Abstract
Conclusions
(1) The immunological identity of 4 strains of W.E.E. virus was confirmed by tests in mice. (2) Results of comparative tests for their antigenic potency suggested that the one most recently isolated may be somewhat superior to the older laboratory strains. (3) Mice vaccinated with comparable doses of either of the 4 strains showed equal degrees of resistance to 3 of the strains, while tests with the 4th, the current R.I. strain, indicated that this strain is not a sensitive indicator of the degree of immunity of vaccinated mice. (4) Parallel tests in which vaccinated mice were challenged intracerebrally with the current and with earlier samples of the R.I. strain indicate that after its 9th or 10th year of propagation in laboratory mice, certain changes may have taken place which rendered this strain increasingly less suitable for use in challenge tests for immunity.
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