A series of rabbits was subcutaneously injected with three measured doses of killed cultures of two strains of Bacterium pneumosintes derived from the nasopharyngeal secretions of influenza patients. These rabbits were subsequently tested for the development of serum antibodies and for the presence of an induced immunity to the living organisms, with the following results.

The serum of eleven of fifteen rabbits, tested from 10 to 27 days after the final subcutaneous injection, specifically agglutinated Bacterium pneumosintes, whereas normal rabbit serum did not.

Nineteen vaccinated rabbits were subjected to protection experiments. Two of them were unaffected by an intratracheal injection of Bacterium pneumosintes, contained in the lung tissues of previously infected animals, in a dose which typically affected the control rabbits. Fifteen of the other seventeen proved to be completely resistant when tested by intratracheal injections of Bacterium pneumosintes cultures that produced typical infections in the controls. Ten of these fifteen rabbits were injected intravenously with living cultures of pneumococcus, Streptococcus hamolyticus, or Bacillus pfeifferi in doses which were non-infective under normal conditions, but infective, as experience has shown, in the presence of a primary lesion caused by Bacterium pneumosintes. In none of these animals did infection develop. The two remaining rabbits of the seventeen were not protected against Bacterium pneumosintes by the vaccination, and they further developed a secondary pulmonary infection with Bacillus pfeifferi after its intravenous injection. Control rabbits similarly injected intratracheally with Bacterium pneumosintes, and then intravenously with the pneumococcus, streptococcus, or Bacillus pfeifferi in doses that had proved non-infective for normal rabbits, uniformly developed a secondary infection with these organisms.

The mildness of the local reactions and the absence of general signs, following vaccination with Bacterium pneumosintes, indicate that similar injections would be well tolerated in man. There is no evidence that the subcutaneous injection of large doses of the heat-killed organisms reduces the resistance of the animal body to infections with other bacteria. In single rabbit experiments the resistance to intravenously injected pneumococci, streptococci, or Bacillus pfeifferi has been found unreduced immediately after vaccination with Bacterium pneumosintes.

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