RTS,S/AS02A for malaria

Systematic Review of Safety of RTS,S with AS01 and AS02 Adjuvant Systems Using Data from Randomized Controlled Trials in Infants, Children, and Adults

Background

Emergence of antimalarial drugs and insecticides resistance alarms scientists to develop a safe and effective malaria vaccine. A pre-erythrocytic malaria vaccine called RTS,S has made great strides.

Aim

The review was aimed to assess the safety of the candidate malaria vaccine RTS,S with AS01 and AS02 adjuvants using data from Phase I-III randomized controlled clinical trials (RCTs).

Methods

This systematic review was conducted based on PRISMA 2020. Regardless of time of publication year, all articles related with safety of RTS,S, RCTs published in the English language were included in the study. The last search of databases, and registry was conducted on 30 May, 2022. Pubmed, Google Scholar, Cochrane Library, Wiley Online Library, and Clinical trials.gov were thoroughly searched for accessible RCTs on the safety of RTS,S malaria vaccine. The studies were screened in three steps: duplicate removal, title and abstract screening, and full-text review. The included studies' bias risk was assessed using the Cochrane risk of bias tool for RCTs. This systematic review is registered at Prospero (registration number: CRD42021285888). The qualitative descriptive findings from the included published studies were reported stratified by clinical trial phases.

Findings

A total of thirty-five eligible safety studies were identified. Injection site pain and swelling, febrile convulsion, fever, headache, meningitis, fatigue, gastroenteritis, myalgia, pneumonia, reactogenicity, and anemia were the most commonly reported adverse events. Despite few clinical trials reported serious adverse events, none of them were related to vaccination.

Conclusion

Most of the adverse events observed from RTS,S/AS01 and RTS,S/AS02 malaria vaccines were reported in the control group and shared by other vaccines. Hence, the authors concluded that both RTS,S/AS01 and RTS,S/AS02 malaria vaccines are safe.

Mosquirix™ RTS, S/AS01 Vaccine Development, Immunogenicity, and Efficacy

Malaria is a parasitic infection caused by bites from Plasmodium falciparum (P. falciparum)-infected mosquitoes with a present scale of symptoms ranging from moderate fever to neurological disorders. P. falciparum is the most lethal of the five strains of malaria, and is a major case of morbidity and mortality in endemic regions. Recent advancements in malaria diagnostic tools and prevention strategies have improved conjugation antimalarial therapies using fumigation and long-lasting insecticidal sprays, thus lowering malarial infections. Declines in the total number of infected individuals have been correlated with antimalarial drugs. Despite this, malaria remains a major health threat, affecting more than 30 million men, women, and children around the globe, and 20 percent of all children around the globe have malaria parasites in their blood. To overcome this life-threatening condition, novel therapeutic strategies, including immunization, are urgently needed to tackle this infection around the world. In line with this, the development of the RTS, S vaccine was a significant step forward in the fight against malaria. RTS, S is a vaccine for P. falciparum in which R specifies central repeat units, T the T-cell epitopes, and S indicates surface antigen. The RTS, S/AS01 malarial vaccine was synthesized and screened in several clinical trials between 2009 and 2014, involving thousands of young children in seven African countries, showing that children who received the vaccine did not suffer from severe malaria. Mosquirix™ was approved by the World Health Organization in 2021, indicating it to be safe and advocating its integration into routine immunization programs and existing malaria control measures. This paper examines the various stages of the vaccine’s development, including the evaluation of its immunogenicity and efficacy on the basis of a total of 2.3 million administered doses through a routine immunization program. The protection and effectiveness provided by the vaccine are strong, and evidence shows that it can be effectively delivered through the routine child immunization platform. The economic cost of the vaccine remains to be considered.

Conserved immunogens in prime-boost strategies for the next-generation HIV-1 vaccines

INTRODUCTION

Effective vaccines are the best solution for stopping the spread of HIV/AIDS and other infectious diseases. Their development and in-depth understanding of pathogen-host interactions rely on technological advances.

AREAS COVERED

Rational vaccine development can be effectively approached by conceptual separation of, on one hand, design of immunogens from improving their presentation to the immune system and, on the other, induction of antibodies from induction of killer CD8(+) T cells. The biggest roadblock for many vaccines is the pathogens' variability. This is best tackled by focusing both antibodies and T cells on the functionally most conserved regions of proteins common to many variants, including escape mutants. For vectored vaccines, these 'universal' subunit immunogens are most efficiently delivered using heterologous prime-boost regimens, which can be further optimised by adjuvantation and route of delivery.

EXPERT OPINION

Development of vaccines against human diseases has many features in common. Acceleration of vaccine discovery depends on basic research and new technologies. Novel strategies should be safely, but rapidly tested in humans. While out-of-the-box thinking is important, vaccine success largely depends on incremental advances best achieved through small, systematic, iterative clinical studies. Failures are inevitable, but the end rewards are huge. The future will be exciting.

Recent advances in the development of immunotherapies for tauopathies

The use of immunotherapy for Alzheimer's disease (AD) has traditionally focused on the amyloid-β (Aβ) peptide and has shown great potential in both animal and human studies. However, an emerging body of work has begun to concentrate on tau and to develop immunization protocols designed to decrease tau pathology in AD and other tauopathies. This commentary will discuss the use of immunotherapy for AD, focusing on tau immunotherapy in the context of recent reports on the use of tau phospho-peptides in transgenic models of tau pathology.

Safety and immunogenicity of an investigational adjuvanted hepatitis B vaccine (HB-AS02V) in healthy adults

HB-AS02 is an investigational adjuvanted hepatitis B virus (HBV) vaccine for potential use in patients with renal insufficiency and other immunocompromized individuals. In this Phase III lot-to-lot consistency study, 450 healthy adult volunteers who had not previously been vaccinated against HBV were randomized to one of three production lots of HB-AS02 at 0 and 1 month and followed until one month after the last vaccine dose. Lot-to-lot consistency was established. High seroprotection rates were already achieved after the first vaccine dose (75.9%). All subjects were seroprotected (anti-HBs antibody concentrations ≥10 mIU/ml) after two doses, with all but one subject achieving anti-HBs antibody concentrations ≥100 mIU/ml (99.7%). Geometric mean anti-HBs antibody concentration was 4594.5 mIU/ml. Local and general symptoms were reported after 80.7% and 45.5% of doses, respectively. However, these were mainly of mild or moderate severity and no subject withdrew from the study due to adverse events.

Rapid, enhanced, and persistent protection of patients with renal insufficiency by AS02(V)-adjuvanted hepatitis B vaccine

The adjuvanted hepatitis B vaccine, HB-AS04, elicits more rapid and persistent protective antibody concentrations than double doses of conventional recombinant vaccines in patients with renal insufficiency. We compared the immunogenicity, reactogenicity, and safety of the AS02(V)-adjuvanted hepatitis B vaccine HB-AS02 with that of HB-AS04. In this phase III, open, randomized study, 151 hepatitis B vaccine-naïve pre-dialysis, peritoneal dialysis, and hemodialysis patients aged 15 years and older received three doses of HB-AS02 at 0, 1, and 6 months. Another 149 similar patients received four doses of HB-AS04 at 0, 1, 2, and 6 months, and all were followed up for 12 months. HB-AS02 elicited more rapid and persistent seroprotection than HB-AS04, with rates of 77 and 39%, respectively, 1 month after the second vaccine dose, and 94 and 79%, respectively, at 12 months. Superiority of HB-AS02 over HB-AS04 in anti-hepatitis B geometric mean concentrations was found at all time points. HB-AS02 was more reactogenic than HB-AS04, but adverse events were mainly transient, of mild to moderate intensity with no reportable vaccine-related serious events. We conclude that a three-dose primary course of HB-AS02 induced more rapid, enhanced, and persistent protection in patients with renal insufficiency than the licensed four-dose primary schedule of HB-AS04. This adjuvanted vaccine affords greater protection with reduced need for booster doses in patients at high risk of hepatitis B infection.

The development of the RTS,S malaria vaccine candidate: challenges and lessons

RTS,S is the world's most advanced malaria vaccine candidate and is intended to protect infants and young children living in malaria endemic areas of sub-Saharan Africa against clinical disease caused by Plasmodium falciparum. Recently, a pivotal Phase III efficacy trial of RTS,S began in Africa. The goal of the programme has been to develop a vaccine that will be safe and effective when administered via the Expanded Program for Immunization (EPI) and significantly reduce the risk of clinically important malaria disease during the first years of life. If a similar reduction in the risk of severe malaria and other important co-morbidities associated with malaria infection can be achieved, then the vaccine could become a major new tool for reducing the burden of malaria in sub-Saharan Africa. Encouraging data from the ongoing phase II programme suggest that these goals may indeed be achievable. This review discusses some of the unique challenges that were faced during the development of this vaccine, highlights the complexity of developing new vaccine technologies and illustrates the power of partnerships in the ongoing fight against this killer disease.

Sterile protection against Plasmodium knowlesi in rhesus monkeys from a malaria vaccine: comparison of heterologous prime boost strategies

Using newer vaccine platforms which have been effective against malaria in rodent models, we tested five immunization regimens against Plasmodium knowlesi in rhesus monkeys. All vaccines included the same four P. knowlesi antigens: the pre-erythrocytic antigens CSP, SSP2, and erythrocytic antigens AMA1, MSP1. We used four vaccine platforms for prime or boost vaccinations: plasmids (DNA), alphavirus replicons (VRP), attenuated adenovirus serotype 5 (Ad), or attenuated poxvirus (Pox). These four platforms combined to produce five different prime/boost vaccine regimens: Pox alone, VRP/Pox, VRP/Ad, Ad/Pox, and DNA/Pox. Five rhesus monkeys were immunized with each regimen, and five Control monkeys received a mock vaccination. The time to complete vaccinations was 420 days. All monkeys were challenged twice with 100 P. knowlesi sporozoites given IV. The first challenge was given 12 days after the last vaccination, and the monkeys receiving the DNA/Pox vaccine were the best protected, with 3/5 monkeys sterilely protected and 1/5 monkeys that self-cured its parasitemia. There was no protection in monkeys that received Pox malaria vaccine alone without previous priming. The second sporozoite challenge was given 4 months after the first. All 4 monkeys that were protected in the first challenge developed malaria in the second challenge. DNA, VRP and Ad5 vaccines all primed monkeys for strong immune responses after the Pox boost. We discuss the high level but short duration of protection in this experiment and the possible benefits of the long interval between prime and boost.

T-cell epitope polymorphisms of the Plasmodium falciparum circumsporozoite protein among field isolates from Sierra Leone: age-dependent haplotype distribution?

Background

In the context of the development of a successful malaria vaccine, understanding the polymorphisms exhibited by malaria antigens in natural parasite populations is crucial for proper vaccine design. Recent observations have indicated that sequence polymorphisms in the C-terminal T-cell epitopes of the Plasmodium falciparum circumsporozoite protein (Pfcsp) are rather low and apparently stable in low endemic areas. This study sought to assess the pattern in a malaria endemic setting in Africa, using samples from Freetown, Sierra Leone.

Methods

Filter-paper blood samples were collected from subjects at a teaching hospital in Freetown during September–October 2006 and in April–May 2007. The C-terminal portion of the Pfcsp gene spanning the Th2R and Th3R epitopes was amplified and directly sequenced; sequences were analysed with subject parameters and polymorphism patterns in Freetown were compared to that in other malaria endemic areas.

Results and Discussion

Overall, the genetic diversity in Freetown was high. From a total of 99 sequences, 42 haplotypes were identified with at least three accounting for 44.4% (44/99): the 3D7-type (19.2%), a novel type, P-01 (17.2%), and E12 (8.1%). Interestingly, all were unique to the African sub-region and there appeared to be predilection for certain haplotypes to distribute in certain age-groups: the 3D7 type was detected mainly in hospitalized children under 15 years of age, while the P-01 type was common in adult antenatal females (Pearson Chi-square = 48.750, degrees of freedom = 34, P = 0.049). In contrast, the single-haplotype predominance (proportion > 50%) pattern previously identified in Asia was not detected in Freetown.

Conclusion

Haplotype distribution of the T-cell epitopes of Pfcsp in Freetown appeared to vary with age in the study population, and the polymorphism patterns were similar to that observed in neighbouring Gambia, but differed significantly at the sequence level from that observed in Asia. The findings further emphasize the role of local factors in generating polymorphisms in the T-cell epitopes of the P. falciparum circumsporozoite protein.

Heterologous prime-boost vaccinations for poverty-related diseases: advantages and future prospects

Classical vaccination approaches, based on a single vaccine administered in a homologous prime-boost schedule and optimized to induce primarily neutralizing antibodies, are unlikely to be sufficiently efficacious to prevent TB, malaria or HIV infections. Novel vaccines, capable of inducing a more powerful immune response, in particular T-cell immunity, are desperately needed. Combining different vaccine modalities that are able to complement each other and induce broad and sustainable immunity is a promising approach. This review provides an overview of heterologous prime-boost vaccination modalities currently in development for the 'big three' poverty-related diseases and emphasizes the need for innovative vaccination approaches.

Defined tuberculosis vaccine, Mtb72F/AS02A, evidence of protection in cynomolgus monkeys

The development of a vaccine for tuberculosis requires a combination of antigens and adjuvants capable of inducing appropriate and long-lasting T cell immunity. We evaluated Mtb72F formulated in AS02A in the cynomolgus monkey model. The vaccine was immunogenic and caused no adverse reactions. When monkeys were immunized with bacillus Calmette-Guérin (BCG) and then boosted with Mtb72F in AS02A, protection superior to that afforded by using BCG alone was achieved, as measured by clinical parameters, pathology, and survival. We observed long-term survival and evidence of reversal of disease progression in monkeys immunized with the prime-boost regimen. Antigen-specific responses from protected monkeys receiving BCG and Mtb72F/AS02A had a distinctive cytokine profile characterized by an increased ratio between 3 Th1 cytokines, IFN-gamma, TNF, and IL-2 and an innate cytokine, IL-6. To our knowledge, this is an initial report of a vaccine capable of inducing long-term protection against tuberculosis in a nonhuman primate model, as determined by protection against severe disease and death, and by other clinical and histopathological parameters.

Plasmodium pre-erythrocytic stages: what's new?

The pre-erythrocytic (PE) phase of malaria infection, which extends from injection of sporozoites into the skin to the release of the first generation of merozoites, has traditionally been the 'black box' of the Plasmodium life cycle. However, since the advent of parasite transfection technology 13 years ago, our understanding of the PE phase in cellular and molecular terms has dramatically improved. Here, we review and comment on the major developments in the field in the past five years. Progress has been made in many diverse areas, including identifying and characterizing new proteins of interest, imaging parasites in vivo, understanding better the cell biology of hepatocyte infection and developing new vaccines against PE stages of the parasite.

Malaria vaccines: a toy for travelers or a tool for eradication?

The demonstration of efficacy of two candidate malaria vaccines in children living in malaria-endemic areas, namely RTS,S from the circumsporozoite protein that reduced infection and clinical malaria in Mozambique, and an asexual blood-stage vaccine combining MSP1/MSP2/RESA that reduced parasite density in Papua New Guinea, allows one to believe that a malaria vaccine will be available for the fight against malaria in the next decade. Even if long-lasting impregnated bednets and indoor residual spraying have proven to be effective in reducing malaria transmission, these interventions may not be sufficient in the long-run since they rely on too few compounds and are, thus, vulnerable to the emergence of resistance. New tools, such as malaria vaccines, may, therefore, provide an added value to achieve the goal of local elimination and subsequent eradication of malaria. A promising candidate for that purpose would be a highly efficacious multicomponent vaccine that includes at least a sexual-stage antigen, the appropriate initial setting would be an area with low endemicity and limited population exchange, and the most suitable mode of delivery would be mass vaccination. For nonimmune populations, such as travelers visiting malaria-endemic areas, the usefulness of the first generation of malaria vaccine(s) will be limited, since the level of protection that is foreseen is unlikely to achieve that of malaria chemoprophylaxis. Only long-term travelers, expatriates and soldiers might realistically benefit from a pre-erythrocytic and/or blood-stage vaccine with an intermediate level of efficacy.

Apical membrane antigen 1: a malaria vaccine candidate in review

Apical membrane antigen 1 (AMA1) is a micronemal protein of apicomplexan parasites that appears to be essential during the invasion of host cells. Immune responses to Plasmodium AMA1 can have profound parasite-inhibitory effects, both as measured in vitro and in animal challenge models, suggesting AMA1 as a potential vaccine component. However, AMA1 is polymorphic, probably as a result of immune selection operating on an important target of naturally occurring immunity. The current understanding of AMA1 will be presented, particularly in relation to the vaccine potential of AMA1 and the approaches being taken towards clinical development.

Vaccines against malaria - an update

Malaria vaccine discovery and development follow two principal strategies. Most subunit vaccines are designed to mimic naturally acquired immunity that develops over years upon continuous exposure to Plasmodium transmission. Experimental model vaccines, such as attenuated live parasites and transmission-blocking antigens, induce immune responses superior to naturally acquired immunity. The promises and hurdles of the different tracks towards an effective and affordable vaccine against malaria are discussed.

Pre-erythrocytic stage malaria vaccines: time for a change in path

Vaccines against the pre-erythrocytic stages of malaria hold the greatest promise as an effective intervention tool against malaria, as shown by immunization with radiation-attenuated sporozoites over four decades ago. To date, however, the development of subunit vaccines, while generating high expectations and investment, has not lived up at all to the promise. This path has been characterized by insufficient research into both identification of key defense mechanisms in humans and the discovery of better antigens, focusing rather on a technological race of how to present mainly a single antigen. The lack of success has also led, perhaps from desperation, to a revival of the live attenuated sporozoite approach, handicapped, however, by major bottlenecks in production, safety, and regulatory issues. It should now be clear that the field can no longer continue to succeed in mice and fail in the clinic. We advocate here in favor of a third option, relying on an understanding of the basis of attenuated sporozoite immunity in humans, to provide leads to the discovery of critical immunogens and the use of models with validated relevance to the human situation in order to rationalize and renew the promise of pre-erythrocytic subunit vaccines.

New concepts in vaccine development in malaria

PURPOSE OF REVIEW

To focus on recent novel concepts in the development of malaria vaccines.

RECENT FINDINGS

There is a renewed interest in whole attenuated sporozoite vaccines, either as irradiated or genetically modified sporozoites, because they consistently elicit solid protection against challenge infections. Enthusiasm about these vaccines is, however, tempered by technical, logistical, safety and even cultural hurdles that might need to be surmounted. Less than a score of Plasmodium falciparum proteins are currently in the development pipeline as malaria vaccines. There is an urgent need to ratchet up the process of candidate vaccine discovery, and reverse vaccinology and genome-wide surveys remain promising strategies. The development of malaria vaccines for placental malaria is an active area and chondroitin sulfate A-binding epitopes of the variant PfEMP1 have been identified. Live bacteria and viral vectors hold special promise for vaccine delivery.

SUMMARY

Attenuated sporozoite vaccines have made a resurgence to center stage in malaria vaccine development. There is an urgent need to identify more subunit vaccine candidates that can enter into the development pipeline, identify surrogate markers of immunity and design vaccines which induce long-lasting immunity.