Relation between Degree of Immunity of Mice following Vaccination with St. Louis Encephalitis Virus and the Titre of the Protective Antibodies of the Serum

Vaccination of susceptible mice with St. Louis encephalitis virus induced an acquired immunity to intracerebral inoculation which appeared within 1 week. When first demonstrable this immunity was at a very high level and it remained so for about 6 weeks, after which time it declined and disappeared completely between the 12th and 20th week after vaccination. Humoral antibodies of significant titre could not be demonstrated during the first 6 weeks after vaccination when actual immunity to intracerebral inoculation was at its highest level. Antibodies were first detected 8 weeks after vaccination, when immunity was decreasing. The humoral antibody titre reached its peak at 23 weeks, at which time immunity had completely disappeared. A high level of circulating antibody was still present 37 weeks after vaccination. The evidence presented leads to the conclusion that humoral antibodies do not play a major part in the immunity acquired by susceptible mice after vaccination against St. Louis encephalitis virus.