Demonstration of the Efficacy of Vaccination Against Influenza Type A by Experimental Infection of Human Beings

Abstract

Summary The efficacy of allantoic fluid vaccines in the prevention of experimental human infection with the virus of influenza A has been studied. Of 28 control individuals, 10 came down with clinical influenza following inhalation of a recently isolated active virus. Only 1 case, which had not responded to vaccination, occurred among the 44 vaccinated persons, 27 of which had been immunized 4 months prior to the exposure. There was good correlation between the pre-inhalation antibody level and the degree of protection in that most of the clinical cases occurred in the group with the lowest antibody titer as measured both by mouse protection and inhibition of red cell agglutination technic. Increase in the antibody level decreased the morbidity. The clinical picture of the experimental disease confirmed the observation of earlier investigators. Since a number of individuals harbored pathogenic organisms in the upper respiratory tract, all clinical cases were treated with sulfonamides. No complications were encountered. Subclinical infections were numerous, as shown by slight elevations of temperature, mostly on the second or fourth day after inhalation, leucopenia below 5000, or rises in antibodies 2 weeks following the exposure. These symptoms were independent of each other in that either one alone or any combination of the three could be found. Virus was re-isolated only from clinical cases. Various questions concerning the subclinical infections are discussed.