The malaria parasite has survived for more than 50,000 years, and natural selection favors strains of the organism with mutations that help them evade threats. This is why more and more drug-resistant parasites and insecticide-resistant mosquitoes are emerging. Natural selection favors those plasmodium parasites that are able to frequently change their surface antigens to the evade immune attack on their hosts.
This understanding is critical in the development of the vaccine. Since the targeting of surface antigens could be rendered the vaccine only temporarily effective, due to their rapid changes, vaccines could be developed find other targets that are stable such as those involved in the reproduction of the parasite.