Serological Response to Japanese B Encephalitis Vaccine of Children and Horses Immune to St. Louis Virus.

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Abstract
Three children convalescent from St. Louis encephalitis and a group of 16 horses, 13 of which had naturally acquired antibodies to St. Louis virus, were given one injection with a killed Japanese B encephalitis virus vaccine. After 10 or 11 days all individuals previously infected with St. Louis virus showed complement fixing antibody rises to Japanese B and St. Louis viruses and most also showed neutralizing antibody rises, particularly to Japanese virus. Those horses not previously immune to St. Louis virus showed no response. The response of these St. Louis immune hosts was identical to that shown by persons with naturally acquired immunity to Japanese virus. Normal individuals show no such response even after several injections of vaccine. This immediate, secondary type immunologic response to an injection of vaccine is not specific for Japanese virus but to an antigen common to Japanese and St. Louis viruses and probably to other viruses in this complex. It is suggested that immunity acquired by active infection with one member of the group may serve as good basic or primary immunization for other representatives and a high and probably effective protection response can be stimulated subsequently by giving one "booster" injection of a killed virus vaccine representating an immunologically related virus.