Safety, immunogenicity and duration of protection of the RTS,S/AS02(D) malaria vaccine: one year follow-up of a randomized controlled phase I/IIb trial

Background

The RTS,S/AS02_D vaccine has been shown to have a promising safety profile, to be immunogenic and to confer protection against malaria in children and infants.

Methods and Findings

We did a randomized, controlled, phase I/IIb trial of RTS,S/AS02_D given at 10, 14 and 18 weeks of age staggered with routine immunization vaccines in 214 Mozambican infants. The study was double-blind until the young child completed 6 months of follow-up over which period vaccine efficacy against new Plasmodium falciparum infections was estimated at 65.9% (95% CI 42.6–79.8, p<0.0001). We now report safety, immunogenicity and estimated efficacy against clinical malaria up to 14 months after study start. Vaccine efficacy was assessed using Cox regression models. The frequency of serious adverse events was 32.7% in the RTS,S/AS02_D and 31.8% in the control group. The geometric mean titers of anti-circumsporozoite antibodies declined from 199.9 to 7.3 EU/mL from one to 12 months post dose three of RTS,S/AS02_D, remaining 15-fold higher than in the control group. Vaccine efficacy against clinical malaria was 33% (95% CI: −4.3–56.9, p = 0.076) over 14 months of follow-up. The hazard rate of disease per 2-fold increase in anti-CS titters was reduced by 84% (95% CI 35.1–88.2, p = 0.003).

Conclusion

The RTS,S/AS02_D malaria vaccine administered to young infants has a good safety profile and remains efficacious over 14 months. A strong association between anti-CS antibodies and risk of clinical malaria has been described for the first time. The results also suggest a decrease of both anti-CS antibodies and vaccine efficacy over time.

Trial Registration

ClinicalTrials.gov {"type":"clinical-trial","attrs":{"text":"NCT00197028","term_id":"NCT00197028"}}NCT00197028     

RTS,S Vaccination Is Associated With Serologic Evidence of Decreased Exposure to Plasmodium falciparum Liver- and Blood-Stage Parasites

The leading malaria vaccine candidate, RTS,S, targets the sporozoite and liver stages of the Plasmodium falciparum life cycle, yet it provides partial protection against disease associated with the subsequent blood stage of infection. Antibodies against the vaccine target, the circumsporozoite protein, have not shown sufficient correlation with risk of clinical malaria to serve as a surrogate for protection. The mechanism by which a vaccine that targets the asymptomatic sporozoite and liver stages protects against disease caused by blood-stage parasites remains unclear. We hypothesized that vaccination with RTS,S protects from blood-stage disease by reducing the number of parasites emerging from the liver, leading to prolonged exposure to subclinical levels of blood-stage parasites that go undetected and untreated, which in turn boosts pre-existing antibody-mediated blood-stage immunity. To test this hypothesis, we compared antibody responses to 824 P. falciparum antigens by protein array in Mozambican children 6 months after receiving a full course of RTS,S (n = 291) versus comparator vaccine (n = 297) in a Phase IIb trial. Moreover, we used a nested case-control design to compare antibody responses of children who did or did not experience febrile malaria. Unexpectedly, we found that the breadth and magnitude of the antibody response to both liver and asexual blood-stage antigens was significantly lower in RTS,S vaccinees, with the exception of only four antigens, including the RTS,S circumsporozoite antigen. Contrary to our initial hypothesis, these findings suggest that RTS,S confers protection against clinical malaria by blocking sporozoite invasion of hepatocytes, thereby reducing exposure to the blood-stage parasites that cause disease. We also found that antibody profiles 6 months after vaccination did not distinguish protected and susceptible children during the subsequent 12-month follow-up period but were strongly associated with exposure. Together, these data provide insight into the mechanism by which RTS,S protects from malaria.The RTS,S malaria vaccine candidate provides partial protection against clinical malaria in African children, which has been repeatedly demonstrated in Phase IIb and Phase III clinical trials (1–5). The RTS,S target is the Plasmodium falciparum circumsporozoite protein (CSP), and it has been shown to generate high antibody titers that remain above levels acquired naturally for years (6). However, it remains unclear how the vaccine, which targets sporozoites, provides protection against disease caused by blood-stage parasites. A rational mechanism has been proposed, based on antibody and T cell responses to the CSP (7), but antibodies have not consistently correlated with protection when clinical disease was the trial end point (8). We and others hypothesized that partial blockage of pre-erythrocytic development would result in low-level blood-stage infections that go untreated in RTS,S vaccinees and that this would boost the blood-stage immune response, contributing to protection from malaria disease (8–10).We set out to address the question of how the vaccine works by investigating the response to malaria parasites in the context of RTS,S vaccination. However, until recently, the means of assessing the response to malaria parasites has been limited to a sparse selection of recombinant proteins or parasite lysates. The P. falciparum (Pf) proteome contains more than 5,300 proteins, and, until recently, less than 0.5% of them have been closely investigated (11). Similar to the approach taken with gene expression microarrays, protein arrays offer the opportunity to screen antibody responses to partial or complete proteomes (12). This approach was taken in this study to identify the breadth and magnitude of naturally acquired immune responses in Mozambican children vaccinated with RTS,S/AS02¹, the predecessor to the RTS,S/AS01 formulation used in the current Phase III trial, or comparator vaccine.In addition to characterizing the RTS,S mode of action, we aimed to identify biomarker correlates of protection against clinical malaria. Malaria vaccinology is lacking in surrogate markers of protection, and such biomarkers would be a highly useful measure for assessment of vaccine efficacy, especially when control or placebo vaccine groups are no longer available (13). This could mitigate the current inefficient means of measuring efficacy in clinical trials. In the post-genomic era, with systems approaches employed for questions to complex problems in biology and medicine, perhaps alternative thinking is required to tackle the question of how to assess vaccines (14, 15). In this study, we took steps in that direction in order to identify antibody signatures of protection that contribute toward a surrogate marker for the RTS,S and other vaccines.     

Radiological Findings in Young Children Investigated for Tuberculosis in Mozambique

IntroductionChest radiography remains a critical tool for diagnosing intrathoracic tuberculosis (TB) in young children who are unable to expectorate. We describe the radiological findings in children under 3 years of age investigated for TB in the district of Manhiça, southern Mozambique, an area with a high prevalence of TB and HIV.MethodsDigital antero-posterior and lateral projections were performed and reviewed by two independent readers, using a standardized template. Readers included a local pediatrician and a pediatric radiologist blinded to all clinical information. International consensus case definitions for intra-thoracic TB in children were applied.ResultsA total of 766 children were evaluated of whom 43 (5.6%) had TB. The most frequent lesion found in TB cases was air space consolidation (65.1%), followed by suggestive hilar lymphadenopathy (17.1%) and pleural effusion (7.0%). Air space consolidation was significantly more common in TB cases than in non-TB cases (odds ratio 8.9; 95% CI: 1.6-50.5), as were hilar lymphadenopathy (OR 17.2; 95% CI: 5.7-52.1). The only case with miliary infiltrates and 3 with pleural effusions occurred in HIV-infected children.ConclusionFrequent air space consolidation complicates radiological distinction between TB and bacterial pneumonia in young children, underscoring the need for epidemiological contextualization and consideration of all relevant signs and symptoms.     

Impact of the RTS,S malaria vaccine candidate on naturally acquired antibody responses to multiple asexual blood stage antigens

Background

Partial protective efficacy lasting up to 43 months after vaccination with the RTS,S malaria vaccine has been reported in one cohort (C1) of a Phase IIb trial in Mozambique, but waning efficacy was observed in a smaller contemporaneous cohort (C2). We hypothesized that low dose exposure to asexual stage parasites resulting from partial pre-erythrocytic protection afforded by RTS,S may contribute to long-term vaccine efficacy to clinical disease, which was not observed in C2 due to intense active detection of infection and treatment.

Methodology/Principal Findings

Serum collected 6 months post-vaccination was screened for antibodies to asexual blood stage antigens AMA-1, MSP-1₄₂, EBA-175, DBL-α and variant surface antigens of the R29 laboratory strain (VSA_R29). Effect of IgG on the prospective hazard of clinical malaria was estimated. No difference was observed in antibody levels between RTS,S and control vaccine when all children aged 1–4 years at enrollment in both C1 and C2 were analyzed together, and no effects were observed between cohort and vaccine group. RTS,S-vaccinated children <2 years of age at enrollment had lower levels of IgG for AMA-1 and MSP-1₄₂ (p<0.01, all antigens), while no differences were observed in children ≥2 years. Lower risk of clinical malaria was associated with high IgG to EBA-175 and VSA_R29 in C2 only (Hazard Ratio [HR]: 0.76, 95% CI 0.66–0.88; HR: 0.75, 95% CI 0.62–0.92, respectively).

Conclusions

Vaccination with RTS,S modestly reduces anti-AMA-1 and anti-MSP-1 antibodies in very young children. However, for antigens associated with lower risk of clinical malaria, there were no vaccine group or cohort-specific effects, and age did not influence antibody levels between treatment groups for these antigens. The antigens tested do not explain the difference in protective efficacy in C1 and C2. Other less-characterized antigens or VSA may be important to protection.

Trial Registration

ClinicalTrials.gov {"type":"clinical-trial","attrs":{"text":"NCT00197041","term_id":"NCT00197041"}}NCT00197041