Protection against Plasmodium chabaudi malaria. I.--Vaccination of mice with merozoites and Freund's adjuvants.

Rodent malaria can be a useful experimental model for preliminary studies of some of the variables of a future malaria vaccine (e. g. optimal doses of merozoites to be used for immunization, nature of adjuvant, etc.). The present study shows that a partial protection can be achieved in outbred OF-1 mice after vaccination with isolated Plasmodium chabaudi merozoites and Freund's complete adjuvant. These results show the relative importance of parasite virulence, number of merozoites used for immunization and size of challenge inoculum. The optimal conditions for using such a model in vaccination experiments are discussed. In these conditions the major effect of immunization is to transform an acute lethal infection into an acute self-curing infection, which is subsequently followed by a solid state of protection against a secondary challenge.