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SummaryAn attenuated strain of Venezuelan equine encephalomyelitis (VEE) virus has been employed for the immunization of persons at the risk of infection with virulent strains of this virus.Systemic reactions following inoculation of either 50th or 80th passage virus occurred in approximately 40% of persons who had previously received several doses of killed vaccine. In persons who had not received the vaccine such reactions occurred in approximately 85%.The attenuated virus elicited immunity in man against challenge with virulent unmodified virus.A substrain was derived by plaque selection which differed from the original attenuated virus in its reduced capacity to induce untoward reactions in selected laboratory hosts and in man.Animals inoculated with the substrain of attenuated virus resisted challenge with 109 lethal doses of four other strains of virulent VEE virus.
McKinney et al. (Mon,) studied this question.