Correct Option
Developing an effective malaria vaccine is challenging primarily because natural infection in humans does not induce strong, long-lasting, or sterilizing immunity. Individuals living in malaria-endemic areas frequently experience repeated infections throughout their lives, indicating that the human immune system often fails to develop robust protective memory against the Plasmodium parasite. This lack of durable natural immunity makes it difficult to design a vaccine that can elicit a superior and sustained protective response.
Incorrect Options
Option (a): Malaria is caused by several species of Plasmodium. While it is true that multiple Plasmodium species (e.g., P. falciparum, P. vivax) cause malaria and exhibit antigenic diversity, which complicates vaccine design by requiring broader protection, this factor alone does not render vaccine development impossible. Many multi-valent vaccines or vaccines targeting conserved antigens are developed for pathogens with multiple strains or species.
Option (c): Vaccines can be developed only against bacteria. This statement is incorrect. Vaccines have been successfully developed against a wide range of pathogens, including viruses (e.g., Polio, Measles, Hepatitis B), bacteria (e.g., Tetanus, Diphtheria), and even some protozoan parasites (e.g., the RTS,S vaccine for malaria, though partially effective). The ability to develop a vaccine depends on the pathogen's biology and the host's immune response, not solely on its classification as bacteria.
Option (d): Man is only an intermediate host and not the definitive host. While humans serve as the intermediate host for the Plasmodium parasite (where asexual reproduction occurs), and the Anopheles mosquito is the definitive host (where sexual reproduction occurs), this host status does not inherently prevent vaccine development. Vaccines are designed to target specific stages of the parasite within the human host to prevent infection or disease, irrespective of the parasite's full life cycle involving another host.