective influence varies with different individuals.

The same thing happens with regard to the protective influence of an attack of smallpox itself; in most persons it lasts for life; many, after a period more or less prolonged, are liable to a second attack; while cases have occurred in which a third attack has proved fatal. In all cases revaccination seems to be a test of the loss or presence of the protective influence; to render this test certain, where revaccination does not succeed on the first trial, it should be carefully performed a second time. In the Prussian army in 1848, 28,859 individuals were revaccinated, in 6,373 of whom the cicatrices of the preceding vaccination were indistinct or invisible. Of these, 16,862 had regular vesicles, 4,404 irregular vesicles, and in 7,753 cases no effect was produced. On a repetition of the vaccination in these last, it succeeded in 1,579 cases. Among the whole number successfully revaccinated either in 1848 or before, there was but a single case of varioloid, and not one case of smallpox; while seven cases of varioloid occurred either among the recruits or among those revaccinated without success.