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.

BACKGROUND: 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. METHODS: 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. FINDINGS: 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). INTERPRETATION: 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.

Efficacy of the RTS,S/AS02A vaccine against Plasmodium falciparum infection and disease in young African children: randomised controlled trial.

BACKGROUND: Development of an effective malaria vaccine could greatly contribute to disease control. RTS,S/AS02A is a pre-erythrocytic vaccine candidate based on Plasmodium falciparum circumsporozoite surface antigen. We aimed to assess vaccine efficacy, immunogenicity, and safety in young African children. METHODS: We did a double-blind, phase IIb, randomised controlled trial in Mozambique in 2022 children aged 1-4 years. The study included two cohorts of children living in two separate areas which underwent different follow-up schemes. Participants were randomly allocated three doses of either RTS,S/AS02A candidate malaria vaccine or control vaccines. The primary endpoint, determined in cohort 1 (n=1605), was time to first clinical episode of P falciparum malaria (axillary temperature > or =37.5 degrees C and P falciparum asexual parasitaemia >2500 per microL) over a 6-month surveillance period. Efficacy for prevention of new infections was determined in cohort 2 (n=417). Analysis was per protocol. FINDINGS: 115 children in cohort 1 and 50 in cohort 2 did not receive all three doses and were excluded from the per-protocol analysis. Vaccine efficacy for the first clinical episodes was 29.9% (95% CI 11.0-44.8; p=0.004). At the end of the 6-month observation period, prevalence of P falciparum infection was 37% lower in the RTS,S/AS02A group compared with the control group (11.9% vs 18.9%; p=0.0003). Vaccine efficacy for severe malaria was 57.7% (95% CI 16.2-80.6; p=0.019). In cohort 2, vaccine efficacy for extending time to first infection was 45.0% (31.4-55.9; p<0.0001). INTERPRETATION: The RTS,S/AS02A vaccine was safe, well tolerated, and immunogenic. Our results show development of an effective vaccine against malaria is feasible.

Long-term safety and efficacy of the RTS,S/AS02A malaria vaccine in Mozambican children.

BACKGROUND: We previously reported that the RTS,S/AS02A vaccine had an acceptable safety profile, was immunogenic, and demonstrated efficacy against Plasmodium falciparum malaria disease for 21 months. METHODS: We conducted a randomized, controlled, phase 2b trial of RTS,S/AS02A in 2022 Mozambican children aged 1-4 years. We now report safety results for all randomized subjects and vaccine efficacy (VE) findings for children in the Manhiça area over the 45-month surveillance period. RESULTS: During the surveillance period, the VE((2.5-45)) (VE over months 2.5-45 of surveillance) against a first or only episode of clinical malaria disease was 30.5% (95% confidence interval [CI], 18.9%-40.4%; P < .001), and the VE((2.5-45)) against all episodes was 25.6% (95% CI, 11.9%-37.1%; P < .001). When the same period was considered, the VE((2.5-45)) for subjects protected against severe malaria was 38.3% (95% CI, 3.4%-61.3%; P = .045). At study month 45, the prevalence of P. falciparum was 34% lower in the RTS,S/AS02A group than in the control group (66 [12.2%] of 541 patients vs 101 [18.5%] of 547 patients) (P = .004). CONCLUSION: These results show evidence that RTS,S/AS02A maintained protection during the 45-month surveillance period, and they highlight the feasibility of developing an effective vaccine against malaria. In combination with other malaria-control measures, such a vaccine could greatly contribute to reducing the intolerable global burden of this disease. Trial registration. ClinicalTrials.gov identifiers NCT00197041 and NCT00323622 .

Blood stage malaria vaccine eliciting high antigen-specific antibody concentrations confers no protection to young children in Western Kenya.

OBJECTIVE: The antigen, falciparum malaria protein 1 (FMP1), represents the 42-kDa C-terminal fragment of merozoite surface protein-1 (MSP-1) of the 3D7 clone of P. falciparum. Formulated with AS02 (a proprietary Adjuvant System), it constitutes the FMP1/AS02 candidate malaria vaccine. We evaluated this vaccine's safety, immunogenicity, and efficacy in African children. METHODS: A randomised, double-blind, Phase IIb, comparator-controlled trial.The trial was conducted in 13 field stations of one mile radii within Kombewa Division, Nyanza Province, Western Kenya, an area of holoendemic transmission of P. falciparum. We enrolled 400 children aged 12-47 months in general good health.Children were randomised in a 1ratio1 fashion to receive either FMP1/AS02 (50 microg) or Rabipur(R) rabies vaccine. Vaccinations were administered on a 0, 1, and 2 month schedule. The primary study endpoint was time to first clinical episode of P. falciparum malaria (temperature >/=37.5 degrees C with asexual parasitaemia of >/=50,000 parasites/microL of blood) occurring between 14 days and six months after a third dose. Case detection was both active and passive. Safety and immunogenicity were evaluated for eight months after first immunisations; vaccine efficacy (VE) was measured over a six-month period following third vaccinations. RESULTS: 374 of 400 children received all three doses and completed six months of follow-up. FMP1/AS02 had a good safety profile and was well-tolerated but more reactogenic than the comparator. Geometric mean anti-MSP-1(42) antibody concentrations increased from1.3 microg/mL to 27.3 microg/mL in the FMP1/AS02 recipients, but were unchanged in controls. 97 children in the FMP1/AS02 group and 98 controls had a primary endpoint episode. Overall VE was 5.1% (95% CI: -26% to +28%; p-value = 0.7). CONCLUSIONS: FMP1/AS02 is not a promising candidate for further development as a monovalent malaria vaccine. Future MSP-1(42) vaccine development should focus on other formulations and antigen constructs. TRIAL REGISTRATION: Clinicaltrials.gov NCT00223990.

Safety of the RTS,S/AS02A malaria vaccine in Mozambican children during a Phase IIb trial.

RTS,S/AS02A is a pre-erythrocytic vaccine candidate based on the Plasmodium falciparum circumsporozoite surface antigen and is currently the most advanced malaria vaccine candidate in development. A proof of concept phase IIb trial of the RTS,S/AS02A in Mozambican children aged 1-4 years determined a vaccine efficacy against risk of clinical malaria of 35.3% (95% CI 21.6-46.6; p<0.0001) and against severe malaria of 48.6% (95% CI 12.3-71.0; p=0.02). We evaluated the safety of the RTS,S/AS02A vaccine. 2022 children that received at least one vaccine dose of RTS,S/AS02A or control vaccines were included in the intention to treat safety analysis. Vaccine safety was evaluated using active and passive follow-up. Participants were observed for at least 1h after each dose. Trained field workers visited children at home daily for the next 3 days to record solicited and unsolicited local and general symptoms. Investigators followed-up participants with severe adverse events until month 21. Overall, we recorded 1712 unsolicited adverse events after vaccination, 53% in the intervention and 47% in the control group. Most unsolicited adverse events reported with RTS,S/AS02A were self-limited, and participants recovered without sequelae. Local reactogenicity increased with the number of doses. The proportion of children experiencing serious adverse events was lower in the RTS,S/AS02A recipients compared to the control group (Engerix-Btrade mark or Prevnartrade mark and Hiberixtrade mark). Overall, these results indicate that the RTS,S/AS02A vaccine has a good safety profile and well tolerated when given in three doses to semi-immune children living in malaria-endemic areas.

Phase 1 randomized double-blind safety and immunogenicity trial of Plasmodium falciparum malaria merozoite surface protein FMP1 vaccine, adjuvanted with AS02A, in adults in western Kenya.

We report the first trial of candidate malaria vaccine antigen FMP1, a 42kDa fragment from the C-terminus of merozoite surface protein-1 (MSP-1) from the 3D7 strain of Plasmodium falciparum, in an endemic area. Forty adult male and female residents of western Kenya were enrolled to receive 3 doses of either FMP1/AS02A or Imovax rabies vaccine by intra-deltoid injection on a 0, 1, 2 month schedule. Thirty-seven volunteers received all three immunizations and 38 completed the 12-month evaluation period. Slightly more recipients of the FMP1/AS02A vaccine experienced any instance of pain at 24 h post-immunization than in the Imovax group (95% versus 65%), but otherwise the two vaccines were equally safe and well-tolerated. Baseline antibody levels were high in both groups and were boosted in the FMP1/AS02A group. Longitudinal models revealed a highly significant difference between groups for both the average post-baseline antibody responses to MSP-1(42) (F1,335=13.16; P<0.001) and the Day 90 responses to MSP-1(42) (F1,335=16.69; P<0.001). The FMP1/AS02A vaccine is safe and immunogenic in adults and should progress to safety testing in children at greatest risk of malaria.

Evaluation of two formulations of adjuvanted RTS, S malaria vaccine in children aged 3 to 5 years living in a malaria-endemic region of Mozambique: a Phase I/IIb randomized double-blind bridging trial.

BACKGROUND: Previous trials of the RTS, S malaria candidate vaccine have shown that this vaccine is safe, tolerated and immunogenic. The development plan for this vaccine aims at administering it in the first year of life through the Expanded Program on Immunization (EPI). The objective was to evaluate the safety and reactogenicity of RTS, S/AS02D (0.5 ml dose), a pediatric formulation of GlaxoSmithKline Biologicals' current malaria candidate vaccine RTS, S/AS02A (0.25 ml dose). A 0.5 ml dose of AS02D is composed of the same active ingredients in the same quantities as in a 0.25 ml dose of AS02A and has been developed to be easily introduced into routine EPI practices. METHODS: We performed a phase I/IIb randomized double-blind bridging study in a malaria-endemic region of Mozambique, to compare the safety and immunogenicity of both candidate vaccines with the aim of replacing RTS, S/AS02A with RTS, S/AS02D as the candidate pediatric vaccine. 200 Mozambican children aged 3 to 5 years were randomized 1:1 to receive one of the 2 vaccines according to a 0, 1, 2 month schedule. RESULTS: Both vaccines were safe and had similar reactogenicity profiles. All subjects with paired pre and post-vaccination samples showed a vaccine response with respect to anti-circumsporozoite (CS) antibodies irrespective of initial anti-CS serostatus. Geometric mean titers (GMTs) were 191 EU/ml (95% CI 150-242) in recipients of RTS, S/AS02D compared to 180 EU/ml (95% CI 146-221) in recipients of RTS, S/AS02A. For the anti-hepatitis B surface antigen (HBsAg), all subjects were seroprotected at day 90, and the GMTs were 23,978 mIU/ml (95% CI 17,896-32,127) in RTS, S/AS02D recipients and 17,410 mIU/ml (95% CI 13,322-22,752) in RTS, S/AS02A recipients. There was a decrease in anti-CS GMTs between months 3 and 14 in both groups (191 vs 22 EU/mL in RTS, S/AS02D group and 180 vs 29 EU/mL in RTS, S/AS02A group). CONCLUSION: Our data show that the RTS, S/AS02D is safe, well tolerated, and demonstrates non-inferiority (defined as upper limit of the 95% confidence interval of the anti-CS GMT ratio of RTS, S/AS02A to RTS, S/AS02D below 3.0) of the antibody responses to circumsporozoite and HBsAg induced by the RTS, S/AS02D as compared to the RTS, S/AS02A.

RTS,S/AS02A malaria vaccine does not induce parasite CSP T cell epitope selection and reduces multiplicity of infection.

OBJECTIVE: The candidate malaria vaccine RTS,S/AS02A is a recombinant protein containing part of the circumsporozoite protein (CSP) sequence of Plasmodium falciparum, linked to the hepatitis B surface antigen and formulated in the proprietary adjuvant system AS02A. In a recent trial conducted in children younger than age five in southern Mozambique, the vaccine demonstrated significant and sustained efficacy against both infection and clinical disease. In a follow-up study to the main trial, breakthrough infections identified in the trial were examined to determine whether the distribution of csp sequences was affected by the vaccine and to measure the multiplicity of infecting parasite genotypes. DESIGN: P. falciparum DNA from isolates collected during the trial was used for genotype studies. SETTING: The main trial was carried out in the Manhiça district, Maputo province, Mozambique, between April 2003 and May 2004. PARTICIPANTS: Children from the two cohorts of the main trial provided parasite isolates as follows: children from Cohort 1 who were admitted to hospital with clinical malaria; children from Cohort 1 who were parasite-positive in a cross-sectional survey at study month 8.5; children from Cohort 2 identified as parasite-positive during follow-up by active detection of infection. OUTCOME: Divergence of DNA sequence encoding the CSP T cell-epitope region sequence from that of the vaccine sequence was measured in 521 isolates. The number of distinct P. falciparum genotypes was also determined. RESULTS: We found no evidence that parasite genotypes from children in the RTS,S/AS02A arm were more divergent than those receiving control vaccines. For Cohort 1 (survey at study month 8.5) and Cohort 2, infections in the vaccine group contained significantly fewer genotypes than those in the control group, (p = 0.035, p = 0.006), respectively, for the two cohorts. This was not the case for children in Cohort 1 who were admitted to hospital (p = 0.478). CONCLUSIONS: RTS,S/AS02A did not select for genotypes encoding divergent T cell epitopes in the C-terminal region of CSP in this trial. In both cohorts, there was a modest reduction in the mean number of parasite genotypes harboured by vaccinated children compared with controls, but only among those with asymptomatic infections.

Safety and reactogenicity of an MSP-1 malaria vaccine candidate: a randomized phase Ib dose-escalation trial in Kenyan children.

OBJECTIVE: Our aim was to evaluate the safety, reactogenicity, and immunogenicity of an investigational malaria vaccine. DESIGN: This was an age-stratified phase Ib, double-blind, randomized, controlled, dose-escalation trial. Children were recruited into one of three cohorts (dosage groups) and randomized in 2:1 fashion to receive either the test product or a comparator. SETTING: The study was conducted in a rural population in Kombewa Division, western Kenya. PARTICIPANTS: Subjects were 135 children, aged 12-47 mo. INTERVENTIONS: Subjects received 10, 25, or 50 microg of falciparum malaria protein 1 (FMP1) formulated in 100, 250, and 500 microL, respectively, of AS02A, or they received a comparator (Imovax (rabies vaccine). OUTCOME MEASURES: We performed safety and reactogenicity parameters and assessment of adverse events during solicited (7 d) and unsolicited (30 d) periods after each vaccination. Serious adverse events were monitored for 6 mo after the last vaccination. RESULTS: Both vaccines were safe and well tolerated. FMP1/AS02A recipients experienced significantly more pain and injection-site swelling with a dose-effect relationship. Systemic reactogenicity was low at all dose levels. Hemoglobin levels remained stable and similar across arms. Baseline geometric mean titers were comparable in all groups. Anti-FMP1 antibody titers increased in a dose-dependent manner in subjects receiving FMP1/AS02A; no increase in anti-FMP1 titers occurred in subjects who received the comparator. By study end, subjects who received either 25 or 50 microg of FMP1 had similar antibody levels, which remained significantly higher than that of those who received the comparator or 10 microg of FMP1. A longitudinal mixed effects model showed a statistically significant effect of dosage level on immune response (F(3,1047) = 10.78, or F(3, 995) = 11.22, p < 0.001); however, the comparison of 25 microg and 50 microg recipients indicated no significant difference (F(1,1047) = 0.05; p = 0.82). CONCLUSIONS: The FMP1/AS02A vaccine was safe and immunogenic in malaria-exposed 12- to 47-mo-old children and the magnitude of immune response of the 25 and 50 microg doses was superior to that of the 10 microg dose.

Safety and immunogenicty of RTS,S/AS02A candidate malaria vaccine in Gambian children.

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.

Towards an RTS,S-based, multi-stage, multi-antigen vaccine against falciparum malaria: progress at the Walter Reed Army Institute of Research.

The goal of the Malaria Vaccine Program at the Walter Reed Army Institute of Research (WRAIR) is to develop a licensed multi-antigen, multi-stage vaccine against Plasmodium falciparum able to prevent all symptomatic manifestations of malaria by preventing parasitemia. A secondary goal is to limit disease in vaccinees that do develop malaria. Malaria prevention will be achieved by inducing humoral and cellular immunity against the pre-erythrocytic circumsporozoite protein (CSP) and the liver stage antigen-1 (LSA-1). The strategy to limit disease will target immune responses against one or more blood stage antigens, merozoite surface protein-1 (MSP-1) and apical merozoite antigen-1 (AMA-1). The induction of T- and B-cell memory to achieve a sustained vaccine response may additionally require immunization with an adenovirus vector such as adenovirus serotype 35. RTS,S, a CSP-derived antigen developed by GlaxoSmithKline Biologicals in collaboration with the Walter Reed Army Institute of Research over the past 17 years, is the cornerstone of our program. RTS,S formulated in AS02A (a GSK proprietary formulation) is the only vaccine candidate shown in field trials to prevent malaria and, in one instance, to limit disease severity. Our vaccine development plan requires proof of an individual antigen's efficacy in a Phase 2 laboratory challenge or field trial prior to its integration into an RTS,S-based, multi-antigen vaccine. Progress has been accelerated through extensive partnerships with industrial, academic, governmental, and non-governmental organizations. Recent safety, immunogenicity, and efficacy trials in the US and Africa are presented, as well as plans for the development of a multi-antigen vaccine.