The Development of a Formalin-Killed Rift Valley Fever Virus Vaccine for Use in Man

Abstract

Discussion and Summary This study has provided the opportunity to compare and evaluate the immunogenic capacity of several types of formaldehyde-inactivated Rift Valley fever (RVF) vaccines. The vaccines were prepared from chick embryo and mouse brain tissues infected with the neurotropic strain of RVF virus and from monkey kidney cell cultures infected with either the neurotropic or the pantropic viruses. Although the neurotropic virus was readily propagated in embryonated chicken eggs, inactivated vaccines prepared from such tissues were of low immunogenicity. On the other hand, vaccines prepared from brain tissue of mice and monkey kidney cell cultures infected with the neurotropic strain induced in mice and monkeys appreciable levels of neutralizing and complement-fixing antibodies; moreover, immunized mice possessed significant resistance when challenged intracerebrally with the homologous virus or intraperitoneally with the pantropic virus (Tables I and II). Indeed, these two vaccines prepared from the neurotropic strain induced immunity in mice which compared favorably with that elicited by a living attenuated neurotropic virus vaccine. However, both of these vaccines were of limited value in human beings since they elicited a significant concentration of neutralizing antibodies (neutralization index of log 2.0 or greater) in the sera of only about 50% of the persons inoculated. Formaldehyde-inactivated vaccines prepared from monkey kidney cell culture material infected with the pantropic strain of Rift Valley fever virus were safe and effective for the immunization of mice, monkeys and man. Such preparations induced appreciable levels of neutralizing antibodies in the serum of human beings and animals, and, in the latter, a solid resistance to challenge with homologous virus. Moreover, immunization of cynomolgus monkeys with this type of inactivated RVF vaccine prevented viremia in all animals subsequently challenged by the intraperitioneal route with at least 106 mouse i.p. LD50 of the pantropic strain. Laboratory personnel vaccinated with this type of vaccine who subsequently handled pantropic RVF virus in their work developed no evidence of laboratory infection, i.e., neither clinical disease nor an increase in neutralizing antibodies for 18 months postvaccination when at this time they received a 1.0-ml booster dose of vaccine. Among the potency tests adapted for the inactivated pantropic type vaccine one was of the extinction type. Such assays showed that consecutively prepared lots of vaccine were of relatively uniform potency (Table VI) and that a direct relationship existed between the concentration of the vaccine administered and the levels of neutralizing antibody and of resistance induced (Table V). The serologic data indicate that those vaccinated persons who developed appreciable levels of neutralizing antibodies, which persisted for many months, usually failed to develop detectable CF and HI antibodies. This is in contrast to the findings in human cases of RVF where Gear et al. (9) demonstrated high levels of both neutralizing and complement-fixing antibodies during convalescence. It may be noted that vaccinated mice occasionally developed complement-fixing antibodies and less frequently hemagglutination-inhibiting antibodies. None of the lots of formaldehyde-inactivated vaccine discussed in this paper contained any detectable infectious RVF virus using the exhaustive safety tests herein described. The potency of inactivated pantropic virus vaccines remained unchanged during storage for 6 to 8 months at 4 to 6°C. Several lots of the pantropic type of vaccine have been used to immunize more than 1000 persons, none of whom displayed untoward reactions.