Safety and immunogenicty of RTS,S/AS02A candidate malaria vaccine in Gambian children. Efficacy of RTS,S/AS02 malaria vaccine against Plasmodium falciparum infection in semi-immune adult men in The Gambia: a randomised trial. Plasmodium falciparum circumsporozoite vaccine immunogenicity and efficacy trial with natural challenge quantitation in an area of endemic human malaria of Kenya. Efficacy of recombinant circumsporozoite protein vaccine regimens against experimental Plasmodium falciparum malaria. Duration of protection with RTS,S/AS02A malaria vaccine in prevention of Plasmodium falciparum disease in Mozambican children: single-blind extended follow-up of a randomised controlled trial. Safety, immunogenicity and limited efficacy study of a recombinant Plasmodium falciparum circumsporozoite vaccine in Thai soldiers. [Vaccination against malaria: initial trial with an ant-sporozoite vaccine, (NANP)3-TT (RO 40-2361) in Africa (Bobo-Dioulasso, Burkina Faso)]. A randomised, double-blind, controlled vaccine efficacy trial of DNA/MVA ME-TRAP against malaria infection in Gambian adults.
RTS,S/AS02A is a pre-erythrocytic malaria vaccine candidate in which a portion of the circumsporozoite surface protein (CSP) of Plasmodium falciparum is genetically linked to hepatitis B surface antigen (HBsAg) coexpressed in yeast with unfused HBsAg. The resulting particulate antigen is formulated with the adjuvant system AS02A. We have initiated the paediatric clinical development of this vaccine by conducting two sequential Phase I studies in children: a study in older children (6--11 years), followed by a second study in younger children (1--5 years). In each study, a double-blind, randomised controlled, staggered, dose-escalation design was used to evaluate 10 microg RTS,S dose (10 microg RTS,S in 0.1mL AS02A), 25 microg dose (25 microg RTS,S in 0.25mL AS02A) and finally a 50 microg dose (50 microg RTS,S in 0.5mL AS02A) of the RTS,S/AS02A candidate malaria vaccine administered according to a 0-, 1- and 3-month vaccination schedule. Safety and reactogenicity were evaluated before moving to a higher dose level. The RTS,S/AS02A vaccine was safe at all dose levels, in both age groups. No serious adverse events related to vaccination were reported. The frequency of local Grade 3 symptoms was low but tended to increase with increasing dose level. Grade 3 general adverse events in the RTS,S/AS02A groups were infrequent and of short duration. The majority of local and general Grade 3 symptoms resolved or decreased in intensity within 48h. The pattern and intensity of reactogenicity seen in these studies are similar to those of previous studies with RTS,S/AS02A. All doses were highly immunogenic for anti-CSP and anti-HBsAg antibodies. The pooled anti-CSP antibody data from the two studies showed that the 25 microg dose and 50 microg dose anti-CSP antibody response were similar at both dose levels. However, the immunogenicity of the 10 microg dose anti-CSP response was significantly lower than that of either the 50 microg or 25 microg dose. The 25 microg dose was selected for future studies of RTS,S/AS02A in paediatric populations. RTS,S/AS02 is a pre-erythrocytic malaria vaccine based on the circumsporozoite surface protein of Plasmodium falciparum fused to HBsAg, incorporating a new adjuvant (AS02). We did a randomised trial of the efficacy of RTS,S/AS02 against natural P. falciparum infection in semi-immune adult men in The Gambia.
                306 men aged 18-45 years were randomly assigned three doses of either RTS,S/AS02 or rabies vaccine (control). Volunteers were given sulfadoxine/pyrimethamine 2 weeks before dose 3, and kept under surveillance throughout the malaria transmission season. Blood smears were collected once a week and whenever a volunteer developed symptoms compatible with malaria. The primary endpoint was time to first infection with P. falciparum. Analysis was per protocol.
                250 men (131 in the RTS,S/AS02 group and 119 in the control group) received three doses of vaccine and were followed up for 15 weeks. RTS,S/AS02 was safe and well tolerated. P. falciparum infections occurred significantly earlier in the control group than the RTS,S/AS02 group (Wilcoxon's test p=0.018). Vaccine efficacy, adjusted for confounders, was 34% (95% CI 8.0-53, p=0.014). Protection seemed to wane: estimated efficacy during the first 9 weeks of follow-up was 71% (46-85), but decreased to 0% (-52 to 34) in the last 6 weeks. Vaccination induced strong antibody responses to circumsporozoite protein and strong T-cell responses. Protection was not limited to the NF54 parasite genotype from which the vaccine was derived. 158 men received a fourth dose the next year and were followed up for 9 weeks; during this time, vaccine efficacy was 47% (4-71, p=0.037).
                RTS,S/AS02 is safe, immunogenic, and is the first pre-erythrocytic vaccine to show significant protection against natural P. falciparum infection. It has been hypothesized that antibody induced by Plasmodium falciparum circumsporozoite protein vaccine would be effective against endemic human malaria. In a malaria endemic region of Kenya, 76 volunteers, in 38 pairs sleeping adjacently, were immunized with subunit circumsporozoite protein Asn-Ala-Asn-Pro tetrapeptide repeat-pseudomonas toxin A, or hepatitis B vaccine. After quinine and doxcycycline, volunteers were followed for illness daily, parasitemia weekly, antibody, T-lymphocyte responses, and treated if indicated. Anopheles mosquitoes resting in houses were collected, and tested for P. falciparum antigen, or dissected for sporozoites and tested for blood meal ABO type and P. falciparum antigen. Vaccine was safe, with side-effects similar in both groups, and immunogenic, engendering IgG antibody as high as 600 micrograms ml-1, but did not increase the proportion of volunteers with T-lymphocyte responses. Estimation of P. falciparum challenge averaged 0.194 potentially infective Anopheles bites/volunteer/ day. Mosquito blood meals showed no difference in biting intensity between vaccine and control groups. Both groups had similar malaria-free survival curves, cumulative positive blood slides, cumulative parasites mm-3, and numbers of parasites mm-3 on first positive blood slide, during three post-vaccination observation periods. Every volunteer had P. falciparum parastemia at least once. Vaccinees had 82% and controls 89% incidences of symptomatic parasitemia (P = 0.514, efficacy 9%, statistical power 95% probability of efficacy < 50%). Vaccine-induced anti-sporozoite antibody was not protective in this study. Within designed statistical precisions the present study is in agreement with efficacy studies in Colombia, Venezuela and Tanzania. After initial successful evaluation of the circumsporozoite-based vaccine RTS,S/SBAS2, developed by SmithKline Beecham Biologicals with the Walter Reed Army Institute of Research, protective efficacy of several regimens against Plasmodium falciparum challenge was determined. A controlled phase 1/2a study evaluated 1 or 2 standard doses of RTS,S/SBAS2 in 2 groups whose members received open-label therapy and 3 immunizations in blinded groups who received standard, one-half, or one-fifth doses. RTS,S/SBAS2 was safe and immunogenic in all groups. Of the 41 vaccinees and 23 control subjects who underwent sporozoite challenge, malaria developed in 7 of 10 who received 1 dose, in 7 of 14 who received 2 doses, in 3 of 6 who received 3 standard doses, in 3 of 7 who received 3 one-half doses, in 3 of 4 who received 3 one-fifth doses, and in 22 of 23 control subjects. Overall protective efficacy of RTS,S/SBAS2 was 41% (95% confidence interval, 22%-56%; P=.0006). This and previous studies have shown that 2 or 3 doses of RTS,S/SBAS2 protect against challenge with P. falciparum sporozoites. RTS,S/AS02A is a pre-erythrocytic stage malaria vaccine that provides partial protection against infection in malaria-naive adult volunteers and hyperimmune adults. A previous report showed that this vaccine reduced risk of clinical malaria, delayed time to new infection, and reduced episodes of severe malaria over 6 months in African children. An important remaining issue is the durability of protection against clinical disease in these children.
                We did a randomised, controlled, phase IIb trial of RTS,S/AS02A given at 0, 1, and 2 months in 2022 Mozambican children aged 1-4 years. We previously determined vaccine efficacy (VE) against clinical malaria in a double-blind phase that included study months 2.5-8.5 (VE(2.5-8.5)). We now report VE in a single-blind phase up to month 21 (VE(8.5-21)). The primary endpoint was time to first or only clinical episode of Plasmodium falciparum malaria (axillary temperature 37.5 degrees C and P falciparum asexual parasitaemia >2500 per microL) detected through a passive case detection system. We also determined VE for other case definitions and for episodes of severe malaria. This study is registered with the ClinicalTrials.gov identifier NCT00197041.
                During the single-blind phase, VE(8.5-21) was 28.9% (95% CI 8.4-44.8; p=0.008). At month 21, prevalence of P falciparum infection was 29% lower in the RTS,S/AS02A group than in the control (p=0.017). Considering the entire study period, VE(2.5-21) was 35.3% (95% CI 21.6-46.6; p<0.0001) and VE(2.5-21) for severe malaria was 48.6% (95% CI 12.3-71.0; p=0.02).
                These results show that RTS,S/AS02A confers partial protection in African children aged 1-4 years living in rural endemic areas against a range of clinical disease caused by P falciparum for at least 18 months, and confirm the potential of malaria vaccines to become credible control tools for public-health use. Thai soldiers were vaccinated with a recombinant protein derived from the central repeat region of the circumsporozoite (CS) protein of Plasmodium falciparum conjugated to Toxin A (detoxified) of Pseudomonas aeruginosa (R32Tox-A) to evaluate its safety, immunogenicity and efficacy. In a randomized, double-blind manner, 199 volunteers received either R32Tox-A or a control vaccine at 0, 8 and 16 weeks. Immunization was performed in a malaria non-transmission area, after completion of which volunteers were deployed to an endemic border area and monitored closely to allow early detection and treatment of infection. The vaccine was found to be safe and to elicit antibody responses in all vaccinees. Peak CS antibody (IgG) concentrations in malaria-experienced vaccinees exceeded those in malaria-naive vaccinees (mean 40.6 versus 16.1 micrograms ml-1; p = 0.005) as well as those induced by previous CS protein-derived vaccines and observed in association with natural infections. A log-rank comparison of time to falciparum malaria revealed no differences between vaccinated and non-vaccinated subjects. Secondary analyses revealed that CS antibody levels were lower in vaccinee malaria cases than in non-cases, 3 and 5 months after the third dose of vaccine (p = 0.06 and p = 0.014, respectively). Because antibody levels had fallen substantially before peak malaria transmission occurred, the question of whether high levels of CS antibody are protective remains to be resolved. The vaccine (NANP)3-TT is a synthetic peptide of the circumsporozoite protein (CS) of Plasmodium falciparum coupled to tetanus toxoid (TT) as protein carrier and adsorbed to aluminium hydroxide as adjuvant. The objectives of the study were to assess the immunogenicity and the protective efficacy of the vaccine in an area where malaria is endemic. The study was conducted in a zone of irrigated rice cultivation known as the Vallée du Kou to the North of Bobo-Dioulasso. Malaria transmission is permanent in the Vallée with maxima in July and November. The study was conducted from June to December 1988. It was a controlled randomised, double blind, prospective vaccine trial. A total of 123 infants from 3 to 5 months of age were randomly assigned to three groups. Group I (controls) received three doses of TT alone, group II received two doses of TT and one of (NANP)3-TT and group III received three doses of (NANP)3-TT. These vaccines were administered simultaneously with the Enlarged Program of Immunisation (EPI) vaccines. The clinical parasitological and immunological status of the children was then monitored over a period of five months. No systemic reactions to the vaccine were observed in the infants either immediately after administration or during the follow-up. Minor local tumefactions were observed in only 3% of the children. The vaccine was found to be immunogenic with a peak IgG response at day 75, when 56% (group II) and 60% (group III) showed antibody titres of at least four times that seen at day 0. The response, however, was a short duration; by day 150 the average antibody titres were not significantly different between the three groups. The incidence and the level of parasiaemia and the incidence of clinical malaria were also not significantly different for each of the three groups during the period of the study. The association of (NANP)3 with tetanus toxoid was not shown to be immunologically inhibitive. The results, despite not showing a protective effect for the vaccine (NANP)3-TT, have shown its immunogenicity and therefore suggest that further development of this vaccine may be worthwhile. Many malaria vaccines are currently in development, although very few have been evaluated for efficacy in the field. Plasmodium falciparum multiple epitope (ME)- thrombospondin-related adhesion protein (TRAP) candidate vaccines are designed to potently induce effector T cells and so are a departure from earlier malaria vaccines evaluated in the field in terms of their mechanism of action. ME-TRAP vaccines encode a polyepitope string and the TRAP sporozoite antigen. Two vaccine vectors encoding ME-TRAP, plasmid DNA and modified vaccinia virus Ankara (MVA), when used sequentially in a prime-boost immunisation regime, induce high frequencies of effector T cells and partial protection, manifest as delay in time to parasitaemia, in a clinical challenge model.
                A total of 372 Gambian men aged 15-45 y were randomised to receive either DNA ME-TRAP followed by MVA ME-TRAP or rabies vaccine (control). Of these men, 296 received three doses of vaccine timed to coincide with the beginning of the transmission season (141 in the DNA/MVA group and 155 in the rabies group) and were followed up. Volunteers were given sulphadoxine/pyrimethamine 2 wk before the final vaccination. Blood smears were collected weekly for 11 wk and whenever a volunteer developed symptoms compatible with malaria during the transmission season. The primary endpoint was time to first infection with asexual P. falciparum. Analysis was per protocol. DNA ME-TRAP and MVA ME-TRAP were safe and well-tolerated. Effector T cell responses to a non-vaccine strain of TRAP were 50-fold higher postvaccination in the malaria vaccine group than in the rabies vaccine group. Vaccine efficacy, adjusted for confounding factors, was 10.3% (95% confidence interval, -22% to +34%; p = 0.49). Incidence of malaria infection decreased with increasing age and was associated with ethnicity.
                DNA/MVA heterologous prime-boost vaccination is safe and highly immunogenic for effector T cell induction in a malaria-endemic area. But despite having produced a substantial reduction in liver-stage parasites in challenge studies of non-immune volunteers, this first generation T cell-inducing vaccine was ineffective at reducing the natural infection rate in semi-immune African adults.
RTS,S vaccine was effective in preventing a significant number of clinical malaria episodes, including good protection against severe malaria in children for 18 months. No severe adverse events were attributable to the vaccine. Progression of this vaccine towards licensing is justified while efforts to increase its efficacy continue. The other vaccines do not look promising and further research is a priority.