We have adduced evidence that guinea pigs can be completely or partially protected by three injections of typhus tunica material in which there are moderate numbers of Rickettsiae, treated for from 24 to 48 hours with a 0.2 per cent formalin solution.
We believe that the immunization is due to the presence of the Rickettsiae, since in our preceding experiments we have satisfied ourselves that these organisms are the true etiological factors of the disease. For the reasons stated above, we believe that the formalinized vaccine does not contain living, but attenuated organisms, and that the immunizing effect is the result of treatment with formalin-killed Rickettsiae. This point, however, we admit, is not absolutely determined.
These experiments, together with the results obtained in the concentration of Rickettsia material by the diet method of reducing resistance as described in the paper which follows, furnish a hopeful method and a reasonable theoretical basis for a procedure of active immunization against this disease in human beings.