Question: Explain why it has been proven difficult to develop a vaccine for malaria?
Answer: There are a number of reasons why it has been difficult to develop a commercially practical and widely available vaccine against malaria. Most vaccines work by priming the immune system to identify and attack the disease when it enters the body. This is usually achieved by killing or weakening the organism that causes the disease and introducing it via a vaccine, or selecting a specific element or elements from the organism, which would allow the body’s immune system to still recognize the whole organism in the event of a real infection.
Malaria is a parasite, which means it is considerably more complex and large as an organism than a bacteria or virus. Its size and complexity, together with its complicated reproductive process, means that each malaria parasite is slightly different from the next. This is why natural immunity to malaria is not developed straight after the first infection, but requires repeated exposures, and even then, immunity is not complete – people who have been infected multiple times with malaria are still at risk from infection. In contrast, one infection with measles is usually sufficient to give the patient full immunity against the disease. Using this model, the measles vaccine utilizes a weakened version of the measles virus, and one dose is very effective.
Vaccines have been trialed for malaria, using a similar approach; people were exposed to mosquitoes which contained a weakened version of the malaria parasite. While this approach worked reasonably well, it required the patient to be bitten a large number of times by mosquitoes and was therefore not suitable to large-scale roll out. Most other vaccine efforts for malaria have focused on targeting a specific element of the malaria parasite (known as antigens), to stimulate the immune response. Again, the diversity of antigens in malaria has made this a challenge; moreover, malaria has multiple life cycle stages in the human body, meaning that a vaccine developing for an antigen in one life stage might not be effective against a different life stage.
The “holy grail” of malaria vaccine development is to create a vaccine that targets multiple antigens, to account for the diversity of the malaria parasite, across multiple life stages, for holistic protection. In addition, the vaccine needs to be able to be produced in large quantities, and able to be delivered to large numbers of people in a relatively short amount of time. All these considerations and factors contribute to why it has been so difficult to develop an effective malaria vaccine, though some on-going clinical trials are showing some promise for the future.