Studies on the Immunization of Swine against Infection with the Swine Influenza Virus

Abstract

Summary A disease easily characterizable and measurable in severity by clinical observations on the height and duration of fever, loss of weight and anorexia was produced in normal, unvaccinated swine by exposure to active swine influenza virus contained in chick-embryo chorio-allantoic fluid. The animals were uniformly highly susceptible to infection, giving a morbidity score of 94.1 per cent, but no deaths occurred. There was no evidence of synergistic bacterial infection. Vaccination with formalin-inactivated, purified swine influenza virus given subcutaneously produced an overall reduction in the incidence and severity of the clinical disease, resulting in a morbidity score of 59 per cent. No significant difference could be seen between the effects of 1, 2 or 3 injections of 0.5-mg amounts of the inactive virus, though the respective morbidity scores were numerically slightly smaller with repeated vaccinations. The results were not related to the age or size of the animals nor to the time of year of the tests. The antibody titers of the vaccinated animals were consistently greatly increased following exposure to active virus, whether symptoms developed or not. The levels of antibody titer 2 weeks after exposure to active virus were related directly to the number of vaccinations preceding the exposure. The degree of immunity or resistance conferred by vaccination, 37 per cent, was far less than that induced by exposure to active virus, 82 per cent, regardless of the severity or mildness of the disease, and the same was true for animals tested with active virus a second time after previous vaccination and exposure to active virus. The levels of antibody titer of animals in these categories rose little, or not ar all, on repeated exposure to active virus, indicating a very low degree, if any, of asymptomatic infection under these conditions. The findings are discussed in relation to their bearing on the control of influenza in man by vaccination.