In the past 25 years, the world has witnessed outbreaks of illnesses in humans from three different coronaviruses. Both the SARS-CoV outbreak of 2003 and the MERS-CoV outbreak of 2013 resulted in overall low fatalities in part due to inefficient human-to-human spread of each virus. In contrast, SARS-CoV-2, which emerged in 2019, was highly efficient at human-to-human spread and caused a global pandemic resulting in millions of casualties. Zoonotic transmission of viruses, including the three coronaviruses, poses an ongoing threat that cannot be ignored. In this review, we have focused on the diagnostics and therapeutics fronts using SARS-CoV-2 as a model. Specifically, we have selected proteins associated with the virus particles as targets and discussed various platform technologies. These insights hold the potential to inform the development of more effective therapeutics and vaccines not only for SARS-CoV-2 but also for future viral pandemics, thus contributing to global health on a broader scale.

Heterologous boost regimens are being increasingly considered against SARS-CoV-2. We report results for the 32 of 45 participants in the Phase 1 CoV2-001 clinical trial (Kim et al., Int J Iinfect Dis 2023, 128:112- 120) who elected to receive an EUA-approved SARS-CoV-2 mRNA vaccine 6 to 8 months following a two -dose primary vaccination with the GLS-5310 bi-cistronic DNA vaccine given intradermally and followed by application of suction using the GeneDerm device. Receipt of EUA-approved mRNA vaccines after GLS-5310 vaccination was well-tolerated, with no reported adverse events. Immune responses were enhanced such that binding antibody titers, neutralizing antibody titers, and T-cell responses increased 1,187-fold, 110-fold, and 2.9-fold, respectively. This paper is the first description of the immune responses following heterologous vaccination with a DNA primary series and mRNA boost. & COPY; 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Harnessing the immune system to combat disease has revolutionized medical treatment. Monoclonal antibodies (mAbs), in particular, have emerged as important immunotherapeutic agents with clinical relevance in treating a wide range of diseases, including allergies, autoimmune diseases, neurodegenerative disorders, cancer, and infectious diseases. These mAbs are developed from naturally occurring antibodies and target specific epitopes of single molecules, minimizing off-target effects. Antibodies can also be designed to target particular pathogens or modulate immune function by activating or suppressing certain pathways. Despite their benefit for patients, the production and administration of monoclonal antibody therapeutics are laborious, costly, and time-consuming. Administration often requires inpatient stays and repeated dosing to maintain therapeutic levels, limiting their use in underserved populations and developing countries. Researchers are developing alternate methods to deliver monoclonal antibodies, including synthetic nucleic acid-based delivery, to overcome these limitations. These methods allow for in vivo production of monoclonal antibodies, which would significantly reduce costs and simplify administration logistics. This review explores new methods for monoclonal antibody delivery, including synthetic nucleic acids, and their potential to increase the accessibility and utility of life-saving treatments for several diseases.

We highlight the significant progress in developing DNA vaccines during the SARS-CoV-2 pandemic. Specifically, we provide a comprehensive review of the DNA vaccines that have progressed to Phase 2 testing or beyond, including those that have received authorization for use. DNA vaccines have significant advantages with regard to the rapidity of production, thermostability, safety profile, and cellular immune responses. Based on user needs and cost, we compare the three devices used in the SARS-CoV-2 clinical trials. Of the three devices, the GeneDerm suction device offers numerous benefits, particularly for international vaccination campaigns. As such, DNA vaccines represent a promising option for future pandemics.

Nipah virus (NiV) is a re-emerging pathogen that causes severe disease in animals and humans. Current treatment measures for NiV infection are insufficient, and there is no approved vaccine against NiV for either humans or animals. Nipah virus is listed as a high-priority pathogen for vaccine and therapeutic research by the World Health Organization (WHO). In the present study, we employed synthetic enhanced DNA technologies developed to design and produce novel consensus NiV Fusion (NiV-F) and Glycoprotein (NiV-G) antigen sequences for inclusion in synthetic DNA vaccines for NiV. The expression of each vaccine antigen was confirmed in vitro using immune-binding assays. Electroporation-enhanced intramuscular injection of each NiV-F and NiV-G into mice induced potent cellular immune responses to multiple epitopes of NiV-G and NiV-F that included antigen-specific CD8+ T cells. Both vaccines elicited high antibody titers in mice, with a single immunization sufficient to seroconvert 100% of immunized animals. Additionally, the NiV-F vaccine also induced antibodies to neutralize NiV-F-pseudotyped virus particles. These data support further study of these novel synthetic enhanced NiV nucleic acid-based antigens as potential components of an effective vaccine against the Nipah virus.

In the present study, the effect(s) of the immunomodulatory drug GLS-1027 on various cell types involved in inflammation were investigated. GLS-1027 reduced LPS-stimulated secretion of pro-inflammatory cytokines by macrophage or monocytic cells and cell lines. This reduction was likely due in part to decreased activation of NF-κB family transcription factors and inhibition of p38 MAPK signaling in GLS-1027-treated cells. Independent from its effects on macrophages, GLS-1027 inhibited dendritic cell maturation and differentiation of naïve CD4 + T cells into Th17 cells, reducing the production of typical pro-inflammatory cytokines associated with both processes. In vivo administration of GLS-1027 prevented the development of type 1 diabetes in NOD mice which correlated with reduced serum levels of IL17A in GLS-1027 treated animals and reduced ex vivo production of IL17A from both spleen and lymph-node cells. Overall, our data show that GLS-1027 can reduce inflammation through multiple actions, including the reduction of pro-inflammatory cytokine production by innate immune cells, the inhibition of dendritic cells maturation, and the inhibition of Th17 cells polarization.

Vaccines are one of mankind's greatest medical advances, and their use has drastically reduced and in some cases eliminated (e.g., smallpox) disease and death caused by infectious agents. Traditional vaccine modalities including live-attenuated pathogen vaccines, wholly inactivated pathogen vaccines, and protein-based pathogen subunit vaccines have successfully been used to create efficacious vaccines against measles, mumps, rubella, polio, and yellow fever. These traditional vaccine modalities, however, take many months to years to develop and have thus proven less effective for use in creating vaccines to emerging or reemerging infectious diseases (EIDs) including influenza, Human immunodeficiency virus (HIV), dengue virus (DENV), chikungunya virus (CHIKV), West Nile virus (WNV), Middle East respiratory syndrome (MERS), and the severe acute respiratory syndrome coronaviruses 1 and 2 (SARS-CoV and SARS-CoV-2). As factors such as climate change and increased globalization continue to increase the pace of EID development, newer vaccine modalities are required to develop vaccines that can prevent or attenuate EID outbreaks throughout the world. One such modality, DNA vaccines, has been studied for over 30 years and has numerous qualities that make them ideal for meeting the challenge of EIDs including; (1) DNA vaccine candidates can be designed within hours of publishing of a pathogens genetic sequence; (2) they can be manufactured cheaply and rapidly in large quantities; (3) they are thermostable and have reduced requirement for a cold-chain during distribution, and (4) they have a remarkable safety record in the clinic. Optimizations made in plasmid design as well as in DNA vaccine delivery have greatly improved the immunogenicity of these vaccines. Here we describe the process of making a DNA vaccine to an EID pathogen and describe methods used for assessing the immunogenicity and protective efficacy of DNA vaccines in small animal models.

SARS-CoV-2 is the third pathogenic coronavirus to emerge since 2000. Experience from prior outbreaks of SARS-CoV and MERS-CoV has demonstrated the importance of both humoral and cellular immunity to clinical outcome, precepts that have been recapitulated for SARS-CoV-2. Despite the unprecedented rapid development and deployment of vaccines against SARS-CoV-2, more vaccines are needed to meet global demand and to guard against immune evasion by newly emerging SARS-CoV-2 variants. Here we describe the development of pGO-1002, a novel bi-cistronic synthetic DNA vaccine that encodes consensus sequences of two SARS-CoV-2 antigens, Spike and ORF3a. Mice immunized with pGO-1002 developed humoral and cellular responses to both antigens, including antibodies and capable of neutralizing infection by a clinical SARS-CoV-2 isolate. Rats immunized with pGO-1002 by intradermal (ID) injection followed by application of suction with our GeneDerm device also developed humoral responses that included neutralizing antibodies and RBD-ACE2 blocking antibodies as well as robust cellular responses to both antigens. Significantly, in a Syrian hamster vaccination and challenge model, ID+GeneDerm-assisted vaccination prevented viral replication in the lungs and significantly reduced viral replication in the nares of hamsters challenged with either an ancestral SARS-CoV-2 strain or the B.1.351 (Beta) variant of concern. Furthermore, vaccinated immune sera inhibited virus-mediated cytopathic effects in vitro. These data establish the immunogenicity of the SARS-CoV-2 vaccine candidate pGO-1002 which induces potent humoral and cellular responses to the Spike and ORF3a antigens and may provide greater protection against emerging variants.

Objectives: The CoV2-001 phase I randomized trial evaluated the safety and immunogenicity of the GLS-5310 bi-cistronic DNA vaccine through 48 weeks of follow-up. Design: A total of 45 vaccine-naïve participants were recruited between December 31, 2020, and March 30, 2021. GLS-5310, encoding for the SARS-CoV-2 spike and open reading frame 3a (ORF3a) proteins, was administered intradermally at 0.6 mg or 1.2 mg per dose, followed by application of the GeneDerm suction device as part of a two-dose regimen spaced either 8 or 12 weeks between vaccinations. Results: GLS-5310 was well tolerated with no serious adverse events reported. Antibody and T cell responses were dose-independent. Anti-spike antibodies were induced in 95.5% of participants with an average geometric mean titer of ∼480 four weeks after vaccination and declined minimally through 48 weeks. Neutralizing antibodies were induced in 55.5% of participants with post-vaccination geometric mean titer of 28.4. T cell responses were induced in 97.8% of participants, averaging 716 site forming units/106 cells four weeks after vaccination, increasing to 1248 at week 24, and remaining greater than 1000 through 48 weeks. Conclusion: GLS-5310 administered with the GeneDerm suction device was well tolerated and induced high levels of binding antibodies and T-cell responses. Antibody responses were similar to other DNA vaccines, whereas T cell responses were many-fold greater than DNA and non-DNA vaccines.

This work reports a suction-based cutaneous delivery method for in vivo DNA transfection. Following intradermal Mantoux injection of plasmid DNA in a rat model, a moderate negative pressure is applied to the injection site, a technique similar to Chinese báguàn and Middle Eastern hijama cupping therapies. Strong GFP expression was demonstrated with pEGFP-N1 plasmids where fluorescence was observed as early as 1 hour after dosing. Modeling indicates a strong correlation between focal strain/stress and expression patterns. The absence of visible and/or histological tissue injury contrasts with current in vivo transfection systems such as electroporation. Specific utility was demonstrated with a synthetic SARS-CoV-2 DNA vaccine, which generated host humoral immune response in rats with notable antibody production. This method enables an easy-to-use, cost-effective, and highly scalable platform for both laboratorial transfection needs and clinical applications for nucleic acid–based therapeutics and vaccines.

Background Although Zika virus (ZIKV) infection is typically self-limiting, other associated complications such as congenital birth defects and the Guillain-Barré syndrome are well described. There are no approved vaccines against ZIKV infection. Methods In this phase 1, open-label clinical trial, we evaluated the safety and immunogenicity of a synthetic, consensus DNA vaccine (GLS-5700) encoding the ZIKV premembrane and envelope proteins in two groups of 20 participants each. The participants received either 1 mg or 2 mg of vaccine intradermally, with each injection followed by electroporation (the use of a pulsed electric field to introduce the DNA sequence into cells) at baseline, 4 weeks, and 12 weeks. Results The median age of the participants was 38 years, and 60% were women; 78% were white, and 22% black; in addition, 30% were Hispanic. At the interim analysis at 14 weeks (i.e., after the third dose of vaccine), no serious adverse events were reported. Local reactions at the vaccination site (e.g., injection-site pain, redness, swelling, and itching) occurred in approximately 50% of the participants. After the third dose of vaccine, binding antibodies (as measured on enzyme-linked immunosorbent assay) were detected in all the participants, with geometric mean titers of 1642 and 2871 in recipients of 1 mg and 2 mg of vaccine, respectively. Neutralizing antibodies developed in 62% of the samples on Vero-cell assay. On neuronal-cell assay, there was 90% inhibition of ZIKV infection in 70% of the serum samples and 50% inhibition in 95% of the samples. The intraperitoneal injection of postvaccination serum protected 103 of 112 IFNAR knockout mice (bred with deletion of genes encoding interferon-α and interferon-β receptors) (92%) that were challenged with a lethal dose of ZIKV-PR209 strain; none of the mice receiving baseline serum survived the challenge. Survival was independent of the neutralization titer. Conclusions In this phase 1, open-label clinical trial, a DNA vaccine elicited anti-ZIKV immune responses. Further studies are needed to better evaluate the safety and efficacy of the vaccine. (Funded by GeneOne Life Science and others; ZIKA-001 ClinicalTrials.gov number, NCT02809443 .).

The emergence of multiple concurrent infectious diseases localized in the world creates a complex burden on global public health systems. Outbreaks of Ebola, Lassa, and Marburg viruses in overlapping regions of central and West Africa and the co-circulation of Zika, Dengue, and Chikungunya viruses in areas with A. aegypti mosquitos highlight the need for a rapidly deployable, safe, and versatile vaccine platform readily available to respond. The DNA vaccine platform stands out as such an application. Here, we present proof-of-concept studies from mice, guinea pigs, and nonhuman primates for two multivalent DNA vaccines delivered using in vivo electroporation (EP) targeting mosquito-borne (MMBV) and hemorrhagic fever (MHFV) viruses. Immunization with MMBV or MHFV vaccines via intradermal EP delivery generated robust cellular and humoral immune responses against all target viral antigens in all species. MMBV vaccine generated antigen-specific binding antibodies and IFNγ-secreting lymphocytes detected in NHPs up to six months post final immunization, suggesting induction of long-term immune memory. Serum from MHFV vaccinated NHPs demonstrated neutralizing activity in Ebola, Lassa, and Marburg pseudovirus assays indicating the potential to offer protection. Together, these data strongly support and demonstrate the versatility of DNA vaccines as a multivalent vaccine development platform for emerging infectious diseases.

Background: Novel coronaviruses (CoV) caused 3 global outbreaks over the past 2 decades: SARS-CoV (2002), MERS-CoV (2012), and 2019-nCoV in Wuhan, China. Each caused pneumonia with mortality of 10%, 35% and 2%, respectively (2019-nCoV estimated). GLS-5300 DNA vaccine targeting MERS-CoV Spike (S) was first to enter clinical trial, was safe and immunogenic (Lancet ID;2019). In Phase I, a 3 dose series at Day0, 4 and 12 weeks of GLS-5300 at either 0.67, 2 or 6mg was given IM followed by electroporation (EP, IM+EP) with CELLECTRA-5P device. GLS-5300 induced antibodies (Abs) in 94%, Tcell response in 76%, and neutralizing Abs in 50% of participants. No dose response was observed. GLS-5300 response was similar to those recovered from natural MERS-CoV infection. The absence of dose response and prior experience showing benefits of ID+EP vs IM+EP (JID;2019) led us to design this trial of lower ID dosing with an arm for a 2-dose regimen. We report results from MERS-002, the ongoing Phase I/IIa study of GLS-5300. Methods: MERS-002 is an open label, dose ranging, phase I/IIa study of GLS-5300. Participants were enrolled at 2 Korean sites into 3 groups receiving GLS-5300 ID+EP with the CELLECTRA-3P device: Group 1 received three 0.3mg doses at Day0 and weeks 4 and 12;Group 2 received three 0.6mg doses at Day0 and weeks 4 and 12;Group 3 received two 0.6mg doses at Day0 and week 8. Safety and tolerability of GLS-5300 was evaluated at each visit. Samples were collected at baseline, before each dose, and at both 2 and 4 weeks post dose 2 and post dose 3. Study data through 4 weeks after the primary series for a subset of immunoassays were included here. Findings: GLS-5300 given ID+EP was well-tolerated with no vaccine-associated SAEs. Preliminary results were available for: full length S (flS) ELISA, EMC2012-Vero neutralization (MERS-neut) and MERS-CoV S IFNg ELISPOT. GLS-5300 at 0.6mg induced MERS-CoV-specific Abs by flS ELISA and MERS-neut in 74% and 48%, respectively, after 1 dose. After the 2 or 3 dose vaccine series at 0.6mg per dose, flS ELISA response was seen in 100% and 92% of participants, respectively. MERS-neut response was 92% in both 2 and 3 dose 0.6mg groups. Antibody responses and rates were higher during and after primary series in 0.6mg group regardless of regimen than 0.3mg per dose. GLS-5300 induced Tcell responses via MERS-CoV IFNg ELISPOT in 60% and 84% receiving 0.6mg after the 2 or 3 dose series, respectively. Compared to 0.67mg of GLS-5300 given IM+EP in the first trial, 0.6mg of GLS-5300 given ID+EP in MERS-002, binding Abs appeared sooner and neutralizing Abs were observed in a higher fraction of participants (92% vs 50%) while Tcell reactivity was similar between vaccination schema. Conclusions: GLS-5300 was well tolerated with no vaccine-associated SAEs. Like prior studies, DNA vaccines given by ID+EP had fewer injection-related AEs relative to IM+EP. In MERS-002, 0.6mg of GLS-5300 in a 2-dose regimen spanning 8 weeks had similar reactivity and rate to the longer 3-dose regimen. GLS-5300 was safe and immunogenic when given IM+EP and, similarly, when given ID+EP in both 2- and 3-dose regimens in this ongoing MERS-002 Phase I/IIa trial. A Phase II clinical evaluation of the use of GLS-5300 to prevent MERS-CoV infection in endemic regions is planned.

Hepatitis B virus (HBV) causes a potentially life-threatening liver infection that frequently results in life-long chronic infection. HBV is responsible for 887,000 deaths each year, most resulting from chronic liver diseases and hepatocellular carcinoma. Presently, there are 250 million chronic HBV carriers worldwide who are at a high risk for developing cirrhosis and hepatocellular carcinoma (HCC). HCC is the most common type of liver cancer with a strong association with HBV infection. HBV transmission through blood transfusions and perinatal transfer from infected mother to child have been common routes of infection. In the present study, we describe the development of a synthetic DNA plasmid encoding an anti-HBV human monoclonal antibody specific for the common "a determinant region" of HBsAg of hepatitis B virus and demonstrate the ability of this platform at directingin vivoantibody expression.In vivodelivery of this DNA encoded monoclonal antibody (DMAb) plasmid in mice resulted in expression of human IgG over a period of one month following a single injection. Serum antibody was found to recognize the relevant conformational epitope from plasma purified native HBsAg as well as bound HBV in HepG2.2.15 cells. The serum DMAb efficiently neutralized HBV and prevented infection of HepaRG cellsin vitro. Additional study of these HBV-DMAb as a possible therapy or immunoprophylaxis for HBV infection is warranted.

Powassan virus (POWV) infection is a tick-borne emerging infectious disease in the United States and North America. Like Zika virus, POWV is a member of the family Flaviviridae. POWV causes severe neurological sequalae, meningitis, encephalitis, and can cause death. Although the risk of human POWV infection is low, its incidence in the U.S. in the past 16 years has increased over 300%, urging immediate attention. Despite the disease severity and its growing potential for threatening larger populations, currently there are no licensed vaccines which provide protection against POWV. We developed a novel synthetic DNA vaccine termed POWV-SEV by focusing on the conserved portions of POWV pre-membrane and envelope (prMEnv) genes. A single immunization of POWV-SEV elicited broad T and B cell immunity in mice with minimal cross-reactivity against other flaviviruses. Antibody epitope mapping demonstrated a similarity between POWV-SEV-induced immune responses and those elicited naturally in POWV-infected patients. Finally, POWV-SEV induced immunity provided protection against POWV disease in lethal challenge experiments.

Zika virus (ZIKV) infection is endemic to several world regions, and many others are at high risk for seasonal outbreaks. Synthetic DNA-encoded monoclonal antibody (DMAb) is an approach that enables in vivo delivery of highly potent mAbs to control infections. We engineered DMAb-ZK190, encoding the mAb ZK190 neutralizing antibody, which targets the ZIKV E protein DIII domain. In vivo-delivered DMAb-ZK190 achieved expression levels persisting >10 weeks in mice and >3 weeks in non-human primate (NHPs), which is protective against ZIKV infectious challenge. This study is the first demonstration of infectious disease control in NHPs following in vivo delivery of a nucleic acid-encoded antibody, supporting the importance of this new platform.

Significant concerns have arisen over the past 3 y from the increased global spread of the mosquito-borne flavivirus, Zika. Accompanying this spread has been an increase in cases of the devastating birth defect microcephaly as well as of Guillain–Barré syndrome in adults in many affected countries. Currently there is no vaccine or therapy for this infection; however, we sought to develop a combination approach that provides more rapid and durable protection than traditional vaccination alone. A novel immune-based prophylaxis/therapy strategy entailing the facilitated delivery of a synthetic DNA consensus prME vaccine along with DNA-encoded anti-ZIKV envelope monoclonal antibodies (dMAb) were developed and evaluated for antiviral efficacy. This immediate and persistent protection strategy confers the ability to overcome shortcomings inherent with conventional active vaccination or passive immunotherapy. A collection of novel dMAbs were developed which were potent against ZIKV and could be expressed in serum within 24–48 h of in vivo administration. The DNA vaccine, from a previous development, was potent after adaptive immunity was developed, protecting against infection, brain and testes pathology in relevant mouse challenge models and in an NHP challenge. Delivery of potent dMAbs protected mice from the same murine viral challenge within days of delivery. Combined injection of dMAb and the DNA vaccine afforded rapid and long-lived protection in this challenge model, providing an important demonstration of the advantage of this synergistic approach to pandemic outbreaks.

Mayaro virus (MAYV) of the genus alphavirus is a mosquito-transmitted emerging infectious disease that causes an acute febrile illness, rash, headaches, and nausea that may turn into incapacitating, persistent arthralgias in some victims. Since its discovery in Trinidad in 1954, cases of MAYV infection have largely been confined there and to the northern countries of South America, but recently, MAYV cases have been reported in some island nations in the Caribbean Sea. Accompanying these reports is evidence that new vectors, including Aedes spp. mosquitos, recently implicated in the global spread of Zika and chikungunya viruses, are competent for MAYV transmission, which, if true, could facilitate the spread of MAYV beyond its current range. Despite its status as an emerging virus, there are no licensed vaccines to prevent MAYV infection nor therapeutics to treat it. Here, we describe the development and testing of a novel DNA vaccine, scMAYV-E, that encodes a synthetically-designed consensus MAYV envelope sequence. In vivo electroporation-enhanced immunization of mice with this vaccine induced potent humoral responses including neutralizing antibodies as well as robust T-cell responses to multiple epitopes in the MAYV envelope. Importantly, these scMAYV-E-induced immune responses protected susceptible mice from morbidity and mortality following a MAYV challenge.

Background Middle East respiratory syndrome (MERS) coronavirus causes a highly fatal lower-respiratory tract infection. There are as yet no licensed MERS vaccines or therapeutics. This study (WRAIR-2274) assessed the safety, tolerability, and immunogenicity of the GLS-5300 MERS coronavirus DNA vaccine in healthy adults. Methods This study was a phase 1, open-label, single-arm, dose-escalation study of GLS-5300 done at the Walter Reed Army Institute for Research Clinical Trials Center (Silver Spring, MD, USA). We enrolled healthy adults aged 18-50 years; exclusion criteria included previous infection or treatment of MERS. Eligible participants were enrolled sequentially using a dose-escalation protocol to receive 0.67 mg, 2 mg, or 6 mg GLS-5300 administered by trained clinical site staff via a single intramuscular 1 mL injection at each vaccination at baseline, week 4, and week 12 followed immediately by co-localised intramuscular electroporation. Enrolment into the higher dose groups occurred after a safety monitoring committee reviewed the data following vaccination of the first five participants at the previous lower dose in each group. The primary outcome of the study was safety, assessed in all participants who received at least one study treatment and for whom post-dose study data were available, during the vaccination period with follow-up through to 48 weeks after dose 3. Safety was measured by the incidence of adverse events; administration site reactions and pain; and changes in safety laboratory parameters. The secondary outcome was immunogenicity. This trial is registered at ClinicalTrials.gov (number NCT 02670187) and is completed. Findings Between Feb 17 and July 22,2016, we enrolled 75 individuals and allocated 25 each to 0.67 mg, 2 mg, or 6 mg GLS-5300. No vaccine-associated serious adverse events were reported. The most common adverse events were injection-site reactions, reported in 70 participants (93%) of 75. Overall, 73 participants (97%) of 75 reported at least one solicited adverse event; the most common systemic symptoms were headache (five [20%] with 0.67 mg, 11 [44%] with 2 mg, and seven 128%1 with 6 mg), and malaise or fatigue (five [20%] with 0.67 mg, seven [28%] with 2 mg, and two [8%] with 6 mg). The most common local solicited symptoms were administration site pain (23 [92%] with all three doses) and tenderness (21 [84%] with all three doses). Most solicited symptoms were reported as mild (19 [76%] with 0.67 mg, 20[80%]with 2 mg, and 17 [68%] with 6 mg) and were self-limiting. Unsolicited symptoms were reported for 56 participants (75%) of 75 and were deemed treatment-related for 26 (35%). The most common unsolicited adverse events were infections, occurring in 27 participants (36%); six (8%) were deemed possibly related to study treatment. There were no laboratory abnormalities of grade 3 or higher that were related to study treatment; laboratory abnormalities were uncommon, except for 15 increases in creatine phosphokinase in 14 participants (three participants in the 0.67 mg group, three in the 2 mg group, and seven in the 6 mg group). Of these 15 increases, five (33%) were deemed possibly related to study treatment (one in the 2 mg group and four in the 6 mg group). Seroconversion measured by S1-ELISA occurred in 59 (86%) of 69 participants and 61 (94%) of 65 participants after two and three vaccinations, respectively. Neutralising antibodies were detected in 34 (50%) of 68 participants. T-cell responses were detected in 47 (71%) of 66 participants after two vaccinations and in 44 (76%) of 58 participants after three vaccinations. There were no differences in immune responses between dose groups after 6 weeks. At week 60, vaccine-induced humoral and cellular responses were detected in 51 (77%) of 66 participants and 42 (64%) of 66, respectively. Interpretation The GLS-5300 MERS coronavirus vaccine was well tolerated with no vaccine-associated serious adverse events. Immune responses were dose-independent, detected in more than 85% of participants after two vaccinations, and durable through 1 year of follow-up. The data support further development of the GLS-5300 vaccine, including additional studies to test the efficacy of GLS-5300 in a region endemic for MERS coronavirus. Copyright (C) 2019 Elsevier Ltd. All rights reserved.