DURATION OF IMMUNITY FOLLOWING MODERN SMALLPOX VACCINE INOCULATION.

IT WAS believed and taught by Jenner that a person was protected for life against smallpox when he developed a typical reaction following inoculation with cowpox vaccine. In this Jenner erred, for even before his death many writers described the observation of short lived immunity, following this form of protection. To-day we limit our statements regarding the duration of such immunity by saying that a person will be immune for several years after successful inoculation, several being as close an estimate of the duration of the immunity as any experienced inoculator cares to make at this time. A century ago it was possible to test Jenner's original contention by subsequently inoculating the person with the virus of smallpox itself and observing the resulting reaction. But direct inoculation of smallpox virus has since become illegal and there now remains only one practical way by which the immunity can be tested; namely, to re-inoculate with vaccine and to observe the resulting reaction. When an inoculator is familiar with the different types of reaction, he can read, from the type of the reaction that develops, the degree of immunity retained by the person at the time of re-inoculation. This method is open to the objection that repeated successful re-inoculations will probably render a person permanently immune. Observations during the past century in Europe tend to show that three successful immunizations at different age periods, as, for instance, the first during the first year of life, the second on entering school at about 7 years, and the third during adolescence, have rendered the great mass of such people immune for life. A good opportunity was offered recently in a community in California to test the degree of immunity retained several years after modern smallpox immunization, but the campaign was half over before this opportunity was realized; otherwise more careful records with regard to the ages of those inoculated, and other information would have been obtained. Many reactions were seen which were so at variance with all my previous experience that they are here recorded.