At the end of 2019, a new coronavirus disease (COVID-19) emerged in Wuhan city, Hubei Province, China, which has quickly spread to the whole of China and more than 20 countries.[1–4] This disease results from infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a member of the family Coronaviridae. It is likely that wild animals serve as the reservoir host of SARS-CoV-2 and that transmission to humans occurred via wildlife trading[4,6]; however, the natural host of this novel coronavirus remains unknown. The challenges to stopping this epidemic need to be dissected and further strategies should be explored.
Sporadic outbreaks may continue
Although epidemics wane over time, sporadic outbreaks of case clusters may continue. As identification of the natural host of SARS-CoV-2 remains elusive, transmission from the natural host to humans may occur via occasional contact. As wildlife trading has been prohibited in China, such transmission will be greatly diminished; however, occasional contact may trigger an outbreak of cases. Identifying a natural host is critical for the final control of this disease, which requires enormous and continued effort. Transmission from cases of latent and silent virus infection has fueled the challenges. It has recently been reported that transmission could occur during latent infection. A father and daughter with a travel history to Wuhan returned to their hometown; although neither showed symptoms, members of their family developed COVID-19, suggesting that transmission could occur from silent infection.[7,8] The occurrence of transmission from an unknown natural host and possibly from latent and silent infections pose great challenges to disrupting the chain of infection and thereby terminating this epidemic.
Vaccine against SARS-CoV-2 is the best approach to stop the epidemic
A vaccine is composed of, at minimum, an immunogen and an adjuvant. The immunogen comprises features of a potential invading pathogen and thereby serves as the target for immune responses. Adjuvant regulates the strength of the immune response, meaning how vigorously the immune system attacks the target.
Inactivated virus vaccine
As the threat from SARS-CoV-2 will likely continue, it is urgent to develop an effective vaccine. A traditional inactivated vaccine could be rapidly developed as vaccine development technologies have matured and production facilities are well established.[10–12] However, the risk of residual virus infectivity remains a consideration, and over-inactivation may be implemented for safety reasons, as in the diphtheria-tetanus vaccine.[13,14] Because the virus conformational structure is usually sensitive to the inactivation process, over-inactivation may destroy the conformational neutralizing epitopes and result in the induction of only marginal—or even failure to induce—neutralizing antibody responses by the inactivated vaccine. Therefore, it is reasonable to develop a vaccine using alternative approaches.
Nucleic acid vaccines
The great advantages of DNA- and RNA- based vaccines lie in the possibility of rapid early development and effective tools to screen protective immunogens. As construction of a DNA/RNA vaccine takes only weeks, it is possible to surpass development of an inactivated vaccine that is in early development, in a manner that is even more efficient. However, the high cost of producing and delivering these vaccines as well as the problem of low immunogenicity dampens enthusiasm to scale up production capacity for DNA and RNA vaccines.[17,18] However, it is worth exploring the use of these vaccines as a screening tool to identify and optimize immunogens. In addition, DNA vaccines may function as good primers to concentrate host immune responses on the viral-derived immunogen and help to improve the efficacy of a subsequent boost immunization. Whereas it is unlikely a DNA/RNA vaccine alone could be used against SARS-CoV-2, it would be very useful to merge this tool with the overall vaccine development strategies.
Subunit protein/nanoparticle vaccines
Since the success of the HBV subunit protein vaccine in the 1980s, subunit vaccines have been considered to have enhanced safety but less effectiveness. To further improve the efficacy of subunit protein vaccines, different adjuvants have been developed and incorporated into immunization formulations, including AS02, MF59, and CpG.[21,22] Nanoparticle vaccines have been tested for many years since introduction of the uman papillomavirus vaccine, which uses virus-like nanoparticles assembled from different genotypes.[23,24] As the high immunogenicity of these virus-like particles is very attractive for vaccine development, researchers have tried to apply a similar strategy in the development of vaccines against membraned viruses, which are unable to assemble virus-like particles, such as SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV).[25,26] The auto-assembling capacity of ferritin into nanoparticles has been explored in vaccine development against membraned viruses. By fusing virus-derived immunogens with ferritin, the chimeric proteins could assemble into nanoparticles as an immunogenic vaccine. Adjuvant molecules may be further incorporated into the nanoparticles to enhance immunogenicity. A nanoparticle-based vaccine against SARS-CoV-2 would have enhanced safety and efficacy, but would likely involve a prolonged development process.
Viral/bacterial vectored vaccines
Vectored vaccines were introduced in the mid-1990s and have been extensively used in vaccine development in the United States and Europe. However, this area remains undeveloped in China for human vaccines, although the potential future use of this category vaccine in humans is likely. Use of vectored vaccines is the most effective approach to elicit vigorous T-cell responses, which confers the advantage of being capable of protect against a broader spectrum of pathogens[28,29]; thus, vectored vaccines have an indispensable role in development of a universal vaccine. In this regard, a universal vaccine against coronaviruses, such as SARS-CoV, MERS-CoV, and Bat-CoV, will probably include the vectored vaccine in a vaccination regimen.
Human-derived cell-based vaccines
The expression of hemagglutinin protein by influenza viruses, the envelope glycoprotein by human immunodeficiency virus-1, or spike protein by SARS-CoV on the virus membrane can elicit neutralizing antibody responses. As is known, the viral membrane is derived from the host cell membrane; therefore, expression of these proteins on human-derived cell membranes can induce neutralizing antibodies. By expressing spike protein on a human cell membrane, human-derived cells could be used in a vaccine. As these protein-expressing human cells do not contain other viral proteins, such vaccines would be safer than virus vaccines, which include the entire set of viral proteins. Indeed, a human cell-based vaccine could also be subjected to an inactivation process, such as via X-ray, heat, or chemical inactivation. Interestingly, X-ray-based inactivation mainly destroys the nuclear DNA and retains the structure of membrane protein, thereby protecting conformational neutralizing epitopes from damage and ensuring the vaccine's capacity to mount good neutralizing antibody responses.
Challenges in vaccine development
Although there is consensus that development of an effective vaccine is urgent, challenges exist in both research and development as well as policies aiming to establish precautionary, preparatory steps in the development and use of a vaccine against SARS-CoV-2.
It has been observed that rather than protecting host cells from a pathogen attack, antibodies may also mediate enhanced pathogenesis, a phenomenon known as antibody-dependent enhancement (ADE). ADE has been reported in SARS-associated viruses,[34,35] dengue viruses,[34,35] and hand-foot-mouth disease viruses. The underlying mechanism may originate from fragment crystallizable (Fc) receptor-mediated activation of macrophages. In addition to neutralizing antibodies blocking viral binding to receptors to protect host cells, non-neutralizing antibody binding to its epitope on the virus surface could direct the antigen–antibody complex to Fc receptor-positive cells (such as macrophages) via the antibody's Fc domain. This process will activate macrophages to produce inflammatory cytokines, resulting in immune pathogenesis. As macrophage cells are distributed all over the body, inflammation may occur in different organs and could cause organ failure. It is known that the interaction between T cells and macrophages could calm the inflammatory response; however, the exhaustion of T cells may enable macrophages to launch an enhanced inflammatory response. Indeed, one-third of patients with COVID-19 are characterized by T-cell exhaustion.[38–41] It has been noted that non-receptor binding domain (RBD)-specific antibodies represent the main resource for ADE; it would therefore be optimal to use RBDs as vaccine immunogen to reduce the potential for ADE. However, it remains unknown whether the RBD alone would be sufficient to elicit neutralizing antibodies.
Difficulty with a SARS-CoV-2 animal challenge model
An animal infection model is under development. ACE2 transgenic mice have been developed, but more time and expanded facilities are needed, problems that are unlikely to be resolved soon. In addition, according to regulations, this challenge should be implemented in an animal biosafety level 3 (BSL-3) facility, only a few of which are available. Moreover, live strains of SARS-CoV-2 have only been successfully isolated in a limited number of laboratories. Therefore, it is very difficult to bring together all the elements required for an animal challenge model. Similar challenges exist for a non-human primate model, which requires a large-animal BSL-3 facility. In the near future, competition for access to such animal facilities will be a great challenge in vaccine development. Alternative surrogates should be considered, such as neutralization in vitro assays, which could greatly augment vaccine development. The neutralization antibody titers against RBD and the ratio of antibody responses targeting RBD to the whole S protein may be considered as good surrogates.
Policy challenges to clinical trials
For safety considerations, vaccine development must go through a long, complex process, from early development to marketing of a vaccine. It is likely this will remain the case for development of a SARS-CoV-2 vaccine. How to speed up this process while maintaining safety will be an important issue to resolve. As proposed above, alternative surrogates for protection against SARS-CoV-2 should be included as part of the guidelines for vaccine development. In addition, if no severe side-effects are observed in primate immunization, a phase 0 trial could be approved, which may greatly encourage innovation and stimulate the field of new vaccine development.
The author wrote the manuscript and approved the final version of the manuscript.
Conflicts of interest
The author has no conflicts of interest to declare.
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