Cross Immunity Among Strains of Chlamydia Psittaci

The degree of cross protection induced by immunization with vaccines derived from a representative group of psittacosis, human pneumonitis, and related chlamydiae is reported. Nonviable vaccines were prepared by growing the organisms in human diploid cell culture WI-38, followed by inactivation with formaldehyde. Swiss-Webster mice were immunized by three intraperioteneal injections of vaccine and challenged intraperitoneally after 3 weeks with graded doses of the infectious chlamydiae. Results were expressed as the protective index, which represents the difference between the log lethal titers of the challenge suspension in normal and in immunized mice.