Vaccinologists pursue different approaches to developing malaria vaccines.
Malaria vaccine scientists evaluate different vaccine candidates and formulations that are designed to stimulate the immune system to destroy or arrest the malaria parasite. A malaria vaccine's desired actions can take place at several points during the life cycle of the parasite.
The most significant challenge that malaria vaccine scientists face is a lack of understanding of the specific immune responses associated with protection against the parasitic disease. Because the malaria parasite is so complex, scientists pursue a diversity of vaccine development approaches. Many believe that a malaria vaccine will need to encompass more than a single approach to reach a high degree of efficacy.
Early malaria vaccine development efforts focused on the parasite's pre-erythrocytic stage—the period during which the organism, in the form of a sporozoite, enters a person's blood stream and heads for the liver, where it matures and begins a prolific multiplication process. Today, malaria vaccinologists are also trying to develop different types of vaccines:
Pre-erythrocytic vaccine candidates
Pre-erythrocytic vaccine candidates aim to protect against the early stage of malaria infection—the stage at which the parasite enters or matures in an infected person's liver cells. These vaccines would elicit an immune response that would either prevent infection or attack the infected liver cell if infection does occur. These candidates include:
- Recombinant or genetically engineered proteins or antigens from the surface of the parasite or from the infected liver cell.
- DNA vaccines that contain the genetic information for producing the vaccine antigen in the vaccine recipient.
- Live, attenuated vaccines that consist of a weakened form of the whole parasite (the sporozoite) as the vaccine's main component.
Blood-stage vaccine candidates
Blood-stage vaccine candidates target the malaria parasite at its most destructive stage—the rapid replication of the organism in human red blood cells. Blood-stage vaccines do not aim to block all infection. They are expected to decrease the number of parasites in the blood, and in so doing, reduce the severity of disease. Evidence suggests that people who have survived regular exposure to malaria develop natural immunity over time. The goal of a vaccine that contains antigens or proteins from the surface of the blood-stage parasite (the merozoite) would be to allow the body to develop that natural immunity with much less risk of getting ill.
Transmission-blocking vaccine candidates
Transmission-blocking vaccine (TBV) candidates seek to interrupt the life cycle of the parasite by inducing antibodies that prevent the parasite from maturing in the mosquito after it takes a blood meal from a vaccinated person. TBV candidates aim to prevent mosquitoes from becoming infected by malaria-causing parasites when they feed on infected people.
While the TBV approach would not necessarily prevent malaria infection in the immunized person, such a vaccine would reduce the number of mosquitoes carrying the parasite and thus the number of people in a community who are infected. A successful transmission-blocking vaccine would be expected to reduce deaths and illness related to malaria in at-risk communities. See the preferred product characteristics for the TBV approach.