Malaria

Mosquitoes carrying the deadly malaria parasite have flown along next to humans for thousands of years, and the disease appears in documented reports as early as 2700 B.C. Malaria continues to plague humans today, causing hundreds of thousands of deaths each year.

Malaria is caused by the parasites in the Plasmodium species, single-celled organisms with multiple life stages, requiring more than one host for its survival. Five species of the parasite cause disease in humans – Plasmodium falciparum, P. vivax, P. ovale, P. malariae, and P. knowlelsi. Plasmodium falciparum is the most dangerous strain in humans and the target of most scientific research today. In 2002, scientists succeeded in sequencing the P. falciparum genome, which has allowed researchers to make great strides in better understanding ways to target it.

The name malaria comes from mal’aria, which is Italian for “bad air.” Before the development of germ theory in the late 1800s, many people thought the disease was transmitted via miasmas, or contaminated air. By 1897, British physician Ronald Ross discovered that mosquitoes were the vectors that transmit the disease. Scientists then found that only the female Anopheles mosquito transmits the parasite (males do not feed on blood). Females of 60 species of Anopheles mosquitoes can serve as malaria vectors.

More than half of the world’s population lives in areas vulnerable to malaria, with cases documented in more than 109 countries and the highest mortality (roughly 89% of all deaths) occurring in Africa. The parasite infects around 220 million people each year. Moreover, children under five, pregnant women, and people with HIV/AIDS are most at risk for severe illness and death. 

Malaria’s long history includes many historic attempts to defeat it. Quinine, a substance derived from the bark of the cinchona tree, has been known to be effective against malaria since the 1600s. After the role of mosquitoes in malaria transmission, scientists focused on vector control. They hypothesized that by killing the vector, they could halt the cycle of infection. Consequently, DDT and other insecticides came into vogue in the mid-1900s and have been used ever since. Bed nets to protect sleeping people from mosquito bites are another form of vector control that is not only effective, but also extremely cost effective. Finally, the development of several anti-malaria drugs has changed the way travelers view malaria-endemic countries and the risk associated with travel in general. In fact, and likely due to all the above measures, estimated deaths from malaria fell 13%, from 755,000 in 2000 to 655,000 in 2010. Cases of the disease fell, although less dramatically, from 223 million in 2000 to about 216 million in 2010.

With all these developments, why does malaria remain a problem? The emergence of resistance to drugs and insecticides is a major concern. 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. Today, we are seeing more and more drug-resistant parasites and insecticide-resistant mosquitoes. Global efforts are underway in the next era of malaria prevention: the development of malaria vaccines that can save countless lives and ultimately help eradicate this historic plight.

The Plasmodium Life Cycle

 Malaria is unlike any infectious disease for which we have already created a successful vaccine. Most notable of these differences is that malaria is transmitted via a parasite that passes through multiple life stages, each of which presents a unique challenge to vaccine developers. The three stages in the Plasmodium life cycle can be divided into two distinct categories – in the first two, the parasite undergoes asexual reproduction in the host’s body, and in the third, it undergoes sexual reproduction in the mosquito vector gut. Because the parasite can reproduce both asexually and sexually, it has many advantages over the viruses and bacteria that we currently vaccinate against.

The three stages of the Plasmodium life cycle are (1) the pre-erythrocytic stage, better known as the liver stage, or the stage before the parasite infects human red blood cells, (2) the erythrocytic stage, or the blood stage when the parasite infects the red blood cells, and (3) the sexual stage, the stage when a mosquito has taken up the parasite, and the parasite is sexually reproducing in the mosquito gut.

It’s important to remember that each life stage occurs in a different part of the infected human or vector. First, when a mosquito infected with Plasmodium bites a human host, the parasite goes directly to the liver. Second, once the parasite has matured in the liver, it will enter the bloodstream and invade blood cells. Finally, when it is ready to infect the next host, it will be sucked up by another female Anopheles mosquito and sexually reproduce in the mosquito’s gut.

Before we go into detail on how a vaccine could prevent malaria, it’s helpful to review the stages of the parasite’s life cycle. The first form is known as the sporozoite (pronounced spore-o-zo-ite). When a mosquito containing the malaria parasite bites a person, the parasite enters the human’s body as a sporozoite. Once the sporozoite gets to the liver, it quickly infects the liver cells and goes through multiple rounds of asexual reproduction to produce merozoites (pronounced mer-o-zo-ites). These developments all make up the pre-erythrocytic stage of the parasite’s life cycle. One sporozoite can asexually reproduce to form up to 40,000 merozoites, a large enough number to seriously challenge the immune system's ability to control the parasite.

The erythrocytic stage is the next stage, which occurs once the merozoites leave the liver cells and enter the bloodstream. Here, a merozoite infects a red blood cell and begins asexually reproducing and releasing hundreds of new merozoites. This is the stage when an individual experiences symptoms such as malaria-associated periodic fever. The symptoms are the result of the bursting of red blood cells, which is why symptoms often occur periodically. When the parasite is inside the red blood cell reproducing, the fever will decrease and the patient appears to be improving, but will start up again when the merozoites are released.

In the third stage, or the sexual stage, a few merozoite-infected blood cells will stop asexually reproducing and instead mature into sexual forms of the parasites – known as male and female gametocytes (pronounced gam-eat-o-cytes). Under a microscope, P. falciparum gametocytes are distinguishable by their unique banana shape. When an Anopheles mosquito bites a human with malaria, it will take up the gametocytes along with the blood. These gametocytes can then further sexually reproduce in the mosquito gut, develop mature sex cells or gametes, and finally fusing as they move up the mosquito gut wall to become an oocyst. The oocyst grows, divides, and eventually bursts and produces thousands of haploid sporozoites, which will travel to the mosquito’s salivary glands to be injected into the next individual during the mosquito’s next blood meal. (If you will recall, the sporozoite is the form of the parasite that infects the liver.) Thus, the parasite’s life cycle comes full circle, allowing malaria to continue to spread and infect people across the globe.

A Malaria Vaccine: An Immunological Approach Against the Parasite

The complicated life cycle of Plasmodium presents a challenge to malaria vaccine development. Researchers must determine which life stage of the parasite to target, or whether the vaccine needs to combine elements that target more than one life stage. However, recent findings allow us to be optimistic about an effective malaria vaccine.

Malaria is different from many of the diseases we currently vaccinate for, because it does not confer so-called sterile immunity. This means that if you become ill from malaria and recover, you can be infected over and over again. The fact that your immune system responded to malaria in the past will not prevent future infection. This is different from a disease such as measles: most people who contract measles will be immune to future measles infection for life. With malaria, there is evidence of a degree of naturally acquired immunity – someone who has had malaria in the past can still get it again, but she will probably get a less severe case. In many African countries where malaria is common, most people who are re-infected with malaria experience only mild symptoms due to this partial acquired immunity. This is also the reason that malaria is so deadly for children under five. These children have not yet acquired any immunity to the parasite, and they are much more likely to experience a severe case that may lead to fatal complications. Moreover, this is also the reason that foreigners who have never experienced malaria must be careful – they may develop a serious case when they first are infected. Finally, naturally acquired partial immunity does not last long. In fact, when someone has lived in Africa his entire life and leaves for even a year, he will lose this partial immunity and again be as vulnerable to malaria as someone who had never been infected. So, one approach to developing a malaria vaccine would be to understand the mechanism of partial immunity and develop a vaccine based on that principle.

Another avenue that has given direction to malaria vaccine researchers is the concept of immunizing with a live attenuated (weakened) whole parasite in its sporozoite form. Support for this idea came in a 1967 study in which Nussenzweig et al. immunized mice with radiation-attenuated Plasmodium berghei (a non-human form of malaria) sporozoites, and saw that the mice were protected in a later challenge with infectious sporozoites.

Adapting this idea to humans in 2002, Hoffman et al. showed that they could use gamma radiation to attenuate the sporozoites inside infected Anopheles mosquitoes, and thus almost completely protect humans. Human subjects were exposed to the bites of infected mosquitoes, which injected the irradiated sporozoites into the subjects. The sporozoites could travel into the liver cells, but could mature no further. These weakened sporozoites were still able to elicit an immune response in the human host, but because they could not develop further than the liver, the host would not get sick. As a result, the next time an infected mosquito took a blood meal from the immunized person and injected the person with Plasmodium sporozoites, the immune system recognized the threat and eliminated the parasite before it caused disease.

This irradiation approach had two major flaws: it was not cost effective and not practical on a large scale. Nevertheless, it served as a proof of principle, giving scientists hope for the future and helping stimulate a lot of research into the field.

Current Research

Scientists have expanded on what was learned in the 2002 study to develop many potential malaria vaccines. Instead of attempting a live attenuated vaccine, most scientists today use technologies to isolate and deliver specific antigens in a vaccine. And because the parasite has three different life stages, there are three distinct vaccine approaches being investigated.

Pre-erythrocytic vaccines target the infectious phase and aim either to prevent the sporozoites from getting into the liver cells or to destroy infected liver cells. The most significant challenge for a pre-erythrocytic vaccine is the time frame: sporozoites reach the liver less than an hour after being injected by the mosquito. Therefore, the immune system has limited time to eliminate the parasite. Although most potential pre-erythrocytic vaccines are still in Phase I or Phase II trials, one vaccine is currently in Phase III trials and shows promise: the RTS,S vaccine. (Note that Phase I studies evaluate for safety, Phase II tests evaluate dosing, and Phase III tests assess efficacy. )

To develop the RTS,S vaccine, developers identified the protein most responsible for protection in the irradiated sporozoite trial from 2002. This antigen is known as the circumsporozoite protein, or CS protein. Although this antigen is protective, it is not very immunogenic on its own, meaning that it is not good at stimulating an immune response. Thus, scientists fused the Hepatitis B surface antigen (the antigen responsible for providing protection in the Hepatitis B vaccine) with an antigen from the CS protein. To stimulate the immune system even further, scientists employed a compound called an adjuvant that boosts the immune system’s response to the antigen. The goal is to induce high levels of antibodies to both block the sporozoites from entering the liver cells and to tag specific infected cells for destruction.

The RTS,S vaccine was tested in Phase III trials in 11 African countries. These trials have had some successes. , released in October 2011, showed that vaccination in children aged 5-17 months with RTS,S reduced the risk of clinical malaria and severe malaria by 56% and 47%, respectively.  However, in , the vaccine was less effective in infants aged 6-12 weeks at first vaccination. In that group, vaccination with RTS,S led to one-third fewer episodes of both clinical and severe malaria. Final results from the trial, which followed young children for about three years, showed a reduction in clinical malaria cases by 26% for the youngest children to to 36% for children up to age 17 months at first vaccination. In July 2015, the European Medicines Agency recommended the vaccine be licensed for use in young children in Africa. The World Health Organization is considering the recommendation on the vaccine. In the meantime, a WHO advisory group has recommended pilot implementation of the vaccine in 3-5 sub-Saharan African countries. The chief developer of RTS,S, the Malaria Vaccine Initiative, a nonprofit based in Seattle, Washington, hopes to develop an even better vaccine that is 80% effective by 2025.

Several other pre-erythrocytic vaccines are in trials, but none have shown the promise or success of RTS,S. Scientists are working on improving the efficacy of the RTS,S vaccine to be more than 50% effective by employing prime boost technology, adjuvants, and antigen optimization.

The World Health Organization recommended a second pre-erythrocytic vaccine, R21/Matrix-M, on October 2, 2023. According to WHO, “In areas with highly seasonal malaria transmission (where malaria transmission is largely limited to 4 or 5 months per year), the R21 vaccine was shown to reduce symptomatic cases of malaria by 75% during the 12 months following a 3-dose series. A fourth dose given a year after the third maintained efficacy. This high efficacy is similar to the efficacy demonstrated when RTS,S is given seasonally.” This is based on an ongoing clinical trial.

Erythrocytic vaccines, or blood-stage vaccines, aim to stop the rapid invasion and asexual reproduction of the parasite in the red blood cells. Recall the blood stage is the time when symptoms appear, and is also the most destructive to the patient due to the bursting of red blood cells. Because of the huge number of merozoites produced during this stage – 40,000 merozoites are released for each infected liver cell – a blood-stage vaccine can only aim to reduce the number of merozoites infecting red blood cells, rather than completely block their replication. Currently, there are no blood-stage vaccines that have had the success of the RTS,S vaccine, and most are still undergoing Phase I or II trials.

Finally, another type of vaccine targets the stage of sexual reproduction that occurs in the mosquito gut. This approach is known as a transmission blocking vaccine (TBV) because it aims to kill the vector, the Anopheles mosquito, to stop further spread of the parasite. This is an indirect approach to a vaccine, because it will not directly protect an individual who gets the parasite, but rather stop the continued spread.

One TBV candidate vaccine is the Pfs25-EPA which is being developed by the US National Institute of Allergy and Infectious Diseases Laboratory of Malaria Immunology and Virology and Johns Hopkins University Center for Vaccine Research. The idea behind this vaccine is that if the body can develop antibodies against the Pfs25 antigen, a mosquito taking a blood meal will take up some of these antibodies into its stomach. There, the antibodies will encounter the antigen, enabling them to interfere with development and kill the parasite.

Many scientists think the next step is to combine multiple approaches to develop a malaria vaccine. But these individual stage vaccines must show their efficacy on their own before scientists can develop a vaccine combining approaches. Moreover, the major challenge scientists will face in the future is that there are no known correlates for immunity, meaning there is no method other than costly clinical trials in humans to demonstrate a vaccine’s efficacy. Thus, although great progress has been made, malaria vaccine development will continue to be a costly and multidimensional effort.

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