Abstract
The purpose of this paper is to present additional evidence that antigenically different, although related, strains of virus may be prevalent during the same outbreak of influenza. It previously has been observed that cases of influenza that occur during the same large epidemic are not all caused by antigenically identical agents. 1 The present data confirm that previous observation, and are of especial interest because they deal with an outbreak of influenza that was a particularly well localized one.
The influenza outbreak with which the investigation was concerned and which consisted of 40 clinically diagnosed cases of epidemic influenza, was observed in the nurses' infirmary of The New York Hospital during January and February, 1941. Throat washings obtained from 11 of the persons were tested for virus, and in 5 instances strains of influenza virus were isolated and adapted to Swiss mice. The 5 strains were compared with each other and with the PR8 2 by means of mouse protection tests in which each strain of virus was tested against the same group of antiserums. The antiserums were obtained from 6 rabbits each of which had been inoculated with a different strain of virus: bleedings were obtained from each rabbit 9 or 10 days and 3 to 4 weeks after a single intraperitoneal injection of a suspension containing 2% infected mouse lung in 0.85% saline. The virus suspensions used in the different protection tests were all comparable in that they contained the same magnitude of lethal doses of virus (from 300 to 600) for mice. The results of the tests are presented in Chart 1.
In Chart 1, the results are summarized in the form of graphs in which the points represent the protective antibody titers of the different serums against the particular strain of virus. The titers are expressed as the initial dilution of serum that would protect 50% of the mice from death.
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