1.The immunity which developed in volunteers as a result of suppressed infection with R. tsutsugamushi, obtained by inoculation of living vaccine followed by a course of chloramphenicol chemoprophylaxis, was essentially the same as that displayed by volunteers who, following inoculation, experienced overt disease which was terminated by specific therapy.
2.Immunity to the homologous strain of rickettsiae persisted at a high level in most persons for at least 1 year and in some for longer periods.
3.Resistance to infection with heterologous strains of R. tsutsugamushi was of a transient nature. Even within a month after inoculation with one strain, an appreciable number of persons became ill following injection with a heterologous strain. By the end of a year, all were again susceptible to heterologous infection. During the period of waning immunity, the disease which resulted from the heterologous strain was modified from the classical picture. Indeed, the illness observed in the test shortly after recovery was so mild that it would probably not have been recognized as scrub typhus if rickettsemia had not been demonstrated. On the other hand, the disease which resulted when the individuals were tested after 1 year generally presented the typical picture of scrub typhus.