Japanese B encephalitis virus vaccines from tissue culture. VII. Formalin inactivated Nakayama strain vaccine.

Abstract

SummaryThe Nakayama strain of JBE virus which, was grown in HKC and harvested in medium 199 with 2% H. Al., was inactivated by 1:4000 formalin and tested for potency. The inactivation curves at 30° and 37°C were biphasic with the inflection points occurring late in the reaction at a low survival ratio of 10-5 or less. The second phase of inactivation proceeded at a rate comparable to that of heat of the same degree alone, resulting in a great prolongation of the time required for inactivation. The antigenic potency appeared to decrease with prolongation of the inactivation time and when the triple inactivation period was achieved the potency of most lots was unacceptable. The OCT-attenuated strain of JBE virus prepared previously in the same manner and with comparable infectivity titers prior to inactivation was inactivated in less than half the time required by the Nakayama virus and consistently yielded a more potent vaccine.