Although various observations suggest that vaccination against malaria may be possible, a number of factors suggest that development of a vaccine conferring effective long-lasting immunity against all or most strains and species will be difficult. Recrudesent infections may last as long as 2 years for P. falciparum to 30 years for P. malariae. Natural immunity builds slowly, and sterile immunity is rarely achieved (Miller, 1985; Perrin et al., 1984).
It may be possible, however, to raise the level of immunity (e.g., to sporozoite antigens) to a greater extent by vaccination since natural exposure to sporozoite before sequestration of the parasite in the liver is brief. In the case of antigenic fragments (e.g., from sporozoites), it may be possible to develop vaccines conferring long-lasting immunity by coupling fragments to carriers (e.g., toxoids), or by incorporating fragments into immunogenic bacterial fusion proteins. That protracted exposure to the natural disease is required for the development of immunity suggests that the parasite has evolved mechanisms for evading the human immune response. This may be particularly problematic in selecting merozoite antigens as candidate immunogens. Knowledge of parasite immune variation is incomplete, as is knowledge of the roles of humoral and cell-mediated responses in combatting the disease. These and other potential problems are discussed more fully by Ravetch et al. (1985).
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