Safety and immunogenicity of the synthetic malaria vaccine SPf66 in a large field trial

Plasmodium falciparum merozoite surface protein 1 as asexual blood stage malaria vaccine candidate

INTRODUCTION

Malaria represents a public health challenge in tropical and subtropical regions, and currently deployed control strategies are likely insufficient to drive elimination of malaria. Development and improvement of malaria vaccines might be key to reduce disease burden. Vaccines targeting asexual blood stages of the parasite have shown limited efficacy when studied in human trials conducted over the past decades.

AREAS COVERED

Vaccine candidates based on the merozoite surface protein 1 (MSP1) were initially envisioned as one of the most promising approaches to provide immune protection against asexual blood-stage malaria. Successful immunization studies in monkey involved the use of the full-length MSP1 (MSP1FL) as vaccine construct. Vaccines using MSP1FL for immunization have the potential benefit of including numerous conserved B-cell and T-cell epitopes. This could result in improved parasite strain-transcending, protective immunity in the field. We review outcomes of clinical trials that utilized a variety of MSP1 constructs and formulations, including MSP1FL, either alone or in combination with other antigens, in both animal models and humans.

EXPERT OPINION

Novel approaches to analyze breadth and magnitude of effector functions of MSP1-targeting antibodies in volunteers undergoing experimental vaccination and controlled human malaria infection will help to define correlates of protective immunity.

Progress in the Development of Subunit Vaccines against Malaria

Malaria is a life-threatening disease and one of the main causes of morbidity and mortality in the human population. The disease also results in a major socio-economic burden. The rapid spread of malaria epidemics in developing countries is exacerbated by the rise in drug-resistant parasites and insecticide-resistant mosquitoes. At present, malaria research is focused mainly on the development of drugs with increased therapeutic effects against Plasmodium parasites. However, a vaccine against the disease is preferable over treatment to achieve long-term control. Trials to develop a safe and effective immunization protocol for the control of malaria have been occurring for decades, and continue on today; still, no effective vaccines are available on the market. Recently, peptide-based vaccines have become an attractive alternative approach. These vaccines utilize short protein fragments to induce immune responses against malaria parasites. Peptide-based vaccines are safer than traditional vaccines, relatively inexpensive to produce, and can be composed of multiple T- and B-cell epitopes integrated into one antigenic formulation. Various combinations, based on antigen choice, peptide epitope modification and delivery mechanism, have resulted in numerous potential malaria vaccines candidates; these are presently being studied in both preclinical and clinical trials. This review describes the current landscape of peptide-based vaccines, and addresses obstacles and opportunities in the production of malaria vaccines.

Conserved Binding Regions Provide the Clue for Peptide-Based Vaccine Development: A Chemical Perspective

Synthetic peptides have become invaluable biomedical research and medicinal chemistry tools for studying functional roles, i.e., binding or proteolytic activity, naturally-occurring regions’ immunogenicity in proteins and developing therapeutic agents and vaccines. Synthetic peptides can mimic protein sites; their structure and function can be easily modulated by specific amino acid replacement. They have major advantages, i.e., they are cheap, easily-produced and chemically stable, lack infectious and secondary adverse reactions and can induce immune responses via T- and B-cell epitopes. Our group has previously shown that using synthetic peptides and adopting a functional approach has led to identifying Plasmodium falciparumconserved regions binding to host cells. Conserved high activity binding peptides’ (cHABPs) physicochemical, structural and immunological characteristics have been taken into account for properly modifying and converting them into highly immunogenic, protection-inducing peptides (mHABPs) in the experimental Aotus monkey model. This article describes stereo–electron and topochemical characteristics regarding major histocompatibility complex (MHC)-mHABP-T-cell receptor (TCR) complex formation. Some mHABPs in this complex inducing long-lasting, protective immunity have been named immune protection-inducing protein structures (IMPIPS), forming the subunit components in chemically synthesized vaccines. This manuscript summarizes this particular field and adds our recent findings concerning intramolecular interactions (H-bonds or π-interactions) enabling proper IMPIPS structure as well as the peripheral flanking residues (PFR) to stabilize the MHCII-IMPIPS-TCR interaction, aimed at inducing long-lasting, protective immunological memory.

Analysis of a Multi-component Multi-stage Malaria Vaccine Candidate--Tackling the Cocktail Challenge

Combining key antigens from the different stages of the P. falciparum life cycle in the context of a multi-stage-specific cocktail offers a promising approach towards the development of a malaria vaccine ideally capable of preventing initial infection, the clinical manifestation as well as the transmission of the disease. To investigate the potential of such an approach we combined proteins and domains (11 in total) from the pre-erythrocytic, blood and sexual stages of P. falciparum into a cocktail of four different components recombinantly produced in plants. After immunization of rabbits we determined the domain-specific antibody titers as well as component-specific antibody concentrations and correlated them with stage specific in vitro efficacy. Using purified rabbit immune IgG we observed strong inhibition in functional in vitro assays addressing the pre-erythrocytic (up to 80%), blood (up to 90%) and sexual parasite stages (100%). Based on the component-specific antibody concentrations we calculated the IC₅₀ values for the pre-erythrocytic stage (17–25 μg/ml), the blood stage (40–60 μg/ml) and the sexual stage (1.75 μg/ml). While the results underline the feasibility of a multi-stage vaccine cocktail, the analysis of component-specific efficacy indicates significant differences in IC₅₀ requirements for stage-specific antibody concentrations providing valuable insights into this complex scenario and will thereby improve future approaches towards malaria vaccine cocktail development regarding the selection of suitable antigens and the ratios of components, to fine tune overall and stage-specific efficacy.

Malaria vaccine development using synthetic peptides as a technical platform

The review covers the development of synthetic peptides as vaccine candidates for Plasmodium falciparum- and Plasmodium vivax-induced malaria from its beginning up to date and the concomitant progress of solid phase peptide synthesis (SPPS) that enables the production of long peptides in a routine fashion. The review also stresses the development of other complementary tools and actions in order to achieve the long sought goal of an efficacious malaria vaccine.

Synthetic vaccine update: Applying lessons learned from recent SPf66 malarial vaccine physicochemical, structural and immunological characterization

The SPf66 synthetic malaria vaccine, developed and obtained almost 2 decades ago, represents the first approach towards developing a multi-antigenic, multi-stage synthetic malarial vaccine composed of subunits derived from different Plasmodium falciparum stage proteins. It is shown here that batches 03, 04, 05, 06, 07, 08, 09, 10, 11, 12, 13, 14, 15 and 16 produced from a few milligrams to kilogram amounts and used in assays on monkeys and humans showed high reproducibility in physicochemical analysis. (1)H NMR two-dimensional studies also revealed high similarity, even in non-oxidized batches. Reproducibility was also high, especially in preclinical studies carried out on Aotus, clinical trials Phase I, IIa and IIb and field-studies carried out in La Tola, Rio Rosario (Colombia), Majadas (Venezuela), La Te (Ecuador), Ifakara (Tanzania) in which there was high antibody titer production, having similar population distribution when done with different batches. These results provide great support for peptide-synthesized vaccines containing minimal epitopes from protection-inducing antigens which have several advantages, such as low cost, safety, reproducibility, stability, being straightforwardly scaled-up from milligram to kilogram amounts; make them the vaccines of choice for the future in a worldwide attempt to scourge diseases such as malaria.

High non-protective, long-lasting antibody levels in malaria are associated with haplotype shifting in MHC–peptide–TCR complex formation: a new mechanism for immune evasion

An effective malarial vaccine must contain multiple immunogenic, protection-inducing epitopes able to block and destroy the P. falciparum malaria parasite, the most lethal form of this disease in the world. Our strategy has consisted in using conserved peptides blocking parasite binding to red blood cells; however, these peptides are non-immunogenic and non-protection-inducing. Modifying their critical residues can make them immunogenic. Such peptides induced antibody titers (determined by immunofluorescence antibody test, IFA) and made the latter reactive (determined by Western blot) and protection inducing against experimental challenge with a highly infective Aotus monkey adapted P. falciparum strain. Modified peptides also induce highly non-protective long-lasting antibody levels. Modifications performed might allow them to bind specifically to different HLA-DRbeta purified molecules. These immunological and biological activities are associated with modifications in their three-dimensional structure as determined by (1)H-NMR. It was found that modified, high non-protective long-lasting antibody level peptides bound to HLA-DR molecules from a different haplotype (to which immunogenic, protection-inducers bind) and had 4.6 +/- 1.4 A shorter distances between residues fitting into these molecules' Pocket 1 to Pocket 9, suggesting fitting into an inappropriate HLA-DR molecule. A multi-component, subunit-based, malarial vaccine is therefore feasible if modified peptides are suitably modified for an appropriate fit into the correct HLA-DRbeta1* molecule in order to form a proper MHC-II-peptide-TCR complex.

Protective cellular immunity against P. falciparum malaria merozoites is associated with a different P7 and P8 residue orientation in the MHC–peptide–TCR complex

Developing a logical and rational methodology for obtaining vaccines, especially against the main parasite causing human malaria (P. falciparum), consists of blocking receptor-ligand interactions. Conserved peptides derived from proteins involved in invasion and having high red blood cell binding ability have thus been identified. Immunization studies using Aotus monkeys have revealed that these peptides were neither immunogenic nor protection inducing. When modified in their critical binding residues, previously identified by Glycine scanning, some of these peptides were immunogenic and non-protection inducers; others induced short-lived antibodies whilst a few were both immunogenic and protection inducing. However, very few of these modified high activity binding peptides (HABPs) reproducibly induced protection without inducing antibody production, but with high cytokine liberation, suggesting that cellular mechanisms had been activated in the protection process. The three-dimensional structure of these peptides inducing protection without producing antibodies was determined by 1H-NMR. Their HLA-DRbeta1* molecule binding ability was also determined to ascertain association between their 3D structure and ability to bind to Major Histocompatibility Complex Class-II molecules (MHC-II). 1H Nuclear Magnetic Resonance analysis and structure calculations clearly showed that these modified HABPs inducing protective cellular immune responses (but not producing antibodies against malaria) adopted special structural configuration to fit into the MHC II-peptide-TCR complex. A different orientation for P7 and P8 TCR contacting residues was clearly recognized when comparing their structure with modified peptides, which induced high antibody titers and protection, suggesting that these residues are involved in activating the immune system associated with antibody production and protection.

Current status of malaria vaccine development

There is an urgent need to develop an effective vaccine against malaria--a disease that has approximately 10% of the world population at risk of infection at any given time. The economic burden this disease puts on the medico-social set-up of countries in Sub-Saharan Africa and South East Asia is phenomenal. Increasing drug resistance and failure of vector control strategies have necessitated the search for a suitable vaccine that could be integrated into the extended program of immunization for countries in the endemic regions. Malaria vaccine development has seen a surge of activity in the last decade or so owing largely to the advances made in the fields of genetic engineering and biotechnology. This revolution has brought sweeping changes in the understanding of the biology of the parasite and has helped formulate newer more effective strategies to combat the disease. Latest developments in the field of malaria vaccine development will be discussed in this chapter.

Immediate-type hypersensitivity and other clinical reactions in volunteers immunized with a synthetic multi-antigen peptide vaccine (PfCS-MAP1NYU) against Plasmodium falciparum sporozoites

We tested the clinical reactions to a synthetic, Plasmodium falciparum, circumsporozoite multiple antigen peptide (MAP) vaccine in 39 volunteers immunized two to three times over 2-8 months using a dose escalation design. Immediate pain at the injection site was associated with the adjuvant QS-21 (P<0.001), and delayed local inflammatory reactions were associated with high-titered circulating IgG anti-MAP antibody (P=0.03). Because two volunteers developed acute, systemic urticaria after the third immunization associated with development of serum IgE MAP antibody, we employed immediate-type hypersensitivity skin tests (ITH-STs) using intradermal injections of diluted MAP vaccine to identify persons sensitized to the vaccine. ITH-STs were negative in seven volunteers tested 27 days after the first vaccination, but six of these individuals developed positive wheal and flare reactions when tested 14 or 83 days after the second vaccination; IgE MAP antibody was detected in only one of them. Another cohort of 16 volunteers, including the 2 allergic individuals, were ITH-ST negative when first tested late after their second or third vaccination at 6-7 months. Five of five non-immunized persons were also ITH-ST negative. ITH-STs may help identify individuals sensitized to malaria peptides and at potential risk of developing systemic allergic reactions after re-vaccination.

The development and use of vaccine adjuvants

Interest in vaccine adjuvants is intense and growing, because many of the new subunit vaccine candidates lack sufficient immunogenicity to be clinically useful. In this review, I have emphasized modern vaccine adjuvants injected parenterally, or administered orally, intranasally, or transcutaneously with licensed or experimental vaccines in humans. Every adjuvant has a complex and often multi-factorial immunological mechanism, usually poorly understood in vivo. Many determinants of adjuvanticity exist, and each adjuvanted vaccine is unique. Adjuvant safety is critical and can enhance, retard, or stop development of an adjuvanted vaccine. The choice of an adjuvant often depends upon expensive experimental trial and error, upon cost, and upon commercial availability. Extensive regulatory and administrative support is required to conduct clinical trials of adjuvanted vaccines. Finally, comparative adjuvant trials where one antigen is formulated with different adjuvants and administered by a common protocol to animals and humans can accelerate vaccine development.

Safety, tolerability and immunogenicity of new formulations of the Plasmodium falciparum malaria peptide vaccine SPf66 combined with the immunological adjuvant QS-21

SPf66 is a synthetic malaria peptide vaccine, which has been widely tested in combination with aluminium hydroxide (alum) as the adjuvant. Since this formulation is weakly immunogenic, we sought to improve its immunogenicity by using the saponin adjuvant QS-21. SPf66/QS-21 vaccines were evaluated for safety, tolerability and immunogenicity in healthy adults. The vaccines were found to be safe in 87/89 (97.8%) volunteers studied. However, two individuals developed severe vaccine allergy following the third dose of 1/3 SPf66/QS-21 formulations tested. Vaccine formulations containing QS-21 induced a 45- to over 200-fold increase in anti-SPf66 IgG titres over the alum formulation after the second and third doses, respectively. Anti-SPf66 antibody from some subjects reacted against asexual blood stage parasites, as demonstrated by immunofluorescence and immunoblotting. Antibody responses generated by the QS-21 formulations were of longer duration compared to those evoked by the alum formulation. While SPf66/alum has been found to induce only CD4+ T cell response, the QS-21 formulations exhibited the potential to also elicit SPf66-specific CD8+ responses. These observations demonstrate that the use of QS-21 can substantially enhance the immunogenicity of peptide vaccines, such as SPf66.

High immunogenicity in chimpanzees of peptides and lipopeptides derived from four new Plasmodium falciparum pre-erythrocytic molecules

We have investigated the immunogenicity in chimpanzees of twelve synthetic peptides derived from four new Plasmodium falciparum molecules expressed at pre-erythrocytic stages of the human malaria parasite. These parasite molecules were initially selected through their ability to be recognized by stage restricted human antibodies. Twelve 20- to 41-mer peptides representing potential human B- or T-cell epitopes were selected from these proteins, and synthesized. Six of these were modified by a C-terminal lipidic chain in order to re-inforce their immunogenicity. Strong B- and T-helper cell responses were induced in chimpanzees by lipopeptides injected without adjuvant and by peptides in Montanide. All twelve peptides induced CD4(+) T-cell proliferative responses, as well as the secretion of IFN-gamma (some of them at very high levels) and eleven peptides induced antibody responses. The immune responses elicited in this way were reactive with native parasite proteins, as shown by recall studies with sporozoite stage proteins, and proved to be long-lasting (up to 10 months after immunization). Our results support the strategy employed to select these four new malarial antigens and the corresponding peptides, and suggest that the immunizing formulations are both efficient and clinically acceptable.

Safety in infants of SPf66, a synthetic malaria vaccine, delivered alongside the EPI

The most likely mechanism to deliver a malaria vaccine in African countries is through the Expanded Program of Immunization (EPI). So far only SPf66, a multistage synthetic peptide, has shown any evidence of protection in Phase III field trials. In Tanzania, SPf66 reduced the risk of clinical malaria by 31% in children aged 1-5 years. In order to progress in the critical path of vaccine development and testing towards the implementation of a new vaccine in malaria control programs, we carried out a randomized double-blind placebo controlled efficacy trial of SPf66 when given alongside the EPI scheme. Monitoring of safety and reactogenicity during this trial included detailed clinical and laboratory assessments on 98 infants and assessment of adverse effects within 1 h of vaccination for all 1207 children vaccinated. Surveillance systems monitored attendances as outpatients, admissions to hospital and fatal events in the community. No serious adverse effects were detected more frequently amongst SPf66 recipients compared to placebo. This first assessment in very young infants of a synthetic vaccine provides evidence of a good safety profile.

Reduced amide pseudopeptide analogues of a malaria peptide possess secondary structural elements responsible for induction of functional antibodies which react with native proteins expressed in Plasmodium falciparum erythrocyte stages

A psi[CH2NH] isoster bond was introduced by replacing one peptide bond at a time within the 1513 malaria peptide KEKMV motif to obtain a set of five pseudopeptides. The motif belongs to a Plasmodium falciparum malarial peptide coded 1513, derived from the MSP-1 protein. This high-binding motif included in the 1513 peptide is involved in the attachment of the malarial parasite to human erythrocytes. The novel malaria 1513 psi[CH2NH] surrogates were analyzed using RP-HPLC and MALDI-TOF mass spectrometry techniques. Nuclear magnetic resonance experiments allowed definition of the five pseudopeptide analogues' secondary structural features. Such structures are present in only a very few molecules in the 1513 parent peptide. A molecular model demonstrating the solution of the three-dimensional structure of the 1 513 peptide Pse-437 analogue was constructed on the basis of 1H-NMR spectral parameters. Monoclonal antibodies were generated to the five 1513 malaria peptide pseudopeptide analogues. These antibodies not only recognize the native MSP-1 (195 kDa) and its 83 kDa and 42 kDa proteolytic processing proteins but also different SPf(66)n malaria vaccine batches containing the native sequence. In addition, the mAbs were able to modify the kinetics of Plasmodium falciparum parasites' intraerythrocytic development and their ability to invade new RBCs. The presented evidence suggests that peptide bond-modified peptides could reproduce a transient state in 1513's native sequence and represent useful candidates in the development of a second generation of effective malarial vaccines.

Comparison of prevalence of anti-hepatitis C virus antibodies in differing South American populations

Very little is known about the distribution of hepatitis C virus (HCV) within South America. To assess the exposure of the general population to this virus, a number of sera obtained from three distinct geographical and racial groups were screened using a combination of immunoassays. Initial screening was undertaken with an inhouse immunoassay (core-ELISA) using synthetic peptides based on the N-terminus of the HCV core protein. Sera which were repeatedly positive by core-ELISA were also assessed using a commercial third-generation assay. The highest prevalence rate (2.3%) was seen in sera taken from the Tumaco region of Colombia. Lower rates were found in sera taken from La T, Ecuador (0.7%) and Las Majadas, Venezuela (0.7%). This indicates significantly different prevalence in different racial and geographical groups within the region.

Randomised double-blind placebo-controlled trial of SPf66 malaria vaccine in children in northwestern Thailand. Shoklo SPf66 Malaria Vaccine Trial Group

BACKGROUND

Previous efficacy trials of SPf66 malaria vaccine have produced conflicting results in different populations. We report a randomised double-blind trial of the SPf66 vaccine conducted in Karen children aged 2-15 living in a malarious region of northwestern Thailand. Recombinant hepatitis B vaccine was used as a comparator.

METHODS

The study had a power of 90% to detect an efficacy of 30%, defined as a reduction in the incidence of first cases of symptomatic falciparum malaria after three doses of vaccine. 1221 children received three immunisations and were eligible for the primary efficacy analysis. Intense active and passive case detection continued over 15 months of follow-up.

FINDINGS

The SPf66 vaccine was well tolerated, although 26 children had mild or moderately severe local or systemic allergic reactions, compared with none in the comparator group. The vaccine was immunogenic; after three doses, 73% of recipients had seroconverted. There were no deaths due to malaria during the study. During the 15-month period of evaluation there were 379 first cases of symptomatic falciparum malaria (195 in the SPf66 recipients, 184 in the comparator group); an SPf66 efficacy of -9% (95% CI -33 to 14, p = 0.41). No significant differences between the two study groups in parasite density or any other measure of malaria-related morbidity were detected.

INTERPRETATION

These findings are consistent with a recent study showing lack of efficacy of SPf66 among Gambian infants and differ from earlier positive reports from South America and evidence of borderline efficacy from Tanzania. We conclude that SPf66 does not protect against clinical falciparum malaria and that further efficacy trials are not warranted.

Safety evaluation of SPf66 malaria vaccine in Brazil

The frequency and description of side effects secondary to the subcutaneous application of SPf66 malaria vaccine and placebo are reported for each dose of application in the participants of the vaccine efficacy trial in Brazil. Side effects evaluated two hours after each application were detected in 8.0%, 30.2% and 8.8%, for the 1st, 2nd and 3rd dose, respectively, in the SPf66 group, and in 7.0%, 8.5% and 2.9% in the placebo group. Local reactions such as mild inflammation, nodule and pain or erythema frequently accompanied by pruritus were the most common reactions detected in both groups (3.8%, 29.1% and 8.5% in the SPf66 group and 4.0%, 7.6% and 2.5% in the placebo group). Among vaccinees, local side effects after the 2nd dose were more frequent in females. Systemic side effects were expressed mainly through general symptoms referred by the participants and were most frequent after the 1st dose in both groups (4.3% in the SPf66 group and 3.0% in the placebo group). Muscle aches and fever were referred by few participants. No severe adverse reactions were detected for either dose of application or group.

Malaria vaccine trials: the missing qualitative data

Recent population-based efficacy trials of the synthetic malaria vaccine SPf66 have shown restricted, if any, clinical protection against Plasmodium falciparum infection. Despite the well-established role of antibodies in effector responses against asexual blood-stage malaria parasites, the titres of anti-SPf66 IgG antibodies do not correlate with the ability of sera from vaccine recipients to inhibit parasite growth in vitro nor with partial clinical protection which could be detected in some trials. Qualitative or functional parameters of SP66-induced antibody responses, such as IgG subclass composition and affinity, may be more predictive of clinical protection against malaria than quantitative estimates of antibody concentration or titre. Since these parameters are readily estimated by laboratory techniques currently available, and may be modulated by changes in vaccination protocols and by the use of different adjuvants, a better understanding of qualitative antibody responses induced by SPf66 and other asexual blood-stage malaria vaccine candidates, and of their relationship with clinical protection in vivo, is urgently needed for the improvement of currently used immunization schedules.

Malaria vaccines

Significant progress has been made in the development of the malaria vaccine during the last 20 years. Ninety percent of the 300-500 million clinical cases of malaria per year worldwide occur in Africa. Thus, research must be directed toward the 1 million African children under 5 years of age who die every year of malaria. An asexual blood-stage vaccine, capable of reducing severe and complicated malaria and malaria-related mortality, is therefore an important public health tool in these countries. Although knowledge of the parasite's biology is incomplete, research has allowed insight into some of the mechanisms that the parasite uses to evade host immunity. This is the basis for adopting an "antigenic cocktail" approach toward obtaining a synthetic or recombinant subunit vaccine such as the synthetic Colombian Malaria vaccine SPf 66. During the development of Spf66, field trials under both low and high malaria endemicity areas in Latin America and Africa have been carried out. The results from these studies showed a protective efficacy ranging between 38.8 and 60.2% against Plasmodium falciparum malaria. Given the characteristics of the normal immune response to malaria (relatively short-lived and not completely effective), it is understandable that the main goal is to try to increase the host's natural immunity. The best candidates for designing a malaria vaccine are the proteins required for parasite survival, those with low mutation rates and conserved epitopes. Because these proteins play an important role in multiple or alternative steps during the invasion process, they should be the targets against which a protective immune response should be elicited. The interaction between the malaria parasite and its host is complex. It is therefore crucial to define new ways of improving the immune response-such as directly modifying the chemical structure of epitopes or using new adjuvants or DNA immunization techniques-to produce novel vaccines against this disease.

Efficacy trial of malaria vaccine SPf66 in Gambian infants

SPf66 malaria vaccine is a synthetic protein with aminoacid sequences derived from pre-erythrocytic and asexual blood-stage proteins of Plasmodium falciparum. SPf66 was found to have a 31% protective efficacy in an area of intensive malaria transmission in Tanzanian children, 1-5 years old. We report a randomised, double-blind, placebo-controlled trial of SPf66 against clinical P falciparum malaria in Gambian infants. 630 children, aged 6-11 months at time of the first dose, received three doses of SPf66 or injected polio vaccine (IPV). Morbidity was monitored during the following rainy season by means of active and passive case detection. Cross-sectional surveys were carried out at the beginning and at the end of the rainy season. An episode of clinical malaria was defined as fever (> or = 37.5 degrees C) and a parasite density of 6000/microL or more. Analysis of efficacy was done on 547 children (316 SPf66/231 IPV). No differences in mortality or in health centre admissions were found between the two groups of children. 347 clinical episodes of malaria were detected during the three and a half months of surveillance. SPf66 vaccine was associated with a protective efficacy against the first or only clinical episode of 8% (95% CI -18 to 29, p = 0.50) and against the overall incidence of clinical episodes of malaria of 3% (95% CI -24 to 24, p = 0.81). No significant differences in parasite rates or in any other index of malaria were found between the two groups of children. The findings of this study differ from previous reports on SPf66 efficacy from South America and from Tanzania. In The Gambia, protection against clinical attacks of malaria during the rainy season after immunisation in children 6-11 months old at time of the first dose was not achieved.

Field trials of an asexual blood stage malaria vaccine: studies of the synthetic peptide polymer SPf66 in Thailand and the analytic plan for a phase IIb efficacy study

Several years ago the Walter Reed Army Institute of Research (WRAIR) initiated an independent analysis of the candidate malaria blood stage vaccine SPf66. WRAIR contracted for the synthesis and formulation of SPf66 in United States Food and Drug Administration (FDA) inspected laboratories within the U.S., and in 1992, filed an Investigational New Drug (IND) application with the FDA. Preclinical studies indicated that the vaccine could be synthesized to meet its release specifications, and when adjuvanted with alum, was essentially equivalent to Colombian produced SPf66 in regards to immunogenicity in preclinical studies of rodents and primates, and in human volunteers in Phase I studies. The goal of these efforts was ultimately to conduct a Phase IIb field trial to determine the safety and efficacy of SPf66 produced under current Good Manufacturing Practices (cGMP). Such a trial is currently underway in a malaria endemic refugee camp along the Thai-Burmese border. Here we briefly describe the study and present the formal analytic plan that was submitted to regulatory authorities in the United States for analysis of the study results. We believe such independent confirmatory studies are an essential part of the vaccine development process and are required to provide important data regarding the safety and efficacy of candidate vaccines in diverse geographical regions, and as a means to assess their role in the context of broader malaria control programmes.

Safety, immunogenicity and protective effect of the SPf66 malaria synthetic vaccine against Plasmodium falciparum infection in a randomized double-blind placebo-controlled field trial in an endemic area of Ecuador

A total of 537 subjects were randomized to receive either SPf66 malaria vaccine against Plasmodium falciparum or placebo in three doses (days 0, 30 and 180). Subjects completing the course of vaccination (230 in the vaccine and 238 in the placebo group) were followed up for a further 12 months. Case detection surveillance was implemented by parasitological cross-sectional surveys every 2 months and by monthly household visits to each participant. Symptomatic subjects were also diagnosed in a local health centre. Minor local side-effects were observed mainly after the second dose in about 19% of the vaccinated subjects and in 3.7% of the placebo group. Thirty days after the third dose the prevalence of anti-SPf66 antibodies was 57% in the vaccine and 8.8% in the placebo groups. The prevaccination prevalence of antibodies measured by indirect immunofluorescence assay increased with age and seemed to be inversely related to anti-SPf66 antibody production. Immune response to SPf66 was independent of age. Vaccine efficacy was calculated based on person-time of exposure. The protective effect considering any malaria episode was 66.8% (95% confidence interval = -2.7-89.3%) and considering only one episode per individual was 60.2% (95% confidence interval = -26-87.5%).

SPf66, a chemically synthesized subunit malaria vaccine, is safe and immunogenic in Tanzanians exposed to intense malaria transmission

As part of the first trial of the SPf66 malaria vaccine in Africa, three randomized double-blind placebo-controlled studies of SPf66 have been conducted in a highly endemic area of Tanzania. The objectives were to confirm that the product is immunogenic and safe in highly exposed individuals. Results from ten male adult expatriates indicated that the product used in Tanzania is at least as immunogenic as that used in Colombia. No major side-effects were observed in indigenous SPf66 recipients (18 adults, and 25 children aged 1-4 years). Anti-SPf66 antibody titres in all groups showed clear responses to three doses of the vaccine.

Case studies from the biomedical and health systems research activities of the Swiss Tropical Institute in Africa

Neither high theoretical efficacy of disease control tools, nor diagnostic accuracy, nor good compliance, nor adequate coverage can lead on its own to the final goal of community effectiveness. There is a complex relationship between these factors. The different steps in the process leading to effective health care in the community are discussed on the basis of biomedical and health systems research activities of the Swiss Tropical Institute. Schistosomiasis and malaria control provide the background to problems related to the efficacy of tools. In particular, information on the trial of a malaria vaccine candidate (SPf66) is given. Approaches to the rapid, accurate and economical diagnosis of communities at risk are discussed with reference to Schistosoma haematobium and S. mansoni. Health service support projects in Tanzania and Chad are presented to exemplify problems linked to the compliance of users/providers and coverage. Finally, it is shown that community effectiveness depends on the highest possible success rate for each step. This requires the co-operative efforts of all those involved: the scientist, the manager, the community health worker and, last but not least, the community itself.

Immunity to blood stages of malaria

Those developmental stages of malaria parasites that infect erythrocytes are responsible for the severe morbidity and mortality associated with this disease. The nature and specificity of the slowly acquired immunity seen in endemic populations remain to be defined, but significant progress has been made recently in identifying specific blood-stage proteins, characterizing immune responses to them, and exploring the dynamics of non-specific host responses to infection.

Vaccination with SPf66, a chemically synthesised vaccine, against Plasmodium falciparum malaria in Colombia

Preclinical and clinical studies have established the safety and immunogenicity of the chemically synthesised SPf66 malaria vaccine. The present study is a phase III randomised, double-blind, placebo-controlled, efficacy trial completed in La Tola, Colombia. 1548 volunteers over one year of age received three doses of either the vaccine (n = 738) or placebo (n = 810). Active and passive case detection methods were used to document clinical episodes of malaria among the study population. The follow-up period began one month after the third dose and lasted for one year. 168 and 297 episodes of Plasmodium falciparum malaria were documented in the SPf66 group and the placebo group, respectively; this corresponds to a crude protective efficacy of 38.8%. Incidence rates for first or only P falciparum malarial episodes were 22.3% per annum among the vaccinee group and 33.5% among the placebo group (RR = 1.5; 95% Cl 1.23, 1.84). Therefore, the protective efficacy of SPf66 against first or only episodes was 33.6% (95% Cl 18.8, 45.7), being highest in children aged 1-4 years (77%) and adults older than 45 years (67%). The estimated protective efficacy against second episodes was 50.5% (95% Cl 12.9-71.9). Our study shows that the chemically synthesised SPf66 malaria vaccine is safe, immunogenic, and protective against P falciparum malaria in semi-immune populations subject to natural challenge.

Malaria: protection of the international traveler

Thorough counseling with emphasis on the importance of mosquito bite avoidance, as well as on compliance with prophylactic antimalarial regimens, remains an important task for the physician preparing a patient for international travel. Chloroquine continues to be the drug of choice for prophylaxis in areas not reporting chloroquine resistance. Otherwise, mefloquine is the first-line agent. Under special circumstances, alternatives to mefloquine can be employed and include doxycycline or proguanil plus chloroquine.