ABSTRACT
The recent outbreak of Coronavirus Disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection has been characterized by the World Health Organization (WHO) as a controllable global pandemic. The spike (S) glycoprotein mediates binding to the angiotensin-converting enzyme 2 (ACE2) receptor for virus entry and also services as the target of virus-neutralizing antibodies, making it an attractive and leading viral antigen for vaccine development. No vaccine against any human coronavirus is available to date. In learning from the experience of developing Middle East respiratory syndrome coronavirus (MERS-CoV) and SARS-CoV vaccine candidates in preclinical and clinical trials, the most promising strategies for SARS-CoV-2 vaccines should employ viral-vector platforms, properly adjuvanted recombinant protein or DNA/mRNA encoding an engineered sequence of trimeric S protein in pre-fusion conformation.
The recent outbreak of Coronavirus Disease 2019 (COVID-19) has spread to essentially every country in the world. This prompted the WHO declaration of a public health emergency of international concern on Jan 30 and a global pandemic on March 11. The mortality rate of reported cases is approaching 7%. However, the actual number of infected is unknown and undoubtedly understated since no country has tested more than 50% of its residents. The retrospective investigation of 44,672 confirmed COVID-19 cases in Mainland China demonstrated that the overall case fatality rate was about 2.3% and more than half of cases were over 50-y old, who account for over 90% of deaths.Citation1 The common symptoms of COVID-19 are bilateral pneumonia, fever, cough, myalgia or fatigue.Citation2,Citation3 Thus, this novel SARS-CoV-2 is more likely to result in severe and even fatal respiratory diseases in the elderly with comorbidities.
Thanks to the prompt sharing of the SARS-CoV-2 genome sequence by Chinese scientists, SARS-CoV-2 was identified as the seventh member of coronaviridae family that infects humans, including Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus (SARS-CoV). SARS-CoV-2 is a positive sense, single-stranded RNA virus with a genome of 29,891 nucleotides. Similar to other β coronaviruses, the SARS-CoV-2 genome contains two flanking untranslated regions (UTRs), a single long open reading frame (ORF) and 12 putative, functional ORFs.Citation4 The genomic characterization indicated that SARS-CoV-2 is closely related to the bat-derived SARS-like coronaviruses (Bat-SL-CoV).Citation5 Although the novel SARS-CoV-2 only shares approximately 79% of sequence identity to SARS-CoV, they use the same spike (S) glycoprotein to mediate the angiotensin-converting enzyme 2 (ACE2) receptor binding. In addition, the S protein of SARS-CoV-2 shows about 10- to 20-fold higher binding affinity to ACE2 than SARS-CoV.Citation5,Citation6 The S glycoprotein is crucial for determining host tropism and the target of virus-neutralizing antibodies, making it an attractive and leading viral antigen candidate for vaccine development.
No vaccine against any human coronavirus is available to date, with protective efficacy in fighting against the SARS-CoV-2 pandemic. Since the epidemics of SARS-CoV in 2002 and MERS-CoV in 2012, dozens of vaccine candidates against coronaviruses have been evaluated in preclinical models and in early clinical studies, among them, the S glycoprotein was mostly used as vaccine antigen.Citation7 Vaccine candidates utilizing viral vector, subunit proteins and DNA platform based on the full-length S protein or receptor-binding domain (RBD) of SARS-CoV and MERS-CoV not only induced T-cell and neutralizing-antibody responses but also stimulated protective immunity in animal models.Citation8–17 Nevertheless, other viral antigens like nucleocapsid (N), membrane (M) and envelope (E) proteins failed to induce complete protective immunity against homologous SARS-CoV challenge in mice.Citation18,Citation19 With two of SARS-CoV vaccines withdrawn, only five candidates based on S protein had advanced to early clinical trials, including four MERS-CoV and one SARS-CoV vaccine candidates (). The fact that two trials were withdrawn and only one phase I trial of SARS-CoV vaccine was completed might be due to the rapid disappearance of the virus. The lack of sufficient patients also hindered further clinical trials of MERS-CoV vaccines, while camels become another option to test efficacy of the vaccine ‘ChAdOx1 MERS’.Citation20 It is noteworthy that three of these promising MERS/SARS vaccines are viral vector-based vaccines, and the other two are DNA vaccines. The phase I trial results of GLS-5300 and VRC-SRSDNA015-00-VP indicated that the vaccines were well tolerated and able to induce cellular immune responses and neutralizing antibody in about 80% of healthy adults.Citation21,Citation22
Table 1. Vaccine candidates of SARS-CoV and MERS-CoV in clinical trials
To date, over 120 enterprises and research institutes have claimed in public reports to initiate projects of SARS-CoV-2 vaccine development. Among available new vaccine development technologies, the viral vector and DNA/mRNA platforms are the front-runners in terms of speed to enter clinical trials, having great potential for the vaccine development against this coronavirus pandemic. These platforms have many advantages including low cost, record of safety, allowing rapid vaccine design and construction for different pathogens. The manufacturing processes of these platforms have been demonstrated to be scalable (specially for DNA vaccine) and can meet the need of a quick response to the coronavirus pandemic. The mRNA-based mRNA-1273 and DNA-based INO-4800 COVID-19 vaccine candidates have entered phase I clinical trials in March and April, 2020 (ClinicalTrials.gov Identifier: NCT04283461, NCT04336410). Additionally, these vaccines are capable of inducing not only potent humoral immune responses but also balanced cellular immune responses. However, as a fact, no single candidate of DNA/mRNA vaccine has been approved yet, which might reflect the immature status of the technologies as compared with traditional approaches. Due to the relatively poor immunogenicity induced by naked DNA, DNA vaccines would need a specific, powerful electroporation (EP) device to achieve the desired effectiveness. So far, only limited immunogenicity and safety data of mRNA vaccines are available from clinical trials, and obtaining an optimal formulation for mRNA delivery is still one of the key challenges for human vaccine development.
The approvals of Ebola vaccines based on vesicular stomatitis virus (VSV-EBOV) and human adenovirus serotype 5 (Ad5-EBOV) demonstrate that recombinant viral vectors could be the most promising platform in the SARS-CoV-2 vaccine development and clinical applications.Citation23,Citation24 A single injection of viral vector-based vaccines has been shown to be sufficient to induce high levels and long-term immune responses. An adenovirus (Ad5) vectored COVID-19 candidate vaccine has entered phase I clinical trial in March, 2020 (NCT04313127), and is expected to initiate phase II soon (NCT04341389). However, the preexisting immunity in human to the viral vector may dampen the vaccine efficacy; thus, a high dose of the vaccine has to be used in clinics.Citation25 In the clinical trial of Ad5-EBOV vaccine, the presence of high preexisting adenovirus vector neutralizing antibodies not only weakened the vaccine-elicited antibody responses but also accelerated the decline of antibody titers.Citation24 Alternatively, rare human serotypes or chimpanzee derived adenoviruses have been used for vaccine development with numerous safety data available in clinical trials. The recombinant replication-defective chimpanzee adenoviruses share the desirable features with human adenovirus such as the broad tissue tropism, the ease of large-scale manufacturing, a high foreign gene loading capacity that could easily accommodate the 3,822 bp of SARS-CoV-2 S coding sequence and could be prepared as freeze dried form that facilitates vaccine shelf life and transportation. Furthermore, adenoviral vector-based vaccines have the potential to stimulate mucosal immunity through oral or nasal vaccination, which was thought to be crucial in limiting SARS-CoV replication in the lungs of mice.Citation26,Citation27
Since the whole virus-based attenuated live or inactivated SARS-CoV-2 vaccine requires a BSL-3 GMP facility, recombinant S protein-based subunit vaccines could be an excellent alternative that is not limited by capacity for delivering a large number of vaccine doses. The cryo-EM structure showed that the trimer conformation of the SARS-CoV-2 S protein is much like MERS-CoV and that the prefusion conformation of MERS-CoV S protein is associated with a high titer of neutralization antibodies.Citation6,Citation28 The results of a SARS-CoV DNA vaccine in mice indicated that the transmembrane domain of S protein, which gives rise to a more native trimer structure, could induce desired immunogenicity.Citation17 The RBD of S protein could also induce highly potent antibody responses but might be insufficient in inducing high levels of cellular immune response.Citation10 Additional concern with RBD-based vaccine is the possible immune evasion due to mutations; fortunately, recent sequence analysis of over 1,000 clinical isolates in China by the WHO team did not reveal significant mutations. Another important aspect of the S protein-based vaccine development is the vaccine adjuvant selection. The vaccines composed of the same recombinant S protein of SARS-CoV with different adjuvants induced not only different levels of antibody responses but also different profiles of T-cell immune responses. Although the formulation with either Advax-1 (delta inulin) or Advax-2 (delta inulin + CpG 2006) adjuvants enhanced S protein-specific total IgG antibody responses significantly, the addition of CpG 2006 resulted in a significant increase in IgG2a and IFN-γ responses, but not IgG1 and IL-4, indicating a significant Th1 immune response. Further, the results of SARS-CoV virus challenge study indicated that vaccine-induced Th2-biased response was associated with the vaccine enhanced immunopathology with eosinophilic infiltrates within the lungs of challenged mice.Citation29 Similar associations with possible vaccine enhanced diseases were also reported in mice vaccinated by inactivated SARS vaccine alone or adjuvanted with alum or MF59.Citation30–32 Vaccine formulations with novel adjuvants, particularly the Toll-like receptor (TLR) ligands, including Poly(I:C) (TLR3), AS04 (TLR4), Imiquimod (TLR7/8) and CpG (TLR9), significantly triggers Th1 responses in animal studies.Citation33 Nevertheless, the comprehensive role of T-cell immunity in SARS-CoV-2 infection for vaccine development is still under investigation.
In summary, the ongoing pandemic over 200 countries will accelerate the development of SARS-CoV-2 vaccines. But the vaccines may help little in controlling the pandemic due to the long development time. Based on the experience of MERS-CoV and SARS-CoV vaccine development, the most promising strategies for a fast SARS-CoV-2 vaccine development should be employing technology platforms such as VSV/Ad-vector platforms, properly adjuvanted recombinant protein (superior in scale) or DNA/mRNA (superior in speed) encoding an engineered sequence of trimeric S protein in pre-fusion conformation.
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.
Acknowledgments
We are grateful to Mr. Lokender Kaoshik and Dr. Jiang Fan for proofreading of this manuscript.
References
- Team N C P E R E. The epidemiological characteristics of an outbreak of 2019 novel coronavirus diseases (COVID-19)-China, 2020. Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(2):145–51. doi:10.3760/cma.j..0254-6450.2020.02.003. PMID: 32064853.
- Huang C, Wang Y, Li X, Ren L, Zhao J, Hu Y, Zhang L, Fan G, Xu J, Gu X, et al. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. Lancet. 2020;395(10223):497–506. doi:10.1016/S0140-6736(20)30183-5. PMID: 31986264.
- Chen N, Zhou M, Dong X, Qu J, Gong F, Han Y, Qiu Y, Wang J, Liu Y, Wei Y, et al. Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. Lancet. 2020;395(10223):507–13. doi:10.1016/S0140-6736(20)30211-7. PMID: 32007143.
- Chan JFW, Kok K-H, Zhu Z, Chu H, To KKW, Yuan S, Yuen K-Y. Genomic characterization of the 2019 novel human-pathogenic coronavirus isolated from a patient with atypical pneumonia after visiting Wuhan. Emerg Microbes Infect. 2020;9(1):221–36. doi:10.1080/22221751.2020.1719902. PMID: 31987001.
- Lu R, Zhao X, Li J, Niu P, Yang B, Wu H, Wang W, Song H, Huang B, Zhu N, et al. Genomic characterisation and epidemiology of 2019 novel coronavirus: implications for virus origins and receptor binding. Lancet. 2020;395(10224):565–74. doi:10.1016/s0140-6736(20)30251-8. PMID: 32007145.
- Wrapp D, Wang N, Corbett KS, Goldsmith JA, Hsieh C-L, Abiona O, Graham BS, McLellan JS. Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation. Science. 2020;367(6483):1260–63. doi:10.1126/science.abb2507. PMID: 32075877.
- Song Z, Xu Y, Bao L, Zhang L, Yu P, Qu Y, Zhu H, Zhao W, Han Y, Qin C. From SARS to MERS, thrusting coronaviruses into the spotlight. Viruses. 2019;11(1):59. doi:10.3390/v11010059. PMID: 30646565.
- Alharbi NK, Padron-Regalado E, Thompson CP, Kupke A, Wells D, Sloan MA, Grehan K, Temperton N, Lambe T, Warimwe G, et al. ChAdOx1 and MVA based vaccine candidates against MERS-CoV elicit neutralising antibodies and cellular immune responses in mice. Vaccine. 2017;35(30):3780–88. doi:10.1016/j.vaccine.2017.05.032. PMID: 28579232.
- Munster VJ, Wells D, Lambe T, Wright D, Fischer RJ, Bushmaker T, Saturday G, van Doremalen N, Gilbert SC, de Wit E, et al. Protective efficacy of a novel simian adenovirus vaccine against lethal MERS-CoV challenge in a transgenic human DPP4 mouse model. NPJ Vaccines. 2017;2(1):28. doi:10.1038/s41541-017-0029-1. PMID: 29263883.
- Ozharovskaia TA, Zubkova OV, Dolzhikova IV, Gromova AS, Grousova DM, Tukhvatulin AI, Popova O, Shcheblyakov DV, Scherbinin DN, Dzharullaeva AS, et al. Immunogenicity of different forms of Middle East respiratory syndrome S glycoprotein. Acta Naturae. 2019;11(1):38–47. doi:10.32607/20758251-2019-11-1-38-47. PMID: 31024747.
- Chen Z, Zhang L, Qin C, Ba L, Yi CE, Zhang F, Wei Q, He T, Yu W, Yu J, et al. Recombinant modified vaccinia virus Ankara expressing the spike glycoprotein of severe acute respiratory syndrome coronavirus induces protective neutralizing antibodies primarily targeting the receptor binding region. J Virol. 2005;79(5):2678–88. doi:10.1128/JVI.79.5.2678-2688.2005. PMID: 15708987.
- Song F, Fux R, Provacia LB, Volz A, Eickmann M, Becker S, Osterhaus AD, Haagmans BL, Sutter G. Middle East respiratory syndrome coronavirus spike protein delivered by modified vaccinia virus Ankara efficiently induces virus-neutralizing antibodies. J Virol. 2013;87(21):11950–54. doi:10.1128/jvi.01672-13. PMID: 23986586.
- Volz A, Kupke A, Song F, Jany S, Fux R, Shams-Eldin H, Schmidt J, Becker C, Eickmann M, Becker S, et al. Protective efficacy of recombinant modified vaccinia virus Ankara delivering Middle East respiratory syndrome coronavirus spike glycoprotein. J Virol. 2015;89(16):8651–56. doi:10.1128/jvi.00614-15. PMID: 26018172.
- Lan J, Yao Y, Deng Y, Chen H, Lu G, Wang W, Bao L, Deng W, Wei Q, Gao GF, et al. Recombinant receptor binding domain protein induces partial protective immunity in Rhesus Macaques against Middle East respiratory syndrome coronavirus challenge. EBioMedicine. 2015;2(10):1438–46. doi:10.1016/j.ebiom.2015.08.031. PMID: 26629538.
- Jiaming L, Yanfeng Y, Yao D, Yawei H, Linlin B, Baoying H, Jinghua Y, Gao GF, Chuan Q, Wenjie T. The recombinant N-terminal domain of spike proteins is a potential vaccine against Middle East respiratory syndrome coronavirus (MERS-CoV) infection. Vaccine. 2017;35(1):10–18. doi:10.1016/j.vaccine.2016.11.064. PMID: 27899228.
- Muthumani K, Falzarano D, Reuschel EL, Tingey C, Flingai S, Villarreal DO, Wise M, Patel A, Izmirly A, Aljuaid A, et al. A synthetic consensus anti-spike protein DNA vaccine induces protective immunity against Middle East respiratory syndrome coronavirus in nonhuman primates. Sci Transl Med. 2015;7(301):301ra132. doi:10.1126/scitranslmed.aac7462. PMID: 26290414.
- Yang Z, Kong W, Huang Y, Roberts A, Murphy BR, Subbarao KN, Abel GJ. A DNA vaccine induces SARS coronavirus neutralization and protective immunity in mice. Nature. 2004;428(6982):561–64. doi:10.1038/nature02463. PMID: 15024391.
- Deming D, Sheahan T, Heise M, Yount B, Davis N, Sims A, Suthar M, Harkema J, Whitmore A, Pickles R, et al. Vaccine efficacy in senescent mice challenged with recombinant SARS-CoV bearing epidemic and zoonotic spike variants. PLoS Med. 2006;3(12):e525. doi:10.1371/journal.pmed.0030525. PMID: 17194199.
- Yasui F, Kai C, Kitabatake M, Inoue S, Yoneda M, Yokochi S, Kase R, Sekiguchi S, Morita K, Hishima T, et al. Prior immunization with severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV) nucleocapsid protein causes severe pneumonia in mice infected with SARS-CoV. J Immunol. 2008;181(9):6337–48. doi:10.4049/jimmunol.181.9.6337. PMID: 18941225.
- Alharbi NK, Qasim I, Almasoud A, Aljami HA, Alenazi MW, Alhafufi A, Aldibasi OS, Hashem AM, Kasem S, Albrahim R, et al. Humoral immunogenicity and efficacy of a single dose of ChAdOx1 MERS vaccine candidate in dromedary camels. Sci Rep. 2019;9(1):16292. doi:10.1038/s41598-019-52730-4. PMID: 31705137.
- Modjarrad K, Roberts CC, Mills KT, Castellano AR, Paolino K, Muthumani K, Reuschel EL, Robb ML, Racine T, Oh MD, et al. Safety and immunogenicity of an anti-Middle East respiratory syndrome coronavirus DNA vaccine: a phase 1, open-label, single-arm, dose-escalation trial. Lancet Infect Dis. 2019;19(9):1013–22. doi:10.1016/s1473-3099(19)30266-x. PMID: 31351922.
- Martin JE, Louder MK, Holman LA, Gordon IJ, Enama ME, Larkin BD, Andrews CA, Vogel L, Koup RA, Roederer M, et al. A SARS DNA vaccine induces neutralizing antibody and cellular immune responses in healthy adults in a phase I clinical trial. Vaccine. 2008;26(50):6338–43. doi:10.1016/j.vaccine.2008.09.026. PMID: 18824060.
- Poetsch JH, Dahlke C, Zinser ME, Kasonta R, Lunemann S, Rechtien A, Ly ML, Stubbe HC, Krähling V, Biedenkopf N, et al. Detectable vesicular stomatitis virus (VSV)-specific humoral and cellular immune responses following VSV-Ebola virus vaccination in humans. J Infect Dis. 2019;219(4):556–61. doi:10.1093/infdis/jiy565. PMID: 30452666.
- Li J-X, Hou L-H, Meng F-Y, Wu S-P, Hu Y-M, Liang Q, Chu K, Zhang Z, Xu -J-J, Tang R, et al. Immunity duration of a recombinant adenovirus type-5 vector-based Ebola vaccine and a homologous prime-boost immunisation in healthy adults in China: final report of a randomised, double-blind, placebo-controlled, phase 1 trial. Lancet Glob Health. 2017;5(3):e324–e334. doi:10.1016/S2214-109X(16)30367-9. PMID: 28017642.
- Fausther-Bovendo H, Kobinger GP. Pre-existing immunity against Ad vectors: humoral, cellular, and innate response, what’s important? Hum Vaccin Immunother. 2014;10(10):2875–84. doi:10.4161/hv.29594. PMID: 25483662.
- See RH, Zakhartchouk AN, Petric M, Lawrence DJ, Mok CPY, Hogan RJ, Rowe T, Zitzow LA, Karunakaran KP, Hitt MM, et al. Comparative evaluation of two severe acute respiratory syndrome (SARS) vaccine candidates in mice challenged with SARS coronavirus. J Gen Virol. 2006;87(3):641–50. doi:10.1099/vir.0.81579-0. PMID: 16476986.
- Kim L, Liebowitz D, Lin K, Kasparek K, Pasetti MF, Garg SJ, Gottlieb K, Trager G, Tucker SN. Safety and immunogenicity of an oral tablet norovirus vaccine, a phase I randomized, placebo-controlled trial. JCI Insight. 2018;3(13):e121077. doi:10.1172/jci.insight.121077. PMID: 29997294.
- Pallesen J, Wang N, Corbett KS, Wrapp D, Kirchdoerfer RN, Turner HL, Cottrell CA, Becker MM, Wang L, Shi W, et al. Immunogenicity and structures of a rationally designed prefusion MERS-CoV spike antigen. Proc Natl Acad Sci U S A. 2017;114(35):E7348–E7357. doi:10.1073/pnas.1707304114. PMID: 28807998.
- Honda-Okubo Y, Barnard D, Ong CH, Peng B-H, Tseng CTK, Petrovsky N, Perlman S. Severe acute respiratory syndrome-associated coronavirus vaccines formulated with delta inulin adjuvants provide enhanced protection while ameliorating lung eosinophilic immunopathology. J Virol. 2015;89(6):2995–3007. doi:10.1128/JVI.02980-14. PMID: 25520500.
- Bolles M, Deming D, Long K, Agnihothram S, Whitmore A, Ferris M, Funkhouser W, Gralinski L, Totura A, Heise M, et al. A double-inactivated severe acute respiratory syndrome coronavirus vaccine provides incomplete protection in mice and induces increased eosinophilic proinflammatory pulmonary response upon challenge. J Virol. 2011;85(23):12201–15. doi:10.1128/jvi.06048-11. PMID: 21937658.
- Tseng CT, Sbrana E, Iwata-Yoshikawa N, Newman PC, Garron T, Atmar RL, Peters CJ, Couch RB. Immunization with SARS coronavirus vaccines leads to pulmonary immunopathology on challenge with the SARS virus. PLoS One. 2012;7(4):e35421. doi:10.1371/journal.pone.0035421. PMID: 22536382.
- Agrawal AS, Tao X, Algaissi A, Garron T, Narayanan K, Peng BH, Couch RB, Tseng CT. Immunization with inactivated Middle East respiratory syndrome coronavirus vaccine leads to lung immunopathology on challenge with live virus. Hum Vaccin Immunother. 2016;12(9):2351–56. doi:10.1080/21645515.2016.1177688. PMID: 27269431.
- Apostólico J, de S, Lunardelli VA, Coirada FC, Boscardin SB, Rosa DS. Adjuvants: classification, modus operandi, and licensing. J Immunol Res. 2016;2016:1459394. doi:10.1155/2016/1459394. PMID: 27274998.