During the height of the epidemic of influenza in New Orleans more than five thousand persons were vaccinated by us with a specially prepared protein suspension of the Pfeiffer bacillus. Of this number approximately 90 per cent did not contract influenza, either during the period of the epidemic or its recrudescence which occurred two months afterwards. This percentage compared with that of our group controls (those refusing vaccin) and the city statistics indicate that a considerable degree of protection was established in all those who were completely vaccinated and it is reasonable to suppose that the protection was the result of the inoculation with B. influenzae antigen.

The culture used in the preparation of the vaccin was grown upon the surface of hemoglobin nutrient agar at a temperature of 37°C. for thirty-six to forty-eight hours, when the growth was washed off and suspended in normal saline solution and immediately devitalized by being saturated with chemically pure chloroform. In our experience the highest potency vaccin was obtained from the influenza culture, which was killed with chloroform. With this agent the viability of the bacilli was destroyed in a few seconds without apparently causing any deleterious effect upon the protein immunizing substance. While it is recognized that chloroform in the presence of water and under the influence of light (actinic rays) in the course of time decomposes and give rise to COCl2 (carbonyl chloride) and HCl (hydrochloric acid), which are poisonous substances, it can be stated that these compounds are not evolved in the chloroform vaccin prepared in the manner herein described. In this connection it may be said that the devitalizing agent is left in contact with the saline suspension of bacilli for a short period of time and is then completely removed from the mixture.

In regard to the behavior of chloroform upon the vaccin we believe that it devitalizes the bacilli by rapidly absorbing the water and in consequence increasing the permeability which results first in plasmolysis then rupture of the bacterial cell, liberating without effecting in any manner its toxic moiety. There is nothing to show that chloroform per se has any direct chemical action upon bacterial protein whether alone or in combination with water and sodium chloride. On the other hand, this cannot be said for heat and the phenol derivatives when used as vaccine devitalizing agents. Here the destruction of bacterial life is due to a direct action upon the protoplasm, which is coagulated to a greater or lesser extent and in consequence there is destroyed at least a part of the antigenic property of a protein (7, 8). Therefore, in our opinion chloroform as a germicide in the preparation of vaccin has distinct advantages over all other agents commonly employed. The use of heat, tricresol, etc., for killing the culture, while effective from a germicidal standpoint, injures to some degree the immunizing property of the protein and particularly is this the case with B. influenzae.

Influenza vaccin, freshly prepared with strict attention to preserving its maximum amount of antigenic value, will excite in the human host the production of specific immune bodies, the degree depending not so much upon the quantity as the quality of the antigen injected. Therefore, the freshness of the vaccin and particularly the method of its preparation are factors of paramount importance if results are to be obtained in the prophylaxis against epidemic influenza. We believe that failure to obtain sufficient protection is, to a large extent, the fault of the vaccin employed. A contrast study of the results obtained with influenzal vaccins devitalized by heat, tricresol and chloroform will show a wide variation in antigenic effectiveness. The culture killed by heat at 56°C. is practically worthless as an antigen while that prepared by the phenol derivatives even in 0.25 per cent strength is altered in its power to produce antibodies.

While our total number of vaccinated persons is too few to permit of the statement that B. influenzae vaccin is a constant specific in the prevention of the disease clinically known as epidemic influenza, it can be said, however, that the results are interesting and significant from the standpoint of prophylaxis.

While the duration of specific antibodies in the blood of the host varies within wide limits for different individuals, it may be said that these substances remain in the circulation for a period of ten weeks on the average, as shown by the agglutination and complement fixation tests. In many isolated cases, however, these tests are positive six months after vaccination.

There occurred in all of the individuals vaccinated a local reaction at the site of inoculation. Usually this was simply a small sharply circumscribed area of erythema; however, in fully 30 per cent of individuals the reaction involved a greater area and there was considerable oedema of the subcutaneous tissues in the vicinity of the inoculation site. Even in the more severe type of local reaction there was a complete subsidence in three to five days.

Constitutional effects following the administration of the vaccin were noted in 90 per cent of the individuals receiving it. In 30 per cent of the cases this host response simulated in symptom-complex the early toxemia of true influenzal infection; however, the reaction subsided in six to twelve hours after the onset. This type of reaction was so striking in its analogy to the early stage of the natural infection that it strongly suggested proof of the relationship between the clinical diseases known as influenza and the artificial toxemia produced by the injection of the Pfeiffer bacillus vaccin.

From the observations herein reported for our series of vaccinations we believe there is every indication that the influenza protein employed gives rise to the production of protective substances and therefore justifies its use in the prophylaxis of epidemic influenza.

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