The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the most formidable challenge to humanity in a century. It is widely believed that prepandemic normalcy will never return until a safe and effective vaccine strategy becomes available and a global vaccination programme is implemented successfully. Here, we discuss the immunological principles that need to be taken into consideration in the development of COVID-19 vaccine strategies. On the basis of these principles, we examine the current COVID-19 vaccine candidates, their strengths and potential shortfalls, and make inferences about their chances of success. Finally, we discuss the scientific and practical challenges that will be faced in the process of developing a successful vaccine and the ways in which COVID-19 vaccine strategies may evolve over the next few years.

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Immunological considerations for COVID-19 vaccine strategies.

A safe and effective vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be required to end the coronavirus disease 2019 (COVID-19) pandemic1-8. For global deployment and pandemic control, a vaccine that requires only a single immunization would be optimal. Here we show the immunogenicity and protective efficacy of a single dose of adenovirus serotype 26 (Ad26) vector-based vaccines expressing the SARS-CoV-2 spike (S) protein in non-human primates. Fifty-two rhesus macaques (Macaca mulatta) were immunized with Ad26 vectors that encoded S variants or sham control, and then challenged with SARS-CoV-2 by the intranasal and intratracheal routes9,10. The optimal Ad26 vaccine induced robust neutralizing antibody responses and provided complete or near-complete protection in bronchoalveolar lavage and nasal swabs after SARS-CoV-2 challenge. Titres of vaccine-elicited neutralizing antibodies correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate robust single-shot vaccine protection against SARS-CoV-2 in non-human primates. The optimal Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in clinical trials.

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Single-shot Ad26 vaccine protects against SARS-CoV-2 in rhesus macaques.

Throughout its 40-year history, the field of gene therapy has been marked by many transitions. It has seen great strides in combating human disease, has given hope to patients and families with limited treatment options, but has also been subject to many setbacks. Treatment of patients with this class of investigational drugs has resulted in severe adverse effects and, even in rare cases, death. At the heart of this dichotomous field are the viral-based vectors, the delivery vehicles that have allowed researchers and clinicians to develop powerful drug platforms, and have radically changed the face of medicine. Within the past 5 years, the gene therapy field has seen a wave of drugs based on viral vectors that have gained regulatory approval that come in a variety of designs and purposes. These modalities range from vector-based cancer therapies, to treating monogenic diseases with life-altering outcomes. At present, the three key vector strategies are based on adenoviruses, adeno-associated viruses, and lentiviruses. They have led the way in preclinical and clinical successes in the past two decades. However, despite these successes, many challenges still limit these approaches from attaining their full potential. To review the viral vector-based gene therapy landscape, we focus on these three highly regarded vector platforms and describe mechanisms of action and their roles in treating human disease.

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Viral vector platforms within the gene therapy landscape

Development of effective preventative interventions against SARS-CoV-2, the etiologic agent of COVID-19 is urgently needed. The viral surface spike (S) protein of SARS-CoV-2 is a key target for prophylactic measures as it is critical for the viral replication cycle and the primary target of neutralizing antibodies. We evaluated design elements previously shown for other coronavirus S protein-based vaccines to be successful, e.g., prefusion-stabilizing substitutions and heterologous signal peptides, for selection of a S-based SARS-CoV-2 vaccine candidate. In vitro characterization demonstrated that the introduction of stabilizing substitutions (i.e., furin cleavage site mutations and two consecutive prolines in the hinge region of S2) increased the ratio of neutralizing versus non-neutralizing antibody binding, suggestive for a prefusion conformation of the S protein. Furthermore, the wild-type signal peptide was best suited for the correct cleavage needed for a natively folded protein. These observations translated into superior immunogenicity in mice where the Ad26 vector encoding for a membrane-bound stabilized S protein with a wild-type signal peptide elicited potent neutralizing humoral immunity and cellular immunity that was polarized towards Th1 IFN-γ. This optimized Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in a phase I clinical trial (ClinicalTrials.gov Identifier: NCT04436276).

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Ad26 vector-based COVID-19 vaccine encoding a prefusion-stabilized SARS-CoV-2 Spike immunogen induces potent humoral and cellular immune responses.

Importance Developing effective vaccines against Ebola virus is a global priority. Objective To evaluate an adenovirus type 26 vector vaccine encoding Ebola glycoprotein (Ad26.ZEBOV) and a modified vaccinia Ankara vector vaccine, encoding glycoproteins from Ebola virus, Sudan virus, Marburg virus, and Tai Forest virus nucleoprotein (MVA-BN-Filo). Design, Setting, and Participants Single-center, randomized, placebo-controlled, observer-blind, phase 1 trial performed in Oxford, United Kingdom, enrolling healthy 18- to 50-year-olds from December 2014; 8-month follow-up was completed October 2015. Interventions Participants were randomized into 4 groups, within which they were simultaneously randomized 5:1 to receive study vaccines or placebo. Those receiving active vaccines were primed with Ad26.ZEBOV (5 × 10 10 viral particles) or MVA-BN-Filo (1 × 10 8 median tissue culture infective dose) and boosted with the alternative vaccine 28 or 56 days later. A fifth, open-label group received Ad26.ZEBOV boosted by MVA-BN-Filo 14 days later. Main Outcomes and Measures The primary outcomes were safety and tolerability. All adverse events were recorded until 21 days after each immunization; serious adverse events were recorded throughout the trial. Secondary outcomes were humoral and cellular immune responses to immunization, as assessed by enzyme-linked immunosorbent assay and enzyme-linked immunospot performed at baseline and from 7 days after each immunization until 8 months after priming immunizations. Results Among 87 study participants (median age, 38.5 years; 66.7% female), 72 were randomized into 4 groups of 18, and 15 were included in the open-label group. Four participants did not receive a booster dose; 67 of 75 study vaccine recipients were followed up at 8 months. No vaccine-related serious adverse events occurred. No participant became febrile after MVA-BN-Filo, compared with 3 of 60 participants (5%; 95% CI, 1%-14%) receiving Ad26.ZEBOV in the randomized groups. In the open-label group, 4 of 15 Ad26.ZEBOV recipients (27%; 95% CI, 8%-55%) experienced fever. In the randomized groups, 28 of 29 Ad26.ZEBOV recipients (97%; 95% CI, 82%- 99.9%) and 7 of 30 MVA-BN-Filo recipients (23%; 95% CI, 10%-42%) had detectable Ebola glycoprotein-specific IgG 28 days after primary immunization. All vaccine recipients had specific IgG detectable 21 days postboost and at 8-month follow-up. Within randomized groups, at 7 days postboost, at least 86% of vaccine recipients showed Ebola-specific T-cell responses. Conclusions and Relevance In this phase 1 study of healthy volunteers, immunization with Ad26.ZEBOV or MVA-BN-Filo did not result in any vaccine-related serious adverse events. An immune response was observed after primary immunization with Ad26.ZEBOV; boosting by MVA-BN-Filo resulted in sustained elevation of specific immunity. These vaccines are being further assessed in phase 2 and 3 studies. Trial Registration clinicaltrials.gov Identifier:NCT02313077

https://jamanetwork.com/journals/jama/articlepdf/2514196/jpc160004.pdf

Safety and Immunogenicity of Novel Adenovirus Type 26– and Modified Vaccinia Ankara–Vectored Ebola Vaccines: A Randomized Clinical Trial

Background. We report the first-in-human safety and immunogenicity assessment of a prototype Ad26 vector-based human immunodeficiency virus (HIV) vaccine in humans.

Methods. Sixty Ad26-seronegative, healthy, HIV-uninfected subjects were enrolled in a randomized, double-blinded, placebo-controlled, dose-escalation phase 1 study. Five groups of 12 subjects received 109–1011 vp of the Ad26-EnvA vaccine (N = 10/group) or placebo (N = 2/group) at weeks 0 and 24 or weeks 0, 4, and 24. Safety and immunogenicity were assessed.

Results. Self-limited reactogenicity was observed after the initial immunization at the highest (1011 vp) dose. No product-related SAEs were observed. All subjects who received the Ad26-EnvA vaccine developed Ad26 NAb titers, EnvA-specific enzyme-linked immunosorbent assays (ELISA) titers, and EnvA-specific enzyme-linked immunospot assays (ELISPOT) responses. These responses persisted at week 52. At week 28 in the 109, 1010, 1011 vp 3-dose and the 1010 and 5 × 1010 vp 2-dose groups, geometric mean EnvA ELISA titers were 6113, 12 470, 8545, 3470, and 9655 and mean EnvA ELISPOT responses were 397, 178, 736, 196, and 1311 SFC/106 peripheral blood mononuclear cells, respectively.

Conclusion. This Ad26 vectored vaccine was generally safe and immunogenic at all doses tested. Reactogenicity was minimal with doses of 5 × 1010 vp or less. Ad26 is a promising new vaccine vector for HIV-1.

Clinical Trials Registration. {"type":"clinical-trial","attrs":{"text":"NCT00618605","term_id":"NCT00618605"}}NCT00618605.

First-in-Human Evaluation of the Safety and Immunogenicity of a Recombinant Adenovirus Serotype 26 HIV-1 Env Vaccine (IPCAVD 001)

There has been substantial progress in approaches to prevent and treat human immunodeficiency virus type 1 (HIV-1) infection, but the need for a safe and effective HIV-1 vaccine remains a high global health priority [1–4]. However, very few HIV-1 vaccine trials have evaluated mucosal immune responses elicited by candidate vaccines in humans. In particular, it remains unclear whether intramuscular immunization can induce mucosal immune responses in humans. Mucosal sites represent the portal of entry for the majority of HIV-1 infections, and thus it would presumably be desirable to induce mucosal HIV-1–specific immune responses that may be able to block or attenuate infection.

Limited data are currently available regarding the magnitude and phenotype of mucosal immune responses and immune activation following HIV-1 vaccination in humans [5]. This reflects logistic challenges associated with mucosal sampling and obtaining mucosal biopsies in the context of clinical vaccine studies as well as the immunologic challenges associated with accurately evaluating mucosal immune responses in humans.

We have recently reported the first-in-human immunogenicity of a novel adenovirus serotype 26 (Ad26) vector expressing HIV-1 envelop (Env) [6, 7]. Ad26 exhibits lower seroprevalence and neutralizing antibody (NAb) titers compared with adenovirus serotype 5 (Ad5), particularly in the developing world [8, 9]. Moreover, Ad26 and Ad5 differ in terms of their basic virology, innate immune profiles, and adaptive immune phenotypes [10–14]. In addition, Ad26-based vaccine regimens have shown improved protection compared with Ad5 and DNA/Ad5 regimens in stringent SIVmac251 (simian immunodeficiency virus) challenge models in rhesus monkeys [15, 16].

In the present study, we explored mucosal immune responses and inflammation in humans following immunization with an Ad26 vector expressing clade A Env glycoprotein (gp) 140 (Ad26-EnvA) [6, 7] in baseline Ad26-seronegative and Ad26-seropositive subjects.

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Induction of HIV-1–Specific Mucosal Immune Responses Following Intramuscular Recombinant Adenovirus Serotype 26 HIV-1 Vaccination of Humans

Abstract Youth with CHD are at greater risk for neurodevelopmental disorders compared to healthy controls. The aetiology is multi-factorial but includes medical and demographic factors. We sought to characterise the prevalence of neurodevelopmental disorders in patients with CHD. Our population included 206 patients with CHD, aged 3–21, who were referred for neuropsychological evaluation. Neurodevelopmental diagnoses were determined by a licensed psychologist. Rates of neurodevelopmental diagnoses were compared to national prevalence rates. Exploratory analyses (chi-square) examined which medical factors (i.e., cardiac diagnosis, genetic condition, prematurity, seizures, and stroke) were associated with neurodevelopmental diagnosis. There was higher prevalence of neurodevelopmental disorders in CHD when compared to the general population (44%). Rates of attention-deficit/hyperactivity disorder (27.3%), autism spectrum disorder (9.6%), and intellectual disability (5.9%) were notably higher than those seen in the general population (p < .01). Children with a history of aortic obstruction were more likely to be diagnosed with autism (p < .05), and children with genetic conditions were more likely to be diagnosed with an intellectual disability (p < .05). Neurodevelopmental diagnoses were not significantly associated with any other specific medical variables (e.g., cardiac diagnosis, seizures, stroke, prematurity, and antenatal diagnosis). School-aged children were more likely to be diagnosed with any neurodevelopmental disorder and attention-deficit/hyperactivity disorder (31.7%; p < .01) than preschool-age children. In summary, our results confirm that children and adolescents with CHD are at high risk for neurodevelopmental disorders and require ongoing monitoring, care, and support. Children with genetic disorders and those with aortic obstruction may be more at risk for certain neurodevelopmental disorders.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/F5C2AAFE8B44541C6B5656D03B3C2673/S1047951122001469a.pdf/div-class-title-prevalence-of-neurodevelopmental-disorders-in-a-clinically-referred-sample-of-children-with-chd-div.pdf

Hayley Loblein

Papers

First-in-Human Evaluation of the Safety and Immunogenicity of a Recombinant Adenovirus Serotype 26 HIV-1 Env Vaccine (IPCAVD 001)

Induction of HIV-1–Specific Mucosal Immune Responses Following Intramuscular Recombinant Adenovirus Serotype 26 HIV-1 Vaccination of Humans

Prevalence of neurodevelopmental disorders in a clinically referred sample of children with CHD