key: cord-0874891-9sflo9ux
authors: Case, James Brett; Winkler, Emma S.; Errico, John M.; Diamond, Michael S.
title: On the road to ending the COVID-19 pandemic: Are we there yet?
date: 2021-02-26
journal: Virology
DOI: 10.1016/j.virol.2021.02.003
sha: cdd0a8d1e18c274001490d5d45176695ab8ea798
doc_id: 874891
cord_uid: 9sflo9ux

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged into the human population in late 2019 and caused the global COVID-19 pandemic. SARS-CoV-2 has spread to more than 215 countries and infected many millions of people. Despite the introduction of numerous governmental and public health measures to control disease spread, infections continue at an unabated pace, suggesting that effective vaccines and antiviral drugs will be required to curtail disease, end the pandemic, and restore societal norms. Here, we review the current developments in antibody and vaccine countermeasures to limit or prevent disease.

In late 2019, for the third time this century, a highly pathogenic betacoronavirus crossed into the human population Zhu et al., 2020c) . Unlike the first two coronavirus outbreaks, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the etiologic agent of Coronavirus Disease 2019 , caused a global pandemic resulting in immense loss of life and economic hardship. Although the case-fatality rates for SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV) are higher, SARS-CoV-2 is considerably more transmissible, most likely through sustained community and asymptomatic human-to-human spread by direct contact, respiratory droplets, or airborne transmission (Petersen et al., 2020; Day, 2020; Li et al., 2020b) . At present, more than 108 million confirmed cases and over 2.3 million deaths have been reported globally (WHO, 2020a) .

Coronaviruses are enveloped viruses encoded by an extraordinarily large, single-stranded, positive-sense RNA genome. For all human coronaviruses, the first two-thirds of the genome encodes nonstructural proteins that primarily contribute to viral replication and RNA synthesis. The remaining 7 Shi et al., 2020b; Wu et al., 2020d; Zost et al., 2020) . In the majority of these studies, convalescent serum responses are screened for antibody binding to S or RBD in conjunction with neutralization tests against fully infectious or pseudotyped SARS-CoV-2. Sorting or recovery of single antigen-specific memory B cells from donors and subsequent sequencing and cloning of the immunoglobulin heavy and light chain genes has enabled rapid antibody expression and characterization.

To date, all neutralizing mAbs to SARS-CoV-2 target the S protein (1273 residues), which is displayed on the surface of the virus as 15 to 30 trimers . The receptor-binding S1 subunit (residues 14-685) consists of the N-terminal domain (NTD) and the RBD, whereas the S2 subunit (residues 685-1273) contains the fusion peptide and transmembrane domain (Figure 1A-B) . The majority of potently neutralizing mAbs bind epitopes in the RBD (residues 319-541) ( Figure 1C ) (Alsoussi et al., 2020; Barnes et al., 2020b; Brouwer et al., 2020; Hansen et al., 2020; Ju et al., 2020; Kreye et al., 2020; Liu et al., 2020b; Shi et al., 2020b; Wu et al., 2020d; Zost et al., 2020) with a smaller number engaging the NTD (residues 14-305) (Chi et al., 2020; Liu et al., 2020b) . The description of two neutralizing mAbs that bind a cryptic epitope on the S protein, but not the RBD or NTD also was recently reported , whereas neutralizing antibodies to the S2 stalk region of the S trimer have not been described. The limited diversity of neutralizing epitopes reported could be biased by use of RBD to sort virus-specific B cells rather than trimeric S, virus-like particles, or infectious virus. Additionally, most of the B cells obtained are from donors who experienced relatively mild COVD-19, rather than severely ill subjects who have higher neutralization titers likely due to persistent viral antigen, prolonged germinal center reactions, and more extensive somatic hypermutation (Ju et al., 2020; Wu et al., 2020b) .

Although many neutralizing mAbs bind the RBD, distinct, non-overlapping epitopes have been identified. X-ray crystallographic or cryo-electron microscopy structures of Fab fragments of mAbs in complex with either the RBD or trimeric spike have been solved . Each RBD in the S trimer can exist in an 'up' or 'down' conformation ( Figure 1D) , with binding to human ACE2 occurring only in the 'up' conformation, leading to four possible conformational configurations per spike trimer J o u r n a l P r e -p r o o f Walls et al., 2020; Wrapp et al., 2020) . Some RBD-binding mAbs target only 'up' conformation RBDs, whereas others preferentially bind the 'down' conformation; a third group can bind both conformations (Barnes et al., 2020a) .

The majority of neutralizing anti-RBD mAbs are believed to function by directly blocking ACE2 through engagement with the receptor-bind motif (RBM) of the S protein. As more RBM-specific antibodies have been characterized, several trends have been observed. For example, in one study, a large number of the neutralizing RBM antibodies utilized a restricted set of germline heavy chain genes, most commonly IGHV3-53, with many of their paratope contact residues retaining their germline identities . Additionally, these antibodies tended to encode shorter CDRH3 loops, although others described mAbs (e.g., COVA2-39, another IGHV3-53 antibody) that adopt a different binding orientation on the RBM, which accommodates a longer CDRH3 . Antibodies utilizing alternative germline variants also have been reported, but still tend to display relatively low levels of somatic hypermutation (Barnes et al., 2020a; Ju et al., 2020) .

A smaller subset of mAbs target epitopes outside the human ACE2 binding site, and thus may neutralize through alternative mechanisms of action including blockade of one or more other cellular attachment factors (heparan sulfate (Clausen et al., 2020) , HDL scavenger receptor B type 1 ) or possibly alternative receptors (neuropilin (Cantuti-Castelvetri et al., 2020) ), inhibition of conformational changes necessary for fusion, or allosteric blockade of ACE2 interactions. Recently, the structure of potently neutralizing antibodies C144 and S2M11 showed that both target a similar quaternary epitope, binding adjacent RBD subunits, and locking them into an ACE2-inaccessible 'down' conformation (Barnes et al., 2020a; . However, it is unclear how important this mechanism is for neutralization, since both mAbs also directly block ACE2 binding (Barnes et al., 2020a; . Among RBD antibodies, binding affinity to recombinant protein does not necessarily correlate with neutralization potency . This could be due to differences in how epitopes on the trimeric S are displayed or glycan shielding of epitopes present on the native virion Zhao et al., 2020) .

A third subset of antibodies targets epitopes further away from the RBM and typically display low to no neutralizing potency. Some of these antibodies, such as the SARS-CoV derived CR3022, engage cryptic epitopes. Notably, despite its ability to bind SARS-CoV-2 RBD and cause S trimer dissociation, CR3022 does not neutralize SARS-CoV-2 (Huo et al., 2020; Wrobel et al., 2020; . In contrast, the recently characterized mAb COVA1-16 targets an epitope outside the RBM that overlaps with CR3022, but still blocks ACE2 binding as a result of steric hindrance with its light chain, indicating that direct overlap with binding residues is not necessarily a requirement for ACE2 blockade . Although non-RBM antibodies tend to be less potently neutralizing, several have been shown to cross-react and neutralize SARS-CoV due to the increased conservation in the RBD outside of the RBM, highlighting their potential use against future betacoronaviruses (Pinto et al., 2020; Yuan et al., 2020b) .

Although coronaviruses have a slower mutation rate relative to other RNA viruses because of their proofreading 3'-to-5' exoribonuclease (nsp14), escape from antibody neutralization is still a major concern. Multiple groups have described viral escape under the selective pressure of single mAb treatments and identified viral escape mutations in the RBD (Baum et al., 2020b; Greaney et al., 2020; Liu et al., 2020d; Weisblum et al., 2020) . The most concerning viral escape mutations are those that do not cause a loss-of-fitness to the virus and can occur through single nucleotide changes. Two approaches have been employed to prevent viral resistance against mAb therapies: (1) the selection of antibodies that bind highly conserved residues and epitopes that do not tolerate mutations without marked loss-of-function phenotypes and (2) the development of antibody cocktails targeting distinct, non-overlapping regions of the RBD or S, which would require the less likely occurrence of simultaneous mutations at two separate genetic sites.

The emergence of rapidly-spreading SARS-CoV-2 variants in the United Kingdom (B.1.1.7), South Africa (B.1.351), and Brazil (B.1.1.248) as well as enzootic crossover events on mink farms has highlighted the need to monitor frequently whether viral evolution could compromise mAb therapy or even vaccine efficacy. Of particular concern, the South Africa and Brazil J o u r n a l P r e -p r o o f variants contain an amino acid substitution at position 484 in the RBM, which previously was identified as a key monoclonal and polyclonal antibody escape mutation in the context of yeast and chimeric vesicular stomatitis virus screening campaigns (Greaney et al., 2021a , Greaney et al., 2021b , Starr et al., 2020 , Weisblum et al., 2020 . Structural and computational approaches outlining the footprints of different classes of neutralizing antibodies that synergize within the RBD ( Barnes et al., 2020b; Greaney et al., 2020; Hansen et al., 2020) and the discovery of additional classes of neutralizing antibodies, such as the NTD mAbs (Suryadevara et al., 2021) that bind epitopes outside the RBM and RBD, may be important tools in designing mAb cocktails against COVID-19 and minimizing the potential for emergence of resistance.

In the majority of humans, viral replication is followed by clearance and rapid clinical resolution. However, a small, yet important, fraction of patients develop severe COVID-19 requiring hospitalization and intensive care. Severe COVD-19 is thought to be driven by an over-exuberant immune response at a time in which viral replication is waning. Prophylactic and therapeutic efficacy of neutralizing anti-RBD mAbs has been demonstrated in vivo in murine, hamster, and NHP models of SARS-CoV-2 pathogenesis ( Table 2 ) (Alsoussi et al., 2020; Baum et al., 2020a; Hassan et al., 2020; Kreye et al., 2020; Shi et al., 2020b; Wu et al., 2020d; Zost et al., 2020) . Treatment benefit is typically assessed by measuring reductions in viral loads, clinical signs of disease (weight loss and body temperature), and lung inflammation (cytokine levels and histopathology). functions can promote immune-mediated clearance by antibody-dependent cellular cytotoxicity, antibodydependent cellular phagocytosis, and complement activation, enhanced antigen presentation and CD8 + T cell responses, and reshaping of inflammation Lu et al., 2018) . The importance of Fc effector functions for antibody efficacy in vivo has been illustrated in many virus models, including HIV, Ebola, West Nile, hepatitis B, chikungunya, and influenza viruses (Bournazos et al., 2019; DiLillo et al., 2016; Fox et al., 2019; Hessell et al., 2007; Liu et al., 2017; Vogt et al., 2011) . While preliminary in vitro and in vivo studies highlight a possible beneficial role of Fc receptor engagement by mAb therapies against SARS-CoV-2 , further work is needed to identify which FcγRs, Fc effector functions, and cell subsets mediate protection.

Notwithstanding these possible beneficial functions, under certain circumstances, Fc-FcγR interactions can contribute to pathological outcomes. As one example, antibody-dependent enhancement of virus infection (ADE) (Morens, 1994) can occur when sub-neutralizing concentrations or nonneutralizing antibodies promote virus uptake and infection of FcγR-expressing immune cells. Although well-documented in dengue virus infection, ADE is at least a theoretical concern of antibody-based therapies and vaccines against SARS-CoV-2 (Diamond and Pierson, 2020) . Another potential concern is the generation of non-productive Fc-mediated immune responses, such as the pathological T H 2 immune skewing seen with SARS-CoV in NHPs in the setting of poorly neutralizing antibodies (Bolles et al., 2011) . However, in pre-clinical animal models and COVID-19 patients treated with convalescent plasma J o u r n a l P r e -p r o o f or anti-SARS-CoV-2 mAbs, there has been no reported evidence for ADE, immune skewing, or enhancement of disease.

A race for the most sought-after vaccine in history. Vaccination is one of the most effective means by which to prevent infectious diseases (WHO, 2019); with a goal of inducing immune responses that prevent or limit disease upon subsequent natural infection. Smallpox, a lethal disease caused by Historically, developing a safe, efficacious, and cost-effective human vaccine took between 4 to 15 years, if not longer. In comparison, COVID-19 vaccine development has proceeded at an unprecedented pace.

The complete viral genome sequence of SARS-CoV-2 from patient isolates was reported on January 11, 2020 -less than a month after the first cases were documented in Wuhan, China . With this information, academic research laboratories and biotechnology companies globally embarked on a race to generate an effective SARS-CoV-2 vaccine. Due to knowledge gained during previous vaccine development efforts for MERS-CoV and SARS-CoV, the initial step of target identification was essentially skipped without loss. For both SARS-CoV and MERS-CoV, the S protein had been demonstrated as the dominant antigen responsible for inducing neutralizing antibodies and an important target of protective T cell responses (Graham et al., 2013; Yong et al., 2019) . A plethora of vaccine candidates for SARS-CoV-2 were designed rapidly using the SARS-CoV-2 S protein as the primary immunogen ( Figure 2 and Table 3 ).

Nucleic acid vaccines. At present, six DNA-based vaccines and six mRNA-based vaccines against SARS-CoV-2 are in clinical trials (Figure 2 ) (WHO, 2020b) . Although DNA vaccines are currently used in veterinary medicine, due to their relatively modest immunogenicity as well as delivery hurdles, they have not been widely utilized in humans (Suschak et al., 2017) . mRNA vaccines are a new platform commonly consisting of an mRNA, which typically includes a 5' cap, regulatory elements in the 5' and/or 3' untranslated regions (UTRs), a poly(A) tail, and modified nucleosides to increase RNA stability and decrease innate immune activation, that is engineered to encode specific viral antigens and delivered by polymer-based or lipid nanoparticles (Pardi et al., 2018) . Upon entry into the cell cytoplasm, the mRNA is translated by host cell ribosomes similarly to endogenous mRNA, resulting in antigen expression for immune system recognition and response. Two mRNA vaccines, Pfizer/BioNtech's BNT162b2 and Moderna/NIAID's mRNA-1273, are currently in phase III clinical trials and recently received EUA status from the U.S. Food and Drug Administration and other countries (Cohen, 2020; WHO, 2020b) . BNT162b2 is an mRNA-lipid nanoparticle-formulated vaccine that encodes a membranebound, prefusion stabilized form of the full-length SARS-CoV-2 S protein (Walsh et al., 2020) . In a phase I, placebo-controlled, dose escalation trial, BNT162b2 and a similar vaccine candidate, BNT162b1, which encodes a secreted trimerized form of the SARS-CoV-2 RBD, were tested for safety and immunogenicity.

Both vaccine candidates were evaluated at four doses (10, 20, 30, or 100 µg) in a prime-boost strategy with three weeks between immunizations. Whereas serum neutralizing activity was dose-dependent for both vaccine candidates and peaked at levels higher than convalescent serum from COVID-19 infected subjects, BNT162b2 had fewer adverse reactions and was selected for clinical development. A recently published safety and efficacy trial of a 30 µg dose of BNT162b2 reported 95% protective efficacy against symptomatic COVID-19 (Polack et al., 2020) . mRNA-1273 encodes a prefusion stabilized form of the SARS-CoV-2 S protein and is delivered into cells by lipid-encapsulated nanoparticles (Jackson et al., 2020) . In a phase I, dose-escalation (25, 100, or 250 µg), open-label clinical trial in 45 healthy adults using a 28-day prime-boost intramuscularly J o u r n a l P r e -p r o o f administered regimen, mRNA-1273 induced serum neutralizing activity in all participants. Participants receiving the 25 µg or 100 µg doses also elicited strong T H 1-polarized CD4 + T cell responses. In a promising unpublished report from a 30,000-person efficacy trial, 185 placebo group participants developed COVID-19 symptoms over the course of the trial compared to 11 such individuals in the mRNA-1273 vaccinated group, suggesting a 94% efficacy rate (Cohen, 2020) . Moreover, of the 11 mRNA-1273 vaccinated individuals that contracted SARS-CoV-2, none progressed to severe disease Chiuppesi et al., 2020; Logunov et al., 2020; WHO, 2020b) .

Currently, four viral-vectored vaccines against SARS-CoV-2 are in phase III clinical trials, all of which utilize replication-defective adenovirus strains in prime-only or prime-boost regimens (Table 3) (WHO, 2020b). ChAdOx1 nCoV-19, which is jointly developed by the University of Oxford and AstraZeneca, uses a genetically modified Y25 simian (chimpanzee) adenovirus strain to deliver and express a full-length, codon-optimized form of the SARS-CoV-2 S protein .

Preclinical studies showed strong cellular and humoral responses in ChAdOx1 nCoV-19 vaccinated mice and NHPs . Phase I-II and II-III safety and immunogenicity trials in humans reported promising S-specific T cell responses and seroconversion rates of >99% in all ChAdOX1 nCoV-19 prime-boost vaccination groups (Folegatti et al., 2020; Ramasamy et al., 2020) . Adverse events attributed to the vaccine were not detected, and immunogenicity data were similar across the multiple age groups (18-55, 56-69, and ≥70 years). In efficacy analyses, vaccination with two standard doses or a low dose followed by a standard dose resulted in 62% and 90% protection against symptomatic disease, respectively (Voysey et al., 2020) .

Ad5-nCOV, a replication-defective viral vector vaccine developed by CanSino Biologics, uses a human adenovirus serotype-5 vector expressing a full-length, codon-optimized form of the SARS-CoV-2 S protein . Phase I and II clinical trials were conducted at a single center in Wuhan, China between March and April, 2020 2020b) . The highest dose (1.5 x 10 11 viral particles) caused considerable adverse reactions and was excluded from further development. T cell and neutralizing antibody responses peaked at 14-and 28-days post-vaccination, respectively, but were generally modest for the low (5 x 10 10 viral particles) and middle doses (1 x 10 11 viral particles). Indeed, neutralizing antibody responses were observed in only 50% of participants receiving the low or middle doses. In contrast to most viral-vectored vaccines in development, Ad5-nCOV is administered as a single intramuscular injection. While an Ad5-nCOV booster dose could theoretically improve the elicited antibody and T cell responses, this approach might be hampered by immunity generated by the first dose or even pre-existing natural immunity to the Ad5 serotype. Ad5-nCOV is currently in phase III clinical trials in Russia, Pakistan, and several Latin American countries (NCT04526990).

Other replication-defective human adenoviral vectors, such as those based on adenovirus serotype 26 (Ad26), which there is little pre-existing immunity in humans, also are under clinical evaluation.

Janssen developed an Ad26 vector-based vaccine (Ad26.COV2.S) that encodes for a full-length SARS-CoV-2 S protein containing a mutated furin cleavage site between S1 and S2 ( Figure 1A) and two proline stabilizing mutations in S2 (Mercado et al., 2020) . Ad26.COV2.S was selected as the best of seven constructs initially tested in NHPs, with each candidate differing in the leader sequences used, status of the furin cleavage site, introduction of proline stabilizing mutations, and/or truncation of C-terminal components of the spike protein. Viral-vectored vaccines often can induce robust B and T cell responses, and some are suitable for mucosal administration, a key consideration for SARS-CoV-2 immunity (Mudgal et al., 2020) . However, a number of issues such as safety in immunocompromised individuals as well as pre-existing vector immunity (against Ad5 and Ad26) may need to be overcome. ). Inactivated vaccines are relatively easy to produce, most often by generating virus in cell culture on a massive scale, followed by inactivation. In the case of SARS-CoV-2, production is more complex due to the need for BSL3 production facilities prior to inactivation. Since the entire SARS-CoV-2 virion is present in the vaccine, the resulting immunity may be more complete and include responses against S as well as the other structural proteins present in the virion (E, M, and N). The SARS-CoV-2 vaccine candidate PiCoVacc (now known as CoronaVac), an alum-adjuvanted vaccine developed by Sinovac Biotech Ltd., elicited potent neutralizing antibodies in mice, rats, and NHPs after two or three immunizations ( Table 3 ) . NHPs receiving the highest vaccine dose were protected from severe interstitial pneumonia, and viral RNA was absent in the lungs. In another study, the inactivated vaccine candidate BBIBP-CorV, developed by Sinopharm, induced neutralizing antibody responses in a dose-dependent manner in mice, rats, guinea pigs, and rabbits after one, two, or three immunizations . However, CD4 + and CD8 + T cell responses were not observed with either inactivated vaccine Wang et al., 2020a) . While inactivated vaccines are safe when given intramuscularly, they poorly induce mucosal immunity or secretory IgA in the upper respiratory tract.

Live-attenuated vaccines are derived from a pathogenic virus that has been weakened to the point where it no longer causes disease despite replicating in an individual for a few J o u r n a l P r e -p r o o f days. Serial passage of the virus in vitro, rational deletion of virulence genes, chimerizaton, and codon deoptimization techniques are strategies to reduce viral pathogenicity (Barrett, 1997; Coleman et al., 2008; Talon et al., 2000; Theiler and Smith, 1937) . Live-attenuated vaccines often generate strong humoral and cellular immune responses at the site of infection. While several live-attenuated antiviral vaccines are approved for use in humans (Amanna and Slifka, 2018), only three are in development against SARS-CoV-2, and only one, COVI-VAC, has advanced beyond preclinical stages (WHO, 2020b). These vaccine candidates all utilize codon-deoptimization strategies for attenuation. Codon-deoptimization introduces silent mutations into the nucleic acid sequences to yield a virus that uses codons that are relatively rare in human host cells, which attenuates viral infection due to poor genome translation efficiency. The primary concern of live-attenuated vaccines is safety due to possible reversion to virulence by mutation or recombination with a circulating strain. Since coronaviruses are known to recombine in nature (Su et al., 2016) , this issue may be challenging to overcome.

The deployment of effective vaccines against SARS-CoV-2 could prevent disease and transmission, ultimately resulting in community protection. However, large-scale manufacturing of EUA vaccines with a requirement of two doses (for most vaccines) to all countries of the world may take many months if not longer, especially in places where cold-chain distribution infrastructure is lacking. Another consideration is whether the current vaccines against SARS-CoV-2 will prevent transmission. While phase III clinical trial data suggest that Pfizer's BNT162b2 and Moderna's mRNA-1273 are effective against symptomatic disease and prevent progression to severe disease, more data over a longer period of time is needed to determine their durability and ability to prevent or mitigate upper airway infection and transmission.

Prophylactic and/or therapeutic administration of mAbs could have utility in populations that respond poorly to vaccination or remain unvaccinated. Currently two antibody drugs, Lilly's LY-CoV555 Jones et al., 2020) and Regeron's REGN-COV2 (Baum et al., 2020a) have been granted EUA status with at least 11 others undergoing advanced human testing in phase II or III clinical trials. Whereas both REGN-COV2 and LY-CoV555 are potently neutralizing anti-RBD human IgG1s, LY-CoV555 is a monotherapy and REGN-COV2 is a cocktail of two mAbs. Preliminary results with REGN-COV2 and LY-CoV555 demonstrated reduced hospitalization rates in mAb-treated subjects with mild to-moderate COVID-19. However, clinical trials showed no benefit of LY-CoV55 in hospitalized patients (ACTIV-3/TICO LY-CoV555 Study Group, 2020) highlighting that current mAb treatments may have their greatest effect early in the course of COVID-19. As more mAbs are developed and tested, several factors may be considered for progression to human evaluation including production capacity, stability, possible synergy in antibody combination, prevention of resistance, the role and optimization of Fc effector functions, and mechanism of neutralization. Furthermore, as SARS-CoV-2 viral evolution continues, as seen with the emergence of novel strains in Great Britan, South Africa, and Brazil, existing antibody therapies and vaccines may need to be reformulated to remain effective. In the case of vaccineinduced immunity, one might predict that a polyclonal response against the SARS-CoV-2 S would not result in a complete loss of vaccine efficacy due to the small number of point mutations present in the spike of the circulating variants and the likelihood of generating multiple antibodies against different regions. However, preliminary data demonstrating reduced neutralizing activity of plasma from vaccinated individuals against psuedoviruses containing E484K, N501Y or K417N/E484K/N501Y substitutions has raised concern (Wang et al., 2021a; Wang et al., 2021b; Wibmer et al., 2021) . These data contrast with a study utilizing infectious SARS-CoV-2 virus with similar mutations that showed only modest reductions in neutralization potency by mRNA vaccine-elicited sera (Xie et al., 2021) . At present, there is insufficient data to conclude what level of efficacy will be achieved by vaccine immunity against the emerging variants. Updated mAb cocktails and adjustments to the spike sequences of vaccines may be needed to prevent loss of protection.

In summary, while the current pandemic continues to rage, due to the remarkable efforts of numerous groups and partnerships across the world, SARS-CoV-2 countermeasures are advancing at record speed and now being deployed. In the months to come, the ongoing need for public health J o u r n a l P r e -p r o o f practices such as universal mask-wearing and social distancing will continue to be particularly important until a sufficient proportion of the population is immunized or achieve herd immunity, and emerging variants are shown to be controlled by natural or vaccine-induced immunity. With four new countermeasures granted EUA status and others on the horizon, hopefully, by spring 2021, the COVID-19 pandemic will reach a turning point, as long as viral evolution does not overcome our immune-based interventions. These data are from published phase I-II or phase III trials, when available.

1.

SARS-CoV-2 -mNeonGreen reporter virus neutralization assay.

2.

PRNT 80 assay with authentic SARS-CoV-2 3.

Public Health England PRNT 50 assay.

Neutralization assay with authentic SARS-CoV-2 5.

Microneutralization assay with authentic SARS-CoV-2.

6.

Microcytopathic effect assay with authentic SARS-CoV-2.

Neutralization assay with infectious SARS-CoV-2. 

A Double-Inactivated Severe Acute Respiratory Syndrome Coronavirus Vaccine Provides Incomplete Protection in Mice and Induces Increased Eosinophilic Proinflammatory Pulmonary Response upon Challenge

Fc-optimized antibodies elicit CD8 immunity to viral respiratory infection

Differential requirements for FcγR engagement by protective antibodies against Ebola virus

Broadly Neutralizing Anti-HIV-1 Antibodies Require Fc Effector Functions for

Potent neutralizing antibodies from COVID-19 patients define multiple targets of vulnerability

2020. Neuropilin-1 facilitates SARS-CoV-2 cell entry and infectivity

Replication-Competent Vesicular Stomatitis Virus Vaccine Vector Protects against SARS-CoV-2-Mediated Pathogenesis in Mice

Simulation of the Clinical and Pathological Manifestations of Coronavirus Disease

Neutralizing Antibody LY-CoV555 in Outpatients with Covid-19

Development of a multi-antigenic SARS-CoV-2 vaccine candidate using a synthetic poxvirus platform

SARS-CoV-2 Infection Depends on Cellular Heparan Sulfate and ACE2

Absolutely remarkable": No one who got Moderna's vaccine in trial developed severe COVID-19

Virus Attenuation by Genome-Scale Changes in Codon Pair Bias

Covid-19: identifying and isolating asymptomatic people helped eliminate virus in Italian village

The Challenges of Vaccine Development against a New Virus during a Pandemic

Broadly neutralizing anti-influenza antibodies require Fc receptor engagement for in vivo protection

Broadly neutralizing hemagglutinin stalkspecific antibodies require FcγR interactions for protection against influenza virus in vivo

A mouse-adapted model of SARS-CoV-2 to test COVID-19 countermeasures

Effectiveness of convalescent plasma therapy in severe COVID-19 patients

Ferret models of viral pathogenesis. Virology 479-480

Broadly neutralizing antibodies from human survivors target a conserved site in the Ebola virus glycoprotein HR2-MPER region

Safety and immunogenicity of the ChAdOx1 nCoV-19 vaccine against SARS-CoV-2: a preliminary report of a phase 1/2, single-blind, randomised controlled trial

Optimal therapeutic activity of monoclonal antibodies against chikungunya virus requires Fc-FcγR interaction on monocytes

Adaptation of SARS-CoV-2 in BALB/c mice for testing vaccine efficacy

A decade after SARS: strategies for controlling emerging coronaviruses

Complete Mapping of Mutations to the SARS-CoV-2 Spike Receptor-Binding Domain that Escape Antibody Recognition

Comprehensive mapping of mutations to the SARS-CoV-2 receptor-binding domain that affect recognition by polyclonal human serum antibodies

ORF8 and ORF3b antibodies are accurate serological markers of early and late SARS-CoV-2 infection

Towards HIV-1 remission: potential roles for broadly neutralizing antibodies

A SARS-CoV-2 Infection Model in Mice Demonstrates Protection by Neutralizing Antibodies

Fc receptor but not complement binding is important in antibody protection against HIV

Cell Entry Depends on ACE2 and TMPRSS2 and Is Blocked by a Clinically Proven Protease Inhibitor

Neutralization of SARS-CoV-2 by Destruction of the Prefusion Spike

Syrian hamsters as a small animal model for SARS-CoV-2 infection and countermeasure development

Syrian Hamster as an Animal Model for the Study of Human Influenza Virus Infection

An mRNA Vaccine against SARS-CoV-2 -Preliminary Report

Pathogenesis of SARS-CoV-2 in Transgenic Mice Expressing Human Angiotensin-Converting Enzyme 2

Furin Cleavage Site Is Key to SARS-CoV-2 Pathogenesis

Serologic responses of 42 MERS-coronavirus

Tracking Changes in SARS-CoV-2 Spike: Evidence that D614G Increases Infectivity of the COVID-19 Virus

A Therapeutic Non-self-reactive SARS-CoV-2 Antibody Protects from Lung Pathology in a COVID-19 Hamster Model

Functional assessment of cell entry and receptor usage for SARS-CoV-2 and other lineage B betacoronaviruses

A potent human neutralizing antibody Fc-dependently reduces established HBV infections

2020a. Effect of Convalescent Plasma Therapy on Time to Clinical Improvement in Patients With Severe and Life-threatening COVID-19: A Randomized Clinical Trial

Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV-2)

2021. Early High-Titer Plasma Therapy to Prevent Severe Covid-19 in Older Adults

Crossneutralization of a SARS-CoV-2 antibody to a functionally conserved site is mediated by avidity

Antibody-dependent-cellular-cytotoxicity-inducing antibodies significantly affect the post-exposure treatment of Ebola virus infection

Convalescent plasma treatment of severe COVID-19: a propensity score-matched control study

Landscape analysis of escape variants identifies SARS-CoV-2 spike mutations that attenuate monoclonal and serum antibody neutralization

Safety and immunogenicity of an rAd26 and rAd5 vector-based heterologous prime-boost COVID-19 vaccine in two formulations: two open, non-randomised phase 1/2 studies from Russia

Antibody responses to SARS-CoV-2 in patients with COVID-19

Beyond binding: antibody effector functions in infectious diseases

Enhanced isolation of SARS-CoV-2 by TMPRSS2-expressing cells

Syrian Hamster as an Animal Model for the Study on

Antibody-Dependent Enhancement of Infection and the Pathogenesis of Viral Disease

Prospects for mucosal vaccine: shutting the door on SARS-CoV-2

Respiratory disease in rhesus macaques inoculated with SARS-CoV-2

Animal models for COVID-19

Inactivated poliovirus vaccine: past and present experience

Detection of SARS-CoV-2-Specific Humoral and Cellular Immunity

A panel of human neutralizing mAbs targeting SARS-CoV-2 spike at multiple epitopes

Transmission of SARS-CoV-2 on mink farms between humans and mink and back to humans

mRNA vaccines -a new era in vaccinology

Comparing SARS-CoV-2 with SARS-CoV and influenza pandemics

Cross-neutralization of SARS-CoV-2 by a human monoclonal SARS-CoV antibody

Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine

Safety and immunogenicity of ChAdOx1 nCoV-19 vaccine administered in a prime

COVID-19 Genomics Consortium UK, 2020. Preliminary genomic characterization of an emergent SARS-CoV-2 lineage in the UK defined by a novel set of spike mutations

Broad and potent activity against SARS-like viruses by an engineered human monoclonal antibody

SARS-CoV-2 is transmitted via contact and via the air between ferrets

Convergent antibody responses to SARS-CoV-2 in convalescent individuals

Comparative pathogenesis of COVID-19, MERS, and SARS in a nonhuman primate model

Isolation of potent SARS-CoV-2 neutralizing antibodies and protection from disease in a small animal model

Treatment of Coronavirus Disease 2019 Patients with Convalescent Plasma Reveals a Signal of Significantly Decreased Mortality

An immunosuppressed Syrian golden hamster model for SARS-CoV infection

Antibody potency, effector function, and combinations in protection and therapy for SARS-CoV-2 infection in vivo

Susceptibility of ferrets, cats, dogs, and other domesticated animals to SARS-coronavirus 2

A human neutralizing antibody targets the receptor-binding site of SARS-CoV-2

Antibody responses against SARScoronavirus and its nucleocaspid in SARS patients

A Randomized Trial of Convalescent Plasma in Covid-19 Severe Pneumonia

Prospective mapping of viral mutations that escape antibodies used to treat COVID-19

IgA dominates the early neutralizing antibody response to SARS-CoV-2

Epidemiology, Genetic Recombination, and Pathogenesis of Coronaviruses

Generation of a Broadly Useful Model for COVID-19 Pathogenesis, Vaccination, and Treatment

Neutralizing and protective human monoclonal antibodies J o u r n a l P r e -p r o o f recognizing the N-terminal domain of the SARS-CoV-2 spike protein

Advancements in DNA vaccine vectors, nonmechanical delivery methods, and molecular adjuvants to increase immunogenicity

Influenza A and B viruses expressing altered NS1 proteins: A vaccine approach

THE EFFECT OF PROLONGED CULTIVATION IN VITRO UPON THE PATHOGENICITY OF YELLOW FEVER VIRUS

Ultrapotent human antibodies protect against SARS-CoV-2 challenge via multiple mechanisms

Severe Acute Respiratory Syndrome Coronavirus Infection of Mice Transgenic for the Human Angiotensin-Converting Enzyme 2 Virus Receptor

ChAdOx1 nCoV-19 vaccine prevents SARS-CoV-2 pneumonia in rhesus macaques

Safety of trivalent inactivated influenza vaccines in adults: Background for pandemic influenza vaccine safety monitoring

Poorly Neutralizing Cross-Reactive Antibodies against the Fusion Loop of West Nile Virus Envelope Protein Protect <em>In Vivo</em> via Fcγ Receptor and Complement-Dependent Effector Mechanisms

Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK. The Lancet

Structure, Function, and Antigenicity of the SARS-CoV-2 Spike Glycoprotein

Vaccine Candidates

Receptor Recognition by the Novel Coronavirus from Wuhan: an Analysis Based on Decade-Long Structural Studies of SARS Coronavirus

Development of an Inactivated Vaccine Candidate

Increased Resistance of SARS-CoV-2 Variants B.1.351 and B.1.1.7 to Antibody Neutralization

Enhanced SARS-CoV-2 neutralization by dimeric IgA

mRNA vaccine-elicited antibodies to SARS-CoV-2 and circulating variants

Site-specific glycan analysis of the SARS-CoV-2 spike

HDL-scavenger receptor B type 1 facilitates SARS-CoV-2 entry

Escape from neutralizing antibodies by SARS-CoV-2 spike protein variants

WHO Coronavirus Disease (COVID-19) Dashboard [WWW Document

DRAFT landscape of COVID-19 candidate vaccines [WWW Document

SARS-CoV-2 501Y.V2 escapes neutralization by South African COVID-19 donor plasma

SARS-CoV-2 infection of human ACE2-transgenic mice causes severe lung inflammation and impaired function

Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation

Antibody-mediated disruption of the SARS-CoV-2 spike glycoprotein

Genome Composition and Divergence of the Novel Coronavirus (2019-nCoV) Originating in China

Evaluating the Association of Clinical Characteristics With Neutralizing Antibody Levels in Patients Who Have Recovered From Mild COVID-19 in

An alternative binding mode of IGHV3-53 antibodies to the SARS-CoV-2 receptor binding domain

A noncompeting pair of human neutralizing antibodies block COVID-19 virus binding to its receptor ACE2

Effect of an Inactivated Vaccine Against SARS-CoV-2 on Safety and Immunogenicity Outcomes: Interim Analysis of

Safety and immunogenicity of an inactivated SARS-CoV-2 vaccine, BBIBP-CorV: , double-blind

Neutralization of SARS-CoV-2 spike 69/70 deletion, E484K, and N501Y variants by BNT162b2 vaccine-elicited sera

Recent Advances in the Vaccine Development Against Middle East Respiratory Syndrome-Coronavirus

A highly conserved cryptic epitope in the receptor binding domains of SARS-CoV-2 and SARS-CoV

2020. Safety, tolerability, and immunogenicity of an inactivated SARS-CoV-2 vaccine in healthy adults aged 18-59 years: a randomised, double-blind, placebo-controlled, phase 1/2 clinical trial. The Lancet Infectious Diseases

Rapid generation of a mouse model for Middle East respiratory syndrome

Virus-Receptor Interactions of Glycosylated SARS-CoV-2 Spike and Human ACE2 Receptor

A pneumonia outbreak associated with a new coronavirus of probable bat origin

Immunogenicity and safety of a recombinant adenovirus type-5-vectored COVID-19 vaccine in healthy adults aged 18 years or older: a randomised, double-blind, placebo-controlled, phase 2 trial

Safety, tolerability, and immunogenicity of a recombinant adenovirus type-5 vectored COVID-19 vaccine: a dose-escalation, open-label, non-randomised, firstin-human trial

A Novel Coronavirus from Patients with Pneumonia in China