A 1:4000 formalin inactivated Japanese B encephalitis virus (OCT-attenuated 24°C line) grown in hamster kidney cell culture of pre-inactivation infectivity titer of 10-7.5/0.1 ml or more has been shown to be a vaccine which complies with the potency requirement of the Division of Biologics Standards, having a minimal immunogenic dose (MID) of 0.02 ml or less. The antigenicity was not affected by prolonging the inactivation period to three times that required for apparent destruction of infectivity at either 30° or 37°C. The 37° formalin inactivation was selected, for both the shorter period of inactivation needed and the minor improvement in potency obtained. A direct correlation was shown between the preinactivation infectivity titer and the final vaccine potency, at least within certain ranges. Infectious virus rather than hemagglutinin appeared to be responsible for the antigenicity, though under certain circumstances the hemagglutination and the infectivity were parallel. The use and limitations of the mouse potency test were examined and modifications employed. Both the monkey and guinea pig elicited good antibody formation subsequent to vaccine inoculation and good evidence was found to support the possibility of utilizing the latter for an antigen extinction test that may overcome certain limitations of the mouse MID test.

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