World's First Malaria Vaccine Approved By WHO

Will Barbour

The malaria vaccine, RTS,S or Mosquirix, has been approved by the World Health Organisation (WHO) in October 2021. In the most recent, randomised controlled, double-blind trial, the vaccine was found to be up to 77% effective making RTS,S the first malaria vaccine to surpass the WHO’s target of 75% efficacy. The approval of the vaccine is significant despite the advancements in preventative methods such as mosquito nets and insecticides, as they only act as a preventative measure, whereas the vaccine could be the key to the eradication of malaria.

Malaria is a potentially deadly disease that is responsible for around 400,000 deaths per year, with the majority of these being young people in Sub-Saharan Africa. Malaria is caused by the single celled plasmodium protoctista. The Plasmodium pathogen is spread by female Anopheles mosquitoes. Humans become infected when they are bitten by a mosquito carrying the malaria pathogen, and plasmodium cells are released into their bloodstream. The plasmodium cells then travel to the human liver, where it grows and multiplies by mitosis. If not treated, the plasmodium pathogen will emerge from the liver with different antigens on its cell surface, and infect and destroy red blood cells. With a lack of red blood cells there is no longer a constant blood supply taking oxygen to the cells of major organs. This means that respiration will stop, which can lead to strokes, heart attacks or complete organ failure. As the pathogen progresses through each stage of infection it will change its surface proteins to avoid the body's immune system. The diagram adjacent shows the life cycle of the pathogen:

The RTS,S vaccine is composed of the Hepatitis- B virus, which has been modified to express the same cell surface antigens as the plasmodium pathogen before it enters the liver. When the vaccine is injected into the body, the immune system identifies the plasmodium antigens on the surface of the virus, and matches it to the correct antibody on the surface of a white blood cell. Once the correct antibody is identified, lymphocytes (specifically B-cells) produce antibodies and secrete these into the bloodstream. Antibodies work in a variety of ways, including neutralising the virus, or by binding to multiple antigens at once, and therefore linking together many viruses, which eases the process of phagocytosis. Phagocytosis, is the process by which phagocytes (another type of white blood cell) finds, engulfs and destroys (using hydrolytic enzymes) the pathogen. During the primary response (the response due to the vaccination), memory cells are produced, these remain in the bloodstream for a long time and act as an immunological memory, meaning that the secondary response is much faster and far more antibodies are produced.

The principle around the malaria vaccine is that if the body can identify the Plasmodium antigen on the surface of the Hep-B virus, it will trigger a slow primary immune response. The slow response and the limited amount of antibodies produced is not a major concern as the vaccine will not give the patient malaria. However, if the patient is then exposed to the Plasmodium pathogen, after being bitten by a mosquito, it will trigger the much more powerful and effective secondary immune response. This should mean that the Plasmodium cells are destroyed before they are able to reach the liver, multiple and evade the immune system.

The development of this vaccine is a huge and promising step towards the eradication of malaria.

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