A Snapshot of the Global Race for Vaccines Targeting SARS-CoV-2 and the COVID-19 Pandemic

A novel coronavirus SARS-CoV-2 causing Coronavirus disease 2019 (COVID-19) has entered the human population and has spread rapidly around the world in the first half of 2020 causing a global pandemic. The virus uses its spike glycoprotein receptor-binding domain to interact with host cell angiotensin-converting enzyme 2 (ACE2) sites to initiate a cascade of events that culminate in severe acute respiratory syndrome in some individuals. In efforts to curtail viral spread, authorities initiated far-reaching lockdowns that have disrupted global economies. The scientific and medical communities are mounting serious efforts to limit this pandemic and subsequent waves of viral spread by developing preventative vaccines and repurposing existing drugs as potential therapies. In this review, we focus on the latest developments in COVID-19 vaccine development, including results of the first Phase I clinical trials and describe a number of the early candidates that are emerging in the field. We seek to provide a balanced coverage of the seven main platforms used in vaccine development that will lead to a desired target product profile for the “ideal” vaccine. Using tales of past vaccine discovery efforts that have taken many years or that have failed, we temper over exuberant enthusiasm with cautious optimism that the global medical community will reach the elusive target to treat COVID-19 and end the pandemic.


INTRODUCTION
Coronavirus disease 2019  is caused by the novel beta-coronavirus family member coined SARS-CoV-2 (Severe acute respiratory syndrome coronavirus 2) (Oberfeld et al., 2020). In December 2019 (and potentially earlier, though unrecognized), SARS-CoV-2 emerged as a pneumonia-causing disorder in Hubei province, China, most likely the result of natural selection in animal hosts (bats, pangolins) prior to zoonotic transfer (Andersen et al., 2020;Wu et al., 2020a;Zhu N. et al., 2020). There are now seven members of this viral family known to infect humans with three having the potential to cause severe respiratory disease (Andersen et al., 2020). The two outbreaks preceding SARS-CoV-2 include the first SARS virus emerging in late 2002 in Guangdong province in China (now referred to as SARS-CoV-1), followed by the Middle-East Respiratory Syndrome coronavirus (MERS-CoV) in 2012 in Saudi Arabia (de Wit et al., 2016). SARS-CoV-2 has rampaged exponentially around the world since the start of 2020; the World Health Organization (WHO) declared it a Public Health Emergency of International Concern on January 30, 2020 and a pandemic officially on March 7, 2020 (Sohrabi et al., 2020). At the time of revision of this review (June 7, 2020), infections are at 7 million individuals in at least 195 countries with ≈400,000 in the mortality column 1 . Collateral damage to global economies and many professional business and recreational sectors (travel, hospitality/tourism, sport, etc.) is ongoing and widespread due to governmentally imposed lockdowns.

SARS-COV-2
The virus spreads primarily from respiratory droplets of infected individuals ( Figure 1) in enclosed spaces, and to a much lesser extent by fomites, to mucosal epithelial cells in the upper airway and oral cavity (Pambuccian, 2020). Here, the virus uses its trimeric Spike protein to latch onto host cell ACE2 (angiotensinconverting enzyme-2) receptor binding sites (Figure 1), via the receptor-binding domain (RBD) of this glycoprotein in the "prefusion" state (Cyranoski, 2020;Walls et al., 2020;Wrapp et al., 2020;Yuan et al., 2020). Proteases such as TMPRSS-2/furin cleave viral Spike (Figure 1) to enable membranes of the virus and host cell to fuse (Hoffmann et al., 2020;Oberfeld et al., 2020). The virus enters cells by endocytosis. The 30 kb single-stranded plus-strand RNA is released directly into the cytoplasm and hijacks the cell to translate the viral replication-transcription complex (RTC) in a double membrane vesicle. The RTC then produces RNAs that translate into protein, the ORFs coding for 16 nonstructural proteins, four main structural proteins and other special proteins (Oberfeld et al., 2020). Virions are assembled with RNA encased by nucleocapsid (N) and a "coat" consisting of membrane (M), envelope (E) and spike (S) proteins. Once released, the viral particles can infect cells in the lower airways (Type II pneumocytes) and enterocytes in the gastrointestinal tract Lamers et al., 2020).

COVID-19: SYMPTOMATOLOGY, CLINICAL DISEASE, AND POTENTIAL THERAPIES
Clinical symptoms of infection, although highly variable, are fever, dry and persistent cough, fatigue, anosomia/dysgeusia, loss of appetite and dyspnea but a plethora of other signs may, or may not, be present (e.g. headaches, sore throat, myalgia, rigors, intestinal discomfort/diarrhea, ocular manifestations, etc) Oberfeld et al., 2020;Vetter et al., 2020). Severe symptoms leading to hospitalization that progress rapidly to hypoxia and respiratory distress requiring supplemental oxygen and ventilator support are most prevalent in the elderly with underlying comorbidities . However, an unusual presentation in children, similar to Kawasaki Disease, termed MIS-C (multisystem inflammatory disease in children) is emerging (Viner and Whittaker, 2020). Why many viral-positive individuals are asymptomatic, or exhibit only minor cold symptoms, remains incompletely understood. Increasingly clear is the multifocal nature of COVID-19 pathogenesis with SARS-CoV-2 sometimes instigating destruction to blood vessel endothelial cells leading to coagulopathy and strokes, as well as potential kidney and neurological problems (Sardu et al., 2020). Blood sampling in moderate-severe cases may reveal lymphocytopenia, elevation of inflammatory markers like Creactive protein (CRP) and the cytokine interleukin 6 (IL-6), along with a pro-coagulant state exhibiting elevated D-dimer (a fibrin-degradation product), indicative of an immune response out of control called a cytokine storm (Chen G. et al., 2020). There are no therapies or preventative vaccines for our immune naïve global population. Hundreds of existing drugs and a common vaccine, repurposed for COVID-19, are undergoing clinical trials. The existing vaccine being studied in the Netherlands and Australia is for tuberculosis. BCG (Bacille Calmette-Gueŕin) has previously shown mixed, but broadly protective, effects against a variety of respiratory infectious diseases (O'Neill and Netea, 2020). The early hype revolved around (hydroxy)chloroquine (malaria, autoimmune indications with antiviral properties) but the drug appears to offer minimal benefit for COVID-19 resolution, with the potential for harm, and some clinical trials have been shut down (Ferner and Aronson, 2020;Tang et al., 2020). Remdesivir, an antiviral drug currently under FDA Emergency Use Authorization (EUA), has shown modest benefit reducing symptoms and recovery by approximately 4 days in one study 2 but no effect in another . Tocilizumab, a recombinant humanized anti-human IL-6 receptor monoclonal antibody, may be effective at rendering the cytokine storminducing effects of IL-6 less capable of initiating damage to the airways but more studies are required 3 . Interferon (IFN)-a2b is looking promising as early results suggest it reduces viral load in the upper airways and can reduce CRP and IL-6 (Zhou Q. et al., 2020). Drug therapy aside, a major step forward in this current pandemic is the generation of a safe and effective preventative vaccine. Several articles have already appeared on this topic (Ahn et al., 2020;Amanat and Krammer, 2020;Caddy, 2020;Callaway, 2020a;Cohen, 2020;Kim et al., 2020;Lurie et al., 2020;Wang F. et al., 2020;Wu, 2020). This article deals with a late May 2020 snapshot of the global race for a SARS-CoV-2 vaccine that is taking place at breakneck speed, with a brief prelude on diagnostics and history of vaccines.

COVID-19 DIAGNOSTICS
Knowing the genetic makeup of SARS-CoV-2 is essential for COVID-19 diagnostics. The 30 kb SARS-CoV-2 genome sequence analysis of a patient isolate from Wuhan at the epicenter in China, published in early January 2020, took place swiftly after viral isolation . From January-May 2020, the genome has been sequenced a multitude of times in clinical isolates obtained from patients around the world (Mavian et al., 2020). The SARS-CoV-2 genome is about 80% similar to that of SARS-CoV-1 and 50% to MERS-CoV but 96% to a bat coronavirus (Andersen et al., 2020;. Since deciphering the original sequence, some genetic drift is occurring in worldwide cases with mutations even arising in the Spike-encoding region, the key gateway to host entry, which could be altering virulence (Becerra-Flores and Cardozo, 2020). Besides the four structural proteins of SARS-CoV-2 mentioned above (S, E, M, N), and the critical RTC, there are several additional open-reading frames (ORFs) encoding nonstructural proteins and a viral protease within the SARS-CoV-2 genome, some with clear functions deduced from prior study with SARS-CoV-1 but others with unknown function (Ahn et al., 2020). Interestingly, ORF3b appears to suppress strongly the important Type I cytokine host immune response by blocking interferon (Konno et al., 2020).
Reverse transcriptase-polymerase chain reaction (RT-PCR) is the hallmark laboratory diagnostic technology to detect viral nucleic acid obtained from nasal swabs. Each jurisdiction around the world has designated screening assays using primer sets from either N, E, RNA-dependent RNA polymerase, ORF1a or ORF1b sequences (Ahn et al., 2020). Reliable COVID-19 diagnostic tests to detect individuals exposed to the virus is of paramount importance. Many asymptomatic infected people and those that have recovered from COVID-19 should carry immunity to further infection (Danchin and Timmis, 2020). This includes the production of neutralizing antibodies to SARS-CoV-2 . The hunt is on for rapid detection assays for those immune-privileged individuals. Our colleagues have carried out detailed analyses of over 300 viral antigen/serological antibody tests seeking approval (or already with EUA) for this important area (Ghaffari et al., 2020). FIGURE 1 | Transmission and life-cycle of SARS-CoV-2 causing COVID-19. SARS-CoV-2 is transmitted via respiratory droplets of infected cases to oral and respiratory mucosal cells. The virus, possessing a single-stranded RNA genome wrapped in nucleocapsid (N) protein and three major surface proteins: membrane (M), envelope (E) and Spike, replicates and passes to the lower airways potentially leading to severe pneumonia. The gateway to host cell entry (magnified view) is via Spike-converting enzyme 2 (ACE2) interaction with cleavage of Spike in the prefusion state by proteases TMPRSS-2/furin. A simplified depiction of the life cycle of the virus is shown along with potential immune responses elicited.
GENERATING THE "IDEAL" VACCINE FOR PROTECTION AGAINST COVID-19

Timelines for Vaccine Development
Vaccines are one of the monumental achievements in human medical intervention to mitigate the dispersion and impact of infectious disease. Polio was a serious crippling disorder for decades until the development of two separate vaccine candidates by Jonas Salk and Albert Sabin (Melnick, 1996). From the time of the first polio outbreak in the United States to testing and development was almost 60 years 4 ( Figure 2). More recently, a vaccine for the lethal infectious hemorrhagic fever elicited by Ebola disease took on a substantially different timeline with development by a Canadian team (Jones et al., 2005) preceding the worst outbreak in Western Africa in 2014, with the disease first described in isolated cases in 1976. Taking into account the prolonged development and testing phases it still took at least 15 years ( Figure 2) for authorized clinical use of an Ebola vaccine 5 . Looking at the other coronavirus diseasecausing outbreaks (SARS and MERS) that led to regional epidemics in various countries, the impetus was strong initially to develop vaccines but fizzled out with their spontaneous resolution. Although vaccine candidates were brought forward for both SARS-CoV-1 and MERS, and projects continue, there are still no approved vaccines for either infectious agent 17 and 6 years, respectively (de Wit et al., 2016;Song et al., 2019), since the original outbreaks ( Figure 2). In view of this, the bar is set high for developing an effective vaccine targeting COVID-19. Will SARS-CoV-2 go the same direction as the other coronaviruses, or will it become a seasonal outbreak like influenza and persist as a moderately benign to severe nuisance-causing infectious disorder that lasts for years?

Current Context for COVID-19 Vaccine Development
Given the worldwide magnitude of the COVID-19 pandemic and comparisons to the great Spanish flu of 1918, the race for a vaccine candidate has taken on unprecedented urgency and commitment across the globe. Established organizations are maintaining databases of vaccines under development including the World Health Organization 6 (WHO), the Coalition for Epidemic Preparedness Innovations 7 (CEPI), the Milken Institute 8 (a nonprofit think-tank out of California), and Biocentury Inc 9 (a partner to the biopharmaceutical industry). Unparalleled data sharing and collaborative team efforts are breaking down barriers in an attempt to reduce the time from the usual 10 + years for an approved vaccine down to 12-18 months. CEPI, the Biomedical Advanced Research and Development Authority 10 (BARDA), GAVI, The Vaccine Alliance 11 (formerly Global Alliance for Vaccines and Immunization), various governments and other sources are either pouring money into the efforts to fund projects or providing logistical support with additional initiatives. One such initiative is the strategic alliance ACTIV 12 (Accelerating COVID-19 Therapeutic Interventions and Vaccines), a publicprivate initiative bringing together more than a dozen leading biopharmaceutical companies, the CDC (Centers for Disease Control and Prevention), FDA (U.S. Food and Drug Administration), and EMA (European Medicines Agency) to develop an international strategy for a coordinated research response to the COVID-19 pandemic. Another massive undertaking to expedite the process is Operation Warp Speed 13 , which includes scientists, pharmaceutical companies and US federal officials and is being compared to the Manhattan Project 14 What does the "ideal" vaccine look like for COVID-19 prevention? In clinical terms, three main factors are essential: (i) a robust immune response generating long-lasting neutralizing antibodies to SARS-CoV-2 antigens (e.g. S and/ or N) is imperative (Figures 1 and 3). When individuals are infected with foreign antigens on viruses, they evoke both innate and adaptive immune reactions with a coordinated antigen-presenting cell (APC) attack on the virus and Thelper cell activation that leads to B-lymphocytes producing antibodies. In this context, ideally the antibodies will directly interfere with SARS-CoV-2's knack for latching onto epithelial cells and Type II pneumocytes via ACE2 ( Figure 1) in order to neutralize the virus. This would curtail successfully the virus from infecting the host and be protective if an infection resulted. However, there remains critical shortfalls in the current knowledge of what comprises a protective immune response against COVID-19 and how long it lasts [as of May 2020]. (ii) The ideal SARS-CoV-2 vaccine will also induce potent T-lymphocyte immunity. Ideally, this would be a well-coordinated, orchestrated T-cell response that includes T-helper and cytotoxic T-lymphocyte subsets that recognize SARS-CoV-2 infected cells in the body and annihilate them to block viral replication, along with acquisition of memory T-cells to prevent reinfection months to years later. (iii) The candidate vaccine should limit any serious adverse events (SAEs) at the injection site or systemically, for example, fever in infants and young children. In the case of respiratory disorders caused by infectious agents, it is essential that vaccine-associated enhanced respiratory disease (VAERD), antibody-dependent en hanc emen t (ADE), a n tibo dy -depende nt ce llular cytotoxicity (ADCC), complement-dependent cytotoxicity, including cytokine storm-inducing effects are completely avoided (Graham, 2020;Hotez et al., 2020;Vardhana and Wolchok, 2020). Some of these clinical sequelae have been associated with past vaccines in development (Figure 3), with a prominent example that for respiratory syncytial virus (RSV) during the late 1960s (Graham, 2020). Since the virus apparently activates complement deposition in small blood vessels of some infected individuals (Magro et al., 2020), blocking adverse host responses is essential. Complement, inflammation, and coagulation systems are intertwined (Oikonomopoulou et al., 2012) and hyperactive in COVID-19 positive cases and the vaccine should not provoke these host response systems.
In logistical terms, there are a few essential parameters for the ideal candidate vaccine for COVID-19: (i) the vaccine should be easy to administer and preferably in a single dose at the lowest possible amount. An oral or intranasal vaccine would be ideal.
(ii) The vaccine should be facile to produce and scale-up. Manufacturing millions to billions of doses required to immunize the human population must be feasible and rapid. (iii) Long-term storage of the vaccine at room temperature should be a sought after goal to facilitate transport and stockpiling in underdeveloped nations with inadequate supply chains and cold chain capacities (Figure 3). The ideal vaccine targeting COVID-19 would fit into a proposed target product profile as described in Table 1.

The Traditional Process Towards Vaccine Development
The normal strides taken to achieve successful vaccine development are similar to those for any drug. The process should be very stringent seeing that it will culminate in administration of the vaccine candidate to billions of humans. The typical paradigm is depicted in Figure 3 and is explained in the legend.

The Platforms Currently Exploited for SARS-CoV-2 Vaccine Development
The scientific publications to date (Ahn et al., 2020;Amanat and Krammer, 2020;Caddy, 2020;Callaway, 2020a;Callaway, 2020b;Cohen, 2020;Chen WH et al., 2020;Kim et al., 2020;Lurie et al., 2020;Thanh Le et al., 2020;Wang F et al., 2020;Wu, 2020), as well as databases mentioned above, have various formats for classifying vaccine platforms. In our analysis, we consider seven main platforms of vaccine development, along with an eighth catch-all "Other" category ( Figure 4). There are two nucleic acid platforms: DNA (12 candidates) and RNA (20 + candidates), which could be sub-divided further according to particular traits related to delivery and carriers (e.g. electroporation with special devices intradermally vs oral formulation or LNPs vs exosomes) but for simplicity are considered only as two categories. These nucleic acid platforms belong to the new generation of vaccines. Neither has reached licensing for human use but a number are being tested in humans (Cohen, 2020). A third category termed "Protein-based" (also referred to as "subunit" vaccine) includes a broad range of technologies to prepare immune-stimulating viral protein antigens and represents the largest category of all current COVID-19 FIGURE 2 | Timelines for the development of various vaccines for polio, Ebola virus, and three betacoronaviruses (SARS-CoV-1, MERS-CoV and SARS-CoV-2). On the left side of each timeline is a reference point for the first clinical case description and/or first recognized cases) for each type of viral infection. Significant events are depicted along the timeline (not according to scale). Solid horizontal red bars indicate the approximate period from first outbreak/vaccine to clinically approved use. Dotted red lines indicate no availability of vaccines since first outbreak and the green dotted line represents a rapid emergency use authorization timeline for a putative SARS-CoV-2 vaccine.
vaccine candidates (currently 44 + ) in development. Like nucleic acids, protein-based vaccines represent a newer technology but some are already in use in the clinical realm (e.g. Gardasil for human papillomavirus). The fourth and fifth categories are viralbased vectors, similar to those used in gene therapy and include nonreplicating (16 + candidates) and replicating (14 + candidates) vectors. The next two are the SARS-CoV-2 viruses themselves, either inactivated (usually with a chemical such as bpropriolactone, which chemically inactivates enveloped viruses and can inhibit viral membrane fusion in a dose-dependent manner)  or in a live-attenuated version, generated by techniques such as codon deoptimization or serial passaging in cell culture (Mueller et al., 2020). The "Other" category includes virus-like particles (VLPs), the use of non-SARS-CoV-2 virus carriers such as killed rabies (CORAVAX) and live modified horsepox (TX-1800). There are 10 + in this category. Repurposed existing vaccines for polio or tuberculosis to evoke general immunity, various cellular immunotherapies to stimulate the host immune system, encapsulated convalescent serum, and "unknown" platform designation (listed in The Milken Institute database) are excluded from our analyses ( Figure 4). According to these criteria, there are over 125 SARS-CoV-2 vaccines in development, which is astounding, from all across the globe in such a short time-frame.

Advantages and Disadvantages of Vaccine Development Platforms
For the ideal COVID-19 vaccine to reach routine use in humans, it is imperative that the vaccine protects against both clinical disease and viral transmission, in order to break the chain of person-to-person pandemic spread. Some of the advantages/ disadvantages of each platform are considered in Table 2.

Representative COVID-19 Vaccines
In this section, we provide some specific details on 16 (of >125) vaccines in the pipeline that are registered for clinical trials or already being tested (as of May 25, 2020), in addition to those that are currently CEPI-and/or BARDA-sponsored from each platform that are, or not yet, in clinical trials. This is to provide a snapshot of the field in early endeavors to procure a functional SARS-CoV-2 vaccine in man and their proposed developmental pipeline ( Figure 5). One can also refer to Table 3 for an abbreviated account of the 16 candidates. Where the vaccine candidate's name has not been specified, we refer to it as "Lead." FIGURE 3 | Development of the "ideal" vaccine for COVID-19 depicted by concentric circles converging to the target. Normally, development occurs in three discrete phases: Research and Development (R&D) involving platform selection, designing targets (e.g. whether that might be selection of an RNA sequence and decisions on nucleoside substitutions, lipid nanoparticle (LNP) formulation, etc, or decisions on how to create a live-attenuated viral preparation) and preclinical testing in vitro in cell culture and in vivo in animals. For SARS-CoV-2 R&D, some of the animal models used are transgenic mice that overexpress the Spike-binding protein ACE-2, Syrian hamsters, ferrets, and non-human primates (NHPs). If encouraging results are apparent in the preclinical phase (indicated by various parameters in green boxes), the candidate vaccine is taken to the second phase, which consists of testing in human volunteers in three stages of clinical trials (Phase I, Phase II, and Phase III). These may be concatenated to expedite approval (e.g. Phase II/III). Due to the pandemic nature of COVID-19, both these outer concentric phases are being pursued simultaneously under expedited approvals with potential for emergency use authorizations. If, and only when, vaccine safety and efficacy is achieved in human volunteers, the logistical operations become the major hurdles to ensure worldwide distribution in a coordinated and inter-connected manner (manufacturing, supply chain distribution, storage, etc.). Vaccine candidates that do not achieve satisfactory results in clinical trials, due to various factors shown in the red boxes, will be dropped from further development. ADE, antibody-dependent enhancement; ADCC, antibody-dependent cellular cytotoxicity; CDC, complement-dependent cellular cytotoxicity; VAERD, vaccine-associated enhanced respiratory disease; AE, adverse event.
We do not endorse any particular candidate. CEPI/WHO/ BARDA/GAVI and governments are taking the pan-platform approach, recognizing that this is the best path forward. Antigen: SARS-CoV-2 Spike glycoprotein (S) Details of platform: DNA plasmid expressing trimeric S and a hybrid transporter protein within Bifidobacterium longum delivered to colonic epithelial cells to prime an immune response via colonic lymphoid tissues 1 | Target product profile of "ideal" SARS-Cov-2 vaccine targeting COVID-19.

Indication and Clinical Use
Prophylactic vaccination against SARS-CoV-2 infection of susceptible health-care workers (HCW), children, adolescents and adults. Use-case scenarios For use in (current) pandemic response and subsequent wave outbreaks. For use in routine immunization, supplementary immunization and stockpiling of vaccine against COVID-19. Formulation Contains SARS-CoV-2 vaccine in sterile liquid (or other format) for direct oral use or in lyophilized format with simple reconstitution at point of use not requiring end-toend cold chain. Presentation Single-use or multidose vial liquid or lyophilized, stored at room temperature. Stability Thermostable, not light sensitive. The stability of vaccine potency is durable >2 yrs at room temperature. Dose regimen and amount Single dose 0.5 ml or less, if injectable. A second boosting dose 4-8 weeks later, if necessary. Route of administration Oral (preferred). Subcutaneous, intradermal, or intramuscular (acceptable). Target population All individuals aged 9 months and above. Target countries All countries around the world.

Safety
The safety profile should be based on at least two data sets with preferably at least 10,000 subjects over all age groups. Immunogenicity Phase III clinical studies should include an immunogenicity arm in which serum and blood samples are collected and tested for antibody and cell-mediated immunity. If possible, a minimum serological correlate of protection should be determined based on the short-term and long-term efficacy.

Interactions
The SARS-CoV-2 vaccine should not be administered with any other vaccine or it can be administered concomitantly with influenza vaccine at separate sites. Pregnancy There is limited information on administration of SARS-CoV-2 vaccine to pregnant or lactating women. User training requirements Minimal training administered by HCW or trained layperson.
Cost/immunized individual COGS should be reasonable to governments of both high-and low-income nations. Disposal Safe disposal of vials/sharps in biohazard waste at point of use.

Route/mode of administration: oral ingestion of probiotic capsule
Safety of Platform: Extensive worldwide use of probiotics. However, DNA vaccines are not currently on the market for use in humans and this particular strategy is untested.
Advantages: fast design and manufacturing; no cold chain for storage and distribution; robust immune response and mucosal immunity predicted Clinical Trial Registration: NCT04334980 Start date: June 2020 (projected) Official Name of Trial: A Phase 1, Randomized, Observer-Blind, Placebo-Controlled Trial to Evaluate the Safety, Tolerability and Immunogenicity of the bacTRL-Spike Oral Candidate Vaccine for the Prevention of COVID-19 in Healthy Adults Study design: Phase I, dose-escalating with three cohorts, n=84; Dosing Regimen: Single oral dose of BacTRL-Spike at three doses (1, 3, or 10 million bacteria) or placebo in each cohort (with n=63 for vaccine and n=21 for placebo); Endpoints: Primary -Incidence and severity of adverse events (AEs) Secondary: Seroconversion, virus stool shedding, protection from COVID-19 Partners: none announced Funding Partners: none announced Manufacturing partners: none announced Next stage: Obtain results of Phase I study by end of the year and get 10,000 + enrolled in further Phase II/III studies Official Name of Trial: Study to Describe the Safety, Tolerability, Immunogenicity, and Potential Efficacy of RNA Vaccine Candidates against COVID-19 in Healthy Adults Study design: Phase I/II, randomized, placebo-controlled, observer-blind, dose-finding, and vaccine candidate-selection study in healthy adults. 18 experimental arms each with four vaccine targets (BNT162a1, BNT162b1, BNT162b2, BNT162c2) x 3 age groups (18-55 y.o., 65-85 y.o. and 18-85 y.o.) × 2 dosing time points (timing not specified) × 3 doses (low-, mid-, high-; amounts not specified) + 3 placebo comparator arms carried out in three stages; Stage 1: to identify preferred vaccine candidate(s), Timeline: up to several hundred million doses of bulk RNA/ year with their current GMP III facility; a new GMP IV suite will be put into operation within two years where capacity for production of one billion or more doses per year is possible Recombinant Protein-Based Vaccines (i) Name: NVX-CoV2373 Company (Country):

Details of platform: Recombinant protein is expressed in genetically engineered Sf9 insect cells and the properly folded and post-translationally modified protein is incorporated into a nanoparticle formulation along with Novavax's saponin-based Matrix-M adjuvant
Route/mode of administration: i.m. injection to deltoid muscle Safety of Platform: Novavax's platform has been tested in several Phase I, II, III trials for seasonal influenza, Ebola and RSV and appears to be safe Advantages: fast design and relatively rapid production possible Clinical Trial Registration: NCT04368988 Start date: May 15, 2020   Next stage: Preliminary data from Phase I in July, then proceeding to Phase II (2200 volunteers in multiple countries) later in summer with results expected by the end of 2020 Timeline: 100 million doses by year-end and a billion by end of 2021 (ii) Name: SCB-2019 Company (Country): Clover Biopharmaceuticals Inc. (China) Antigen: Recombinant trimeric SARS-CoV-2 Spike protein (S) Details of platform: patented Trimer-Tag ® technology to produce a S-Trimer protein subunit vaccine that resembles the native trimeric viral spike via a rapid mammalian cell-culture based expression system Route/mode of administration: i.m. injection to deltoid muscle Safety of Platform: Clover has previously developed recombinant subunit-Trimer vaccines for RSV and Influenza viruses utilizing its Trimer-Tag ® technology and has demonstrated that they are able to evoke protective neutralizing antibody responses in multiple animal models and appears to be safe.
A d v a n t a g e s : f a s t d e s i g n a n d r e l a t i v e l y r a p i d production possible Clinical Next stage: Get clinical trials underway and evaluate data Timeline: not disclosed yet (iii) Name: "Lead candidate" Organization (Country):

University of Queensland (Australia)
Antigen: Recombinant trimeric SARS-CoV-2 Spike protein (S) Details of platform: molecular clamp platform by synthesizing viral surface proteins, which attach to host cells during infection, and "clamping" them into shape, making it easier for the immune system to recognize them as the correct antigen Route/mode of administration: presumably i.m. injection Safety of Platform: unknown details for this specific platform A d v a n t a g e s : f a s t d e s i g n a n d r e l a t i v e l y r a p i d production possible Clinical Trial Registration: not yet registered Start date: July 2020 projected Official Name of Trial: not yet started but will take place in Australia Study design: Phase I not started yet; Dosing Regimen: not known; Endpoints: not known Partners: Peter Doherty Institute for Infection and Immunity, Melbourne (Vaccine development), Viroclinics Xplore (Preclinical animal studies), GSK (Adjuvant) Funding Partners: CEPI $10.6 M Manufacturing partner: Cytiva (previously GE Healthcare Life Sciences), as well as Lonza and Thermo Fisher Scientific; CSL/Seqirus, Dynavax and GSK will supply adjuvants Next stage: Get clinical trials underway Timeline: not disclosed yet (iv) Name: "Lead candidate" Company (Country): Sanofi/ GSK collaboration (France/UK) Antigen: Recombinant trimeric SARS-CoV-2 Spike protein (S) Details of platform: Sanofi's recombinant DNA technology and their Sf9 insect cell expression system Route/mode of administration: presumably i.m. injection Safety of Platform: the combination of a protein-based antigen together with an adjuvant, is well-established and used in a number of vaccines available today A d v a n t a g e s : f a s t d e s i g n a n d r e l a t i v e l y r a p i d production possible Clinical Trial Registration: not yet registered Start date: July-September 2020 projected Official Name of Trial: not yet named Study design: Phase I not started yet; Dosing Regimen: not known; Endpoints: not disclosed yet Funding Partners: BARDA Manufacturing partners: both Sanofi and GSK are global leaders in vaccine development and they have the capacity to generate millions of doses Next stage: Get clinical trials underway and complete the development required for availability of a vaccine by the second half of 2021 Timeline: not disclosed yet Viral Vector-Based (Nonreplicating) (i) Name: AZD1222 (formerly ChAdOx1 nCoV-19) Company (Country): University of Oxford/Astra Zeneca collaboration (UK) Antigen: SARS-CoV-2 Spike glycoprotein (S) Details of platform: AZD1222 derives from a chimpanzee viral vector (ChAdOx1), which is a weakened version of a common cold adenovirus with the Spike-encoding region cloned into the E1 locus. Although it infects this primate, it is genetically altered so that it is incapable of viral spread in humans. The same vector modality is in vaccine candidates for influenza, tuberculosis, Chikungunya and Zika viruses.
Route/mode of administration: i.m. injection Safety of Platform: Vaccines made from the ChAdOx1 viral vector platform for over 10 different pathogens have been tested in thousands of volunteers (1 week to 90 y.o.) to date and are safe and well tolerated, although they can cause temporary side effects, such as a temperature elevation, headache or sore arm Advantages: favorable safety and tolerability profile of the platform Clinical Trial Registration: NCT04324606 (NCT04400838; PhII/III) Start date: April 23, 2020 Official Name Antigen: Full-length SARS-CoV-2 Spike glycoprotein (S) Details of platform: Ad5 vectors are well studied and can be grown into high titer stable stocks, they infect non-dividing and dividing cells, they are maintained in cells as an episome; the essential E1A and E1B genes are deleted and replaced by an expression cassette with a high activity cytomegalovirus immediate early (CMV) promoter, which drives expression of the target S protein Route/mode of administration: i.m. injection into deltoid Safety of Platform: In general, safe and well tolerated; however, can be dangerous in immunocompromised individuals. One drawback is that there could be pre-existing neutralizing Abs to the Ad5 vector in some adults Advantages: well-tested vector in gene therapy and vaccination trials (MERS and Ebola) but could be difficult for large-scale manufacturing Study design: (i) Phase I, non-randomized open label doseescalating with three cohorts (low-, middle, high-dose for 18-60 y.o, n=108; (ii) Phase II randomized, double-blinded, placebocontrolled with 3 groups (low-and middle-dose, placebo) n=500; Dosing Regimen: (i) Single dose (5 x 10 10 , 10 11 , 1.5 x 10 11 vp Ad5-nCoV n=36/cohort (ii) single dose (5 x 10 10 vp, n=125; 10 11 vp, n=250; placebo n=125); Endpoints: Primary -General safety including any adverse events (AEs) up to 7 (or 14 for Phase II) days after injection; Secondary -Safety up to 6 months and various immunogenicity indices up to 6 months for both Phase I and II Timeline: preliminary data from Phase I just published (see below) but no exact timelines on production and scaleup reported (iii) Name: Ad26 SARS-CoV-2 (+ 2 back ups) Company (Country): Johnson and Johnson (USA) Antigen: undisclosed Details of platform: Janssen's AdVac ® and PER.C6 ® technologies allow for over a million doses to be produced from a 1000 liter bioreactor since the cells grow to high density in suspension culture; the non-replicating Ad26 vector (E1/E3 genes deleted) is likely better than Ad5 since less likelihood of preexisting antibodies to the vector Route/mode of administration: i.m. injection into deltoid Safety of Platform: their particular platform technology is used for an investigational Ebola vaccine in Africa and has also been used for their Zika, RSV and HIV vaccine candidates and appears to be safe Advantages: well-tested vector in gene therapy and vaccination trials with transport/storage at 2-8°C for 6 months Clinical Trial Registration: not yet registered Start date: September 2020 projected Official Name of Trial: not known yet Antigen: whole virus, with initial reports that the RBD within the Spike protein is the main immunogen  Details of platform: the inactivated viral vaccine platform is straightforward and used extensively; here, the CN-2 SARS-CoV-2 virus was plaque-purified and passaged several times in Vero cells prior to inactivation with b-propiolactone, verification of inactivation, followed by stringent purification protocols and mixing with an alum adjuvant  Route/mode of administration: i.m. injection Safety of Platform: vaccines made from inactivated virus are used throughout the world with a generally excellent safety profile In a May 18 press release of interim data 15 and ensuing evaluation (Callaway, 2020c), the company reported that in the initial eight participants mRNA-1273 on the two lower doses (25 and 100 µg) evoked neutralizing antibody titer levels reaching or exceeding neutralizing antibody titers generally seen in convalescent sera as measured by a plaque reduction neutralization assay against live SARS-CoV-2. mRNA-1273 was generally safe and well tolerated and provided full protection against viral replication in the lungs in a mouse challenge model. In view of these data, the company is forging on with Phase II and III studies with an anticipated dose for Phase III using between 25 µg and 100 µg RNA. Since the level of neutralizing antibodies against SARS-CoV-2 is generally quite variable, and in some cases undetectable, in people who have recovered from COVID-19 without hospitalization (Wu et al., 2020b), it remains to be seen how effective the Moderna vaccine candidate will be at providing long-term immunity. This should come into focus in the upcoming large clinical trials.

AZD1222 (ChAdOx1 nCoV-19) University of Oxford Study
In this preclinical trial (van Doremalen et al., 2020), NHPs (rhesus macaques) were administered a very high dose of SARS-CoV-2 after receiving a single dose of the vaccine (very similar to regimen being used in the current clinical trials). The active treatment group of animals (n=6) produced elevated levels of SARS-CoV-2 neutralizing antibodies compared to no increase in the control group (n=3). There were indistinguishable amounts of virus in the nasal compartment compared to control animals but significantly reduced viral load in the lungs. The vaccinated NHPs were free of clinical-grade pneumonia, in contrast to the controls. Although the levels of antibodies and cell-mediated cytokine responses in the vaccinated NHPs were substantial, nobody knows yet whether this level of immune response will be protective in humans and how long that protection will last.

Ad5-nCoV CanSino Study
In this complete analysis of Phase I data  obtained between March 16 and March 27, 2020, n=108 participants (51% male, 49% female; mean age 36 y.o.) with equal numbers receiving low-, mid-and high-doses of the vaccine, there were mild-moderate AEs in 75%-83% in each group (injection site pain, fever, fatigue, headache, and muscle pain) with no SAEs within the first 28 days post-vaccination. Neutralizing antibodies increased significantly in the 14-28 day timeframe post-vaccination, along with specific T-cell responses.
The data indicate that this Ad5-vectored COVID-19 vaccine warrants further investigation and they will continue with their Phase II randomized, double-blinded, placebo-controlled study (see above).

CoronaVac (PiCoVacc) Sinovac Biotech Study
In this preclinical NHP study  there was robust SARS-CoV-2-specific neutralizing antibody responses in the rhesus macaques that received three doses of the inactivated virus, which importantly afforded partial to full protection from clinical signs of lung injury after viral challenge. In addition, there were no signs of ADE.

CONCLUDING REMARKS
The world is anxiously awaiting a safe, effective vaccine to protect against COVID-19 in order to resume a "normal" lifestyle, free from public health agency/government lockdowns and fear of ongoing pandemic waves over the coming months-years. Never before in the modern era of science has the accumulation of scientific papers in preprint archives (BioRxiv/MedRxiv, ≈5000 preprints) and peer-reviewed publications (PubMed, ≈22,000 papers) reached such exponential heights in such a short period with thousands of "all-things-considered" COVID topics. Mobilization of data sharing and joint global efforts toward this vaccine goal are monumental. It is hoped that the collaborative framework (such as ACTIV, and others around the world) for prioritizing vaccine and drug candidates, streamlining clinical trials, coordinating regulatory processes and/or leveraging assets among various partners to rapidly respond to the COVID-19 and future pandemics will soon be a reality. However, a successful path forward will be challenging and is certainly not guaranteed. For instance, no efficacious and approved vaccine for HIV/AIDS has come forward in over 30 years. The main HIV surface protein for host cell entry is covered in sugars, as is the Spike protein of SARS-CoV-2, but to a lesser extent (Watanabe et al., 2020). Will this site-specific glycan shield provide difficulties in target antigen recognition, if not properly reconstituted, in some of the vaccine platforms? Will it be necessary to target more than just the Spike protein, which most approaches are banking on? Cryptic epitopes for antibody recognition need to be considered (Yuan et al., 2020) and multivalent formulations may be required to generate effective long-lasting immunity with the ideal target product profile ( Table 1). SARS-CoV-2 antibody-based therapeutics derived from consortia such as CoVIC 16 (Coronavirus Immunotherapy Consortium) and other research groups/biopharma for COVID-19 have already entered clinical trials 17 . These biologics, as well as repurposed drugs, will likely provide mid-term solutions while we wait patiently for the much-anticipated vaccine.

AUTHOR CONTRIBUTIONS
CF and AA conceived the design and concepts. CF wrote the manuscript. CL contributed key information for Table 1. All authors contributed to the article and approved the submitted version.

FUNDING
This research was funded by Queen's University Special Research Project 379415 (CDF).