An analysis has been made of the effects of exposure of more than 200 human subjects to active viruses of influenza (the PR-8, F-99 and, in a small series, the F-12 strains of influenza A, and the Lee strain of influenza B). The results of the experiments, which were conducted for a number of purposes, may be summarized as follows:

  1. All four strains of influenzal virus caused febrile reactions upon inhalation of atomized allantoic-fluid cultures, but very rarely upon intranasal instillation.

  2. A febrile response was obtained in susceptible subjects with regularity only when undiluted infected allantoic fluids were atomized. Upon ten- or hundredfold dilution of the viral preparations the incidence of fever decreased markedly.

  3. The incubationary period between exposure to the virus and onset of temperature differed with the different strains. It was shorter, as a rule, with the PR-8 and Lee strains (less than 24 hours), and usually more extended (30 to 48 hours) with the F-99 virus. Previous exposure to the F-99 strain seemed to influence the incubationary period in that subjects who were re-exposed after 9 months reacted with fever within 24 hours in contrast to control individuals exposed for the first time, who became febrile generally in 30 to 48 hours.

  4. All four strains caused similar clinical manifestations except for the degree of change in the white blood cell picture. When the F-99 strain was employed leucopenia and relative lymphocytosis were frequently marked.

  5. Following inhalation, virus was re-isolated from over 50 per cent of the febrile subjects and from about 15 per cent of the afebrile subjects in the cases of the F-99 and F-12 strains, but from only one subject in the Lee series and from none in the PR-8 series. After intranasal instillation the Lee and F-12 viruses were recovered from most of the subjects but not the F-99 strain. The PR-8 virus has not been studied in this respect. Virus was re-isolated under the various conditions in some instances as late as 72 to 96 hours after exposure. Re-isolation was successful most frequently 48 hours after inhalation and 24 hours after intranasal instillation.

  6. There was no correlation between the pre-inhalation titer of antibodies and the febrile response in the PR-8 and Lee series. With the F-99 strain a definite inverse relationship was noted in two separate experiments. Thus, fever was observed in almost all subjects who showed a titer of 1:16 as measured by inhibition of agglutination of chicken erythrocytes. With increasingly higher titers the incidence of febrile reactions decreased, and finally no fever was noted in individuals with initial levels higher than 1:128.

  7. After exposure to the influenzal viruses most subjects responded with the production of homologous antibodies, i.e., after inhalation of influenza A virus an increase in anti-influenza A antibodies was noted, and conversely in the case of influenza B. A few subjects who inhaled preparations of PR-8 virus showed a rise in anti-influenza B antibodies either with or without a homologous antibody response. A number of febrile cases did not respond with formation of antibodies. The lack of a serological response in febrile cases was not limited to subjects with high titers of antibodies, but was observed also in some cases with low levels. The implications of these findings are discussed in regard to serological observations made during certain epidemics of influenza.

  8. Calculation of the geometric mean antibody response in the various groups showed that inhalation or intranasal instillation of active, or of concentrated inactivated virus as a means of immunization does not offer an advantage when compared with the serological results of subcutaneous vaccination. Irradiated allantoic-fluid cultures caused only slight or no rises in antibodies upon inhalation.

  9. The clinical data and laboratory findings are discussed in regard to the nature of the febrile response and the suggestion is made that the effects of exposure of human subjects to influenzal viruses may be the composite result of three factors: infection, with propagation of the virus, toxic manifestations, and accelerated reactions in partially immune subjects.

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