It is also true that all forms may be perfectly safe. All forms contain bacteria when prepared, and the majority of these bacteria die within a few weeks or months after preparation. On account of the mildly antiseptic quality of glycerin the bacteria in the glycerinated emulsion usually die sooner than those in the other forms of virus, and so far as bacteria are objectionable in the virus the glycerinated form may therefore be said to be somewhat preferable. It should be added, however, both that glycerinated virus is usually put in the market before the bacteria have disappeared and that the bacteria present in virus issued by well-conducted laboratories are not found to be pathogenic to persons when inoculated by the customary method of vaccination.
The ease of use of any form of virus depends largely upon the custom of the physician. In vaccinating a large number at one time there can be no question that the use of a liquid virus supplied in vials is more rapid than the use of a dried virus, as the latter has to be thoroughly moistened before it can be applied effectively.
Methods of Vaccination.—The usual method of vaccination is to scarify a spot on the skin and to rub the virus on that spot. The choice of place depends partly on æsthetic reasons and partly on convenience. To avoid the formation of an unsightly scar on the arm, the leg may be used instead. If the arm is chosen, the insertion of the deltoid is the place of election on account of the small number of lymphatics there. If the leg is chosen, the area just below the head of the fibula presents the same anatomical advantage; but a spot a short distance above the knee on the outside of the thigh is often thought to offer less opportunity for injury and infection. Choice between the sides depends in an adult on the use to which the vaccinated limb is to be put, and in a baby on the advantage of vaccinating the side which is carried away from the nurse.
The size of the scarification is important. The vesicle is always somewhat larger than the scarification, and the larger the vesicle the greater danger that the surface may be broken, and the more opportunity there is for the introduction of extraneous infection. A spot as large as the head of a medium pin is about as small as can be easily scarified, and vesicles formed on such scarifications are least liable to have inflammatory complications. If, as certain evidence tends to show, a larger area of scar guarantees greater protection, and if a larger area is therefore desired, it is better to vaccinate in two or three small spots than in one large one. It is somewhat difficult to rub the virus from a bone point on a spot of the minute size described, and as this form of virus is usually more dilute than glycerinated virus, a larger area may safely be employed.
The scarification may be made with any sharp instrument, or with the point itself. The only precaution necessary is that the instrument should be free from infection. As a scarifier the ordinary cambric needle presents the advantages that it is usually clean, is easily sterilized, and is so inexpensive that a fresh one can be used for every operation.
It is not necessary that the scarification should draw blood, although blood is not objectionable unless it flows so freely as to wash away the virus, or unless the subject has hæmophilia.
Although with a notably susceptible subject or with especially active virus it may be sufficient simply to smear the virus on the scarified area, it is usually necessary and always advisable to rub in the virus with a wooden slip or with the point firmly and thoroughly.
Other methods of introducing vaccine virus are by puncture, by deep injection, and by the mouth.