Pre-erythrocytic malaria vaccines: RTS,S, R21, and beyond

KEYWORDS:

• Malaria
• falciparum
• circumsporozoite protein
• pediatrics
• malaria vaccines
• RTS,S, R21
• PfSPZ Vaccine
• monoclonal antibodies

1. Introduction

Malaria is one of the oldest known human diseases, and the Plasmodium genus of parasites has plagued vertebrates for millions of years. Although five Plasmodium species infect humans, P. falciparum is the deadliest and causes the greatest disease burden, mainly in young children living in Sub-Saharan Africa. In the early 20th century, vector control programs were a centerpiece of malaria control and eradication efforts, and by 1951, the United States had successfully eliminated malaria from its own borders. Programmatic challenges, insecticide and antimalarial resistance, and a decline in political will led to abandonment of a World Health Organization (WHO) campaign to eradicate malaria in 1969, after which malaria incidence rebounded [1]. The sustained global burden of malaria has necessitated a multipronged malaria control approach, combining the continued emphasis on vector control with intermittent preventive treatment strategies including seasonal malaria chemoprophylaxis (SMC), in which antimalarials are provided monthly to young children during the malaria transmission season, and case management [2,3]. From 2000 to 2015, global malaria cases and deaths declined, but progress has stalled. The COVID-19 pandemic further disrupted malaria elimination programs. Current challenges to malaria control include emerging antimalarial resistance and the vulnerability of vector control and bed net programs to natural disasters and public health and humanitarian emergencies [4]; these challenges can be circumvented by effective malaria vaccines. Indeed, the WHO-preferred product characteristics for an effective malaria vaccine include efficacy in preventing illness of at least 90% and durability beyond one year [5]. The development of such a vaccine has been an elusive goal, although recent advancements have shown promising potential [4].

2. Targets of malaria vaccines

Human infection by Plasmodium parasites begins when a female Anopheles mosquito injects sporozoites into the dermis during a blood meal. The sporozoites traverse the circulatory system, invading hepatocytes within 30 minutes of inoculation. After a period of asexual replication, parasite-laden hepatocytes rupture, releasing merozoites that subsequently invade erythrocytes and undergo cyclical rounds of replication, release, and invasion of new erythrocytes. A subset of merozoites differentiate into gametocytes that are taken up by a mosquito where the parasite sexual stage occurs, ultimately leading to the presence of mature sporozoites in the salivary glands so that the cycle can be repeated in a new host. Malaria vaccines are grouped by the life cycle stages they target: pre-erythrocytic vaccines target the sporozoite and infected hepatocytes and are meant to interrupt infection before the beginning of the blood stage; blood-stage vaccines target the merozoite and/or the parasite proteins expressed on the surface of the infected erythrocytes; and transmission-blocking vaccines elicit human immune responses that act within the mosquito host to inhibit the sexual stage of the parasite. This review will focus on pre-erythrocytic vaccines, which are currently at the forefront of malaria vaccine development due to feasibility advantages: pre-erythrocytic vaccines block the life cycle when the fewest parasites are present in the human host, they prevent malaria illness, and the major target of pre-erythrocytic vaccines, circumsporozoite protein (CSP) elicits protective B- and T-cell responses [6].

3. The world’s first malaria vaccine

The first malaria vaccine approved by the WHO was RTS,S adjuvanted with AS01, a saponin-based adjuvant. RTS,S/AS01 is a sub-unit vaccine based on CSP, the protein that coats the sporozoite surface. RTS,S consists of CSP fused with hepatitis B surface antigen (HBsAg) and additional unfused HBsAg assembled into virus-like particles (VLPs). In 5–17-month-old children given a three-dose primary series followed by a booster dose at month 20, RTS,S had a modest efficacy against malaria illness of 56% at one year that waned to 36% by the end of four years of follow-up [6]. Additional studies evaluating the combined efficacy of RTS,S/AS01 with SMC found improved efficacy when RTS,S/AS01 was given on a seasonal schedule along with SMC and improved efficacy of the combination of both interventions vs. either given alone [7,8].

Following results of the phase 3 RTS,S/AS01 study in children and infants and a favorable rating from the European Medicines Agency in 2015, the WHO requested further studies before considering the vaccine for prequalification. A pilot study began in 2019 through the Malaria Vaccine Implementation Program (MVIP) to evaluate the phased introduction of RTS,S/AS01 through routine childhood immunization programs in Malawi, Ghana, and Kenya.

In October 2021, the WHO recommended RTS,S/AS01 for use in children ≥5 months old living in areas of moderate to high malaria transmission following either a four-dose, age-based schedule or a five-dose seasonal strategy [4]. This marked the first time any malaria vaccine was recommended for use by the WHO.

3.1. RTS,S/AS01 implementation

After the WHO recommendation for RTS,S/AS01, distribution expanded to more areas within the MVIP countries. By March 2023, more than 4.1 million doses of the vaccine had been administered through the MVIP, and more than 1.34 million children had received at least one dose. An estimated 100 million doses would be needed annually to meet the expected demand; however, it is anticipated that only 18 million doses of RTS,S/AS01 will be available over the course of 2023–2025 [9]. Accordingly, the WHO established a framework for allocating malaria vaccines in 2022, and in January 2023, Gavi, the Vaccine Alliance, accepted applications from 12 non-MVIP countries for funding to support rollout of RTS,S/AS01 to children in high-risk areas [9,10]. Gavi has subsequently approved applications for 15 additional countries, and vaccine availability to nine non-implementation countries is anticipated in 2024 [11]. RTS,S, implementation has been feasible and well-accepted, and phase 4 studies have not uncovered concerning safety signals. Moreover, a pooled analysis of RTS,S/AS01 implementation in Ghana, Kenya, and Malawi revealed a 13% reduction (95% confidence intervals 2–22%) in all-cause mortality (excluding deaths due to injury) in children who are age-eligible to receive RTS,S [11].

4. The world’s second malaria vaccine

In April 2023, the Food and Drugs Authority of Ghana became the first regulatory agency in the world to license a second malaria vaccine, R21/Matrix-M, for use in 5–36-month-old children [12]. Nigeria and Burkina Faso subsequently licensed the vaccine. In October 2023, the WHO officially recommended R21/Matrix-M, making it the second malaria vaccine to become widely available for children in high-transmission settings [13]. Much like RTS,S/AS01, R21/Matrix-M combines a VLP expressing CSP-HBsAg fusion protein with a saponin-based adjuvant. Unlike the RTS,S VLP, the R21 VLP does not include unfused HBsAg, allowing for a higher concentration of CSP at a lower dose.

A phase 3 trial began in January 2021 and is ongoing; it enrolled 4800 children ages 5–36 months with a 2:1 randomization of 25 µg R21/Matrix-M vs. rabies vaccine [9,14]. The study is evaluating two administration regimens: 2400 children followed a seasonal administration schedule in areas of Mali and Burkina Faso with highly seasonal malaria transmission, while 2400 children followed an age-based administration schedule in areas with low/moderate transmission in Burkina Faso, Kenya, and Tanzania. Both administration schedules included a three-dose primary series followed by a fourth (booster) dose 12 months after dose 3. Participants will be followed for safety and efficacy against clinical malaria through two years after the third dose. Preliminary review of the data by the WHO that led to approval suggests high efficacy against clinical malaria through 18 months post-dose 3 (six months post-dose 4) following a seasonal administration schedule in areas of highly seasonal transmission and through 12 months post-dose 3 following an age-based administration schedule in areas with low/moderate transmission [14]. As many as 200 million doses of R21/Matrix-M may be delivered annually, some of which may be produced in Ghana [15].

5. More to come

The RTS,S/AS01 and R21/Matrix-M vaccines may be the first to reach the market, but other promising pre-erythrocytic vaccine candidates are in the pipeline. The success of mRNA vaccines for prevention of COVID-19 has accelerated development of CSP-based mRNA vaccines, with one candidate in clinical trials [16].

Whole sporozoite approaches deliver metabolically active whole sporozoites capable of invading hepatocytes but not progressing to blood stage infection [17,18]. PfSPZ Vaccine comprises purified, radiation-attenuated whole sporozoites that arrest early in the liver stage. PfSPZ Vaccine has prevented malaria in phase 2b trials in adults; however, results from a phase 2 study in children in Kenya were mixed [18]. Additional studies are underway, and phase 3 studies are being planned. Delivery of non-attenuated sporozoites with chemoprophylaxis is effective in malaria-naive participants and suggests that further liver-stage development may elicit additional protective immune responses [18]. Genetically attenuated sporozoites possess gene deletions that arrest the parasite’s life cycle at the liver stage and have been tested in humans via infected mosquito bite and by direct venous inoculation of cryopreserved parasites with new candidates in development and further studies planned [18].

Monoclonal antibodies (mAb) that target the CSP protein have been isolated from volunteers who were protected following vaccination with RTS,S and PfSPZ Vaccine [19–22]. A modification to the Fc region of the mAb extends the half-life to ~6 months, making seasonal passive immunoprophylaxis a feasible strategy for malaria prevention. These antibodies have shown efficacy as passive immunization against controlled human malaria infection in malaria-naive volunteers and in the setting of intense seasonal transmission in adults in Mali [23]. Studies are ongoing in children in Mali and Kenya [20]. Structural studies of mAb and their interface with the CSP protein have provided new insight to the design of next-generation pre-erythrocytic vaccines [19].

6. Conclusion

Effective vaccines for malaria are on the horizon and bring a ray of hope for children living in malaria-endemic regions. Vaccine supply and implementation still pose challenges, but coupled with other interventions for malaria control, reducing the malaria disease burden is more attainable.

Declaration of interests

E A Hammershaimb and A A Berry are investigators for studies of COVID-19 vaccines developed by Novavax, and A A Berry is an investigator for studies of PfSPZ Vaccine. In all cases, their institution receives funding through federal agencies, and the authors do not have direct financial relationships with the vaccine manufacturers. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or material discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Author contributions

Both authors contributed substantially to the conception of this editorial and interpretation of relevant literature. E. Adrianne Hammershaimb wrote the initial draft and Andrea A. Berry critically revised the manuscript.

Additional information

Funding

This paper was not funded.