Protection of mice against malaria by a killed vaccine: differences in effectiveness against P. yoelii and P. berghei.

(C57BI x Balb/c)F1 mice are normally killed by a strain of the malaria P. yoelii, but they can be fully protected by a killed vaccine. The best results were obtained with saponin-lysed parasitized cells subsequently fixed with 0-06% formalin and injected intravenously with the adjuvant B. pertussis, though good protection was also obtained without the adjuvant. The protection was specific and at least partly mediated by a serum factor. A similar regime gives little or no protection against P. berghei. Possible reasons for this difference are considered and the mechanisms by which vaccination works against P. yoelii are discussed.