Understanding COVID-19 Vaccines and Their Development

By May 15, 2020, 65 days after the World Health Organization (WHO) declared the novel coronavirus disease 2019 (COVID-19) pandemic, 4.2 million individuals were confirmed as being infected with the causative agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and 294,000 people had died from the disease1. These numbers are almost certainly an underestimation of the true morbidity and mortality of the disease to this point. From the beginning of this public health crisis, attention has focused on the development of vaccines against SARS-CoV-2. Many believe that immunization is the key advance in the war against COVID-19 and that control of SARS-CoV-2 allowing approximations of pre-pandemic social conditions will not be possible without a viable vaccine2. Until a viable vaccine is developed, extensive diagnostic testing, quarantining, and social distancing are the only control methods that we have3. This article was written for musculoskeletal physicians and scientists without immunology backgrounds. It seeks to provide a concise but comprehensive understanding of vaccine development with a special emphasis on efforts to establish immunization directed against SARS-CoV-2. This will help to inform readers of the challenges to be hurdled and allow them to track milestones reached as the global community labors toward an effective vaccine. In-depth reviews of complex immunology topics, viral epidemiology, the myriad historic aspects of vaccine development, and the economics of developing and utilizing different types of vaccines are beyond the scope of this work. Background SARS-CoV-2 To understand the process of vaccine development, one must start with a basic understanding of the pathogen against which it will be directed. SARS-CoV-2 is a beta coronavirus4. The virus has been found to be similar to another coronavirus whose usual hosts are bats. It is believed that wild animals being sold at a market in Wuhan, Hubei Province, the People’s Republic of China, led to the transmission of SARS-CoV-2 to humans5. Members of the Coronaviridae family have a positive-sense, single-stranded RNA ([+]ssRNA) genome6. The (+)ssRNA can serve as messenger RNA (mRNA) in the host cell, allowing utilization of host ribosomal machinery to translate viral proteins. This coronavirus genome is highly conserved6. The first gene (open reading frame [ORF]1a and ORF1b) is involved in replication and transcription. Subsequent genes are related to structural proteins (Fig. 1). The spike protein (S) has an exposed location on the virion and is necessary for entry into the cell. Blocking of the spike protein epitopes that interact with angiotensin-converting enzyme 2 (ACE2), the host cellular receptor for SARS-CoV-2, should lead to neutralization (Fig. 2). ACE2 degrades angiotensin II (AngII), downregulating the renin-angiotensin-aldosterone system. ACE2 is distinct from the angiotensin type-I receptor, which leads to activating signal transduction by AngII7.Fig. 1: Illustration of the SARS-CoV-2 genome showing expressed structural component proteins and their location within the virion. The open reading frame (ORF) codes for nonstructural viral proteins participating in viral genome replication, transcription, and protein processing. The spike protein (S) binds to the ACE2 receptor, allowing virus entry into host cells. The envelope protein (E) has been found to bind to host gene regulatory proteins and putatively influences host cell gene expression. The membrane protein (M) seems to cooperate with S during binding and entry into the cell. The nucleocapsid protein (N) coils the viral mRNA inside the viral particle, organizing and protecting it.Fig. 2: SARS-CoV-2 virion entry into the host cell. The virus enters the host cell via the S protein interacting with ACE2. It is important to generate antibodies to the S protein rather than ACE2. This allows ACE2 to remain open to receive the host ligand and to avoid the possibility that the blocking antibody interferes with its normal physiologic function.COVID-19 is usually characterized by fever, cough, dyspnea, fatigue, and sore throat. Radiographic findings have demonstrated pneumonia with infiltrates on chest imaging8. Typically, radiographic changes appear on presentation. In Wuhan, 76% of diagnosed patients demonstrated changes on chest radiographs9. It is unknown whether radiographic changes arise prior to the onset of symptoms. Currently, there are no known curative treatments for COVID-19. Treatment is centered around supportive care while the afflicted individual clears the infection. In some cases, supplemental oxygen needs to be administered to help those with the associated pneumonia to maintain their oxygen saturation. Severe cases may benefit from antiviral medications, dexamethasone10, and ventilatory support with or without extracorporeal membrane oxygenation11,12. In March 2020, the WHO estimated the global mortality rate for those confirmed to have a SARS-CoV-2 infection to be 4.3%, although this varies by region13. Vaccines Vaccines meet the definition of a drug because they are substances other than food used in the prevention, diagnosis, alleviation, treatment, or cure of a disease14. They are administered to healthy individuals to prevent diseases caused by infectious agents to which they might be exposed in the future or, in some cases, to which they have been recently exposed. Immunization is the process of presenting antigens to a live host to induce an immune response. Vaccines can be utilized therapeutically and prophylactically. Most vaccines are developed as prophylaxis against infection. A primary goal of prophylaxis is to generate neutralizing antibodies that prevent viral entry into cells. However, in some cases, vaccines can be administered for short-term protection. This short-term protection is termed passive immunization. A smaller proportion of vaccines are developed for the therapeutic treatment of infections that have already become initiated. An example of this is the vaccine used after exposure to rabies15. Epidemiology of Infectious Disease and Vaccination During infectious disease outbreaks, vaccines have the ability to break chains of transmission, drastically slowing the propagation of infection and eventually eradicating the disease. The vast majority of individuals in modernized societies are immunized against a host of pathogens as children. This global public health effort has resulted in the worldwide elimination of smallpox16 and substantial progress toward the elimination of polio17. These accomplishments required concerted international efforts that were sustained over many decades. The epidemiologic dynamics of vaccination, especially in a population with a disease presence (either endemic or epidemic), can be described mathematically. The basic reproduction number (R0) is the intrinsic measure of transmissibility of a pathogen. It describes the mean number of secondary infections resulting from a single infection within a fully susceptible population (Fig. 3). For SARS-CoV-2, R0 has been estimated to be between 2 and 318.Fig. 3: Illustration of the basic reproduction number (R0) values for representative infectious diseases, demonstrating increasing infectivity with increasing values of R0. HIV = human immunodeficiency virus.Once substantial immunity, through either vaccination or natural exposure to the disease, starts to be established in the population, R0 becomes a less accurate measure of transmissibility, and a second number, the effective reproduction number (Re), becomes more representative of transmissibility. Re represents the actual number of transmissions per infection. For an explanation of how these numbers are calculated and their effect on viral epidemiology (Fig. 4), please see Appendix I.Fig. 4: The relationship between the basic reproduction number (R0) and the effective reproduction number (Re) of a pathogen. Re changes based on the percentage of the population that is immune, demonstrated by the different lines on the chart. For a disease with an R0 of 3.5, immunization of 70% of the population will be necessary to bring Re below 1.0. This can be compared with an immunization level of only 50% needed to bring Re below 1.0 for a disease with an R0 of 2.0. If Re, a measure of disease transmissibility, is <1, a disease outbreak will end without further measures being taken.Herd Immunity Herd immunity is a condition achieved in a population when the indirect protection of susceptible individuals against a specific pathogen is conferred by immune individuals. In effect, those with immunity within a population shield those without immunity from transmission, thereby limiting the spread of the disease19 (Fig. 5). A further explanation of how R0 and Re affect herd immunity (Figs. 6 and 7) is found in Appendix II.Fig. 5: Illustration demonstrating herd immunity. The spread of a hypothetical illness within a population is shown under 3 different conditions. The top row is a time early after the introduction of the pathogen within a population. The second row represents 1 replication cycle and the third row represents 2 replication cycles following the time represented in the top row. In the left column, the population is fully susceptible (no naturally acquired immunity or immunity through vaccination) to the pathogen. The middle column represents an intermediate state between full susceptibility and achievement of the herd immunity threshold (Pcrit). The column on the right represents how spread is hindered in a population that has already achieved Pcrit.Fig. 6: The relationship between the basic reproduction number of a virus, R0, and the proportion of the population that needs to be immunized to enter into a state in which herd immunity takes effect, Pcrit. R0 is unitless and Pcrit is measured as the percentage of the population. The slope of the curve is greatest between R0 values of 1 and 4, indicating that a large increase in the proportion of immune individuals within the population is needed to achieve Pcrit for an infectious disease with an R0 that is just slightly larger than another. The estimated R0 for SARS-CoV-2 is between 2 and 3, which is demarcated in the area between the dotted lines on the graph. This R0 range indicates that the percentage of the population that needs to become immune to reach Pcrit, either through infection or vaccination, is between 50% and 66.7%.Fig. 7: Vaccination affects the achievement of herd immunity. This graph shows vaccination affecting herd immunity to diphtheria. Diphtheria-tetanus-pertussis (DTP3) immunization coverage is expressed as a percentage of the population from 1982 to 2016. The red line on this graph represents the number of diphtheria cases reported worldwide by year. The blue line represents the immunization coverage as a percentage of the population by year. The dashed orange line represents the herd immunity threshold (Pcrit) for diphtheria assuming that it has an R0 of 6. The graph demonstrates how achieving immunization levels just slightly above Pcrit can drive the number of cases down to just a few thousand per year.SARS-CoV-2 Vaccines Under Development It is through recombinant nucleic acid technology that most efforts against COVID-19 are progressing. Theoretically, vaccines using recombinant nucleic acid technology can be developed and, therefore, deployed more quickly than those using more traditional means. However, a few are based on immunologic techniques used widely in the mid-twentieth century for the development of vaccines used in standard childhood immunization schedules. Table I summarizes the types of vaccines as well as their advantages and disadvantages. By June 2, 2020, >130 candidate vaccines targeting SARS-CoV-2 were under development worldwide according to the WHO20. Ten of these candidate vaccines have entered into human trials. Table II summarizes the SARS-CoV-2 vaccines currently in development. TABLE I - Advantages and Disadvantages of Different Vaccine Types* Type of Vaccine Example Vaccine(s) Advantages Disadvantages Potential Solutions to Development Problems Attenuated Sabin polio (host-range mutant) Smallpox (Jennerian method) Reliable Confer both humoral and cell-mediated immunity Limited need for booster doses Relatively inexpensive technology May yield less protective mutant viruses if passages are performed in non-human cell lines63 Can cause disease in immunocompromised individuals Enhanced storage and maintenance requirements (e.g., refrigeration, culture media) Inactivated Typhoid Salk polio Rabies Influenza Relatively fast Easy to scale up Relatively inexpensive development Alternate routes of administration (e.g., oral) Inactivation can sometimes damage key epitopes Can still theoretically cause disease (if not fully inactivated) Labor-intensive process to ensure no viable virion remains after inactivation Greater amounts of inoculum required to achieve immunity Immune response is less durable, requiring booster doses64 Titration of inactivation methods to prevent overtreatment, maintaining key epitopes Subunit Hepatitis B surface antigen Safe Easy to use Requires large amounts of isolated antigen Immunopotentiation2,65 Recombinant technology has made large-scale production possible Adjuvants can be added to vaccine preparations66 mRNA29 HIV† Zika† Influenza† Rabies† Can replicate using host machinery Highly potent Relatively fast Easy to scale up • Relative low cost of manufacturing Safe Confer both humoral and cell-mediated immunity dsRNA byproducts of production are PAMPs‡ and lead to immune and to of mRNA avoid host immune to dsRNA methods to of and of mRNA Can replicate using host machinery Highly potent Relatively fast Easy to scale up • Safe Confer both humoral and cell-mediated immunity to treatments (e.g., to entry into human and immune recombinant (e.g., HIV† Confer both humoral and cell-mediated immunity host via expressed cell surface Can for Highly potent Relatively fast Easy to scale up in Safe routes of administration production (e.g., immunity to host of If viral are cause infection via may can neutralizing antibodies to thereby different to to immunity to highly of viral can = human immunodeficiency virus, dsRNA = = and = are and in the pathogen but not in the host allow early and immune by the host TABLE II - SARS-CoV-2 Vaccines in Vaccine Type of Development mRNA 2: to 2: to Subunit spike 1 and 2: to mRNA 1 and 2: to June Inactivated with 1 and 2: to Inactivated 1 and 2: to Inactivated 1 and 2: to 1: to Inactivated 1 of June 2, Attenuated Vaccines Vaccines that use a but viable of the pathogen to establish an immune response are termed The of is an rather than a the of which is by the scale of the process and the time necessary to the of the virus in the cell In cases, the can to a resulting in the of the disease in the immunized are different types of 3 different are to vaccines against The most of an vaccine is the host range This is usually achieved by the of the virus in the cell culture to generate the host range mutant it while the necessary antigens to generate immunity. The second is naturally This and of or during of The third of vaccine is the This that at of that can be at is the found at the respiratory but become at the or The of termed is not being in the COVID-19 vaccine development Inactivated Vaccine vaccine large amounts of viral to be in and by either or (Fig. The of inactivation is to the virus without the epitopes to which an immune response is and can virus A needs to be achieved in the of will lead to and of key to a neutralizing antibody response that is and protective with a live However, if inactivation not of the disease can from the vaccination through some vaccine types that are being developed against SARS-CoV-2. Inactivated viral are in and through or means. The of viral allows the of an immune response without of infection. Subunit viral proteins are and These proteins are into patients to be The proteins are by the host and the antigens are that an antibody response is mRNA mRNA the viral S protein is in the and is using technology as These mRNA are and are into viral proteins using host and the antigens are that an antibody response is for the viral S protein is into a This enters host and machinery viral proteins. These viral proteins generate an antibody response. the gene for the viral S protein is into an genome in These with entry into the host cell and the host machinery to viral proteins that generate an antibody Vaccines Subunit vaccines generate immunity by administration of a virus antigen (Fig. These antigens are by the immune to generate an antibody response. Typically, antigens are that will generate an immune response. For SARS-CoV-2, one of the candidate antigens for vaccine development is the spike A number of the of and are on developing spike protein mRNA Vaccines In this mRNA for the SARS-CoV-2 spike protein is into the host to it (Fig. proteins made by host will be as by the immune which will generate both an antibody and the response to mRNA vaccines have had that effective but have of these mRNA vaccine and the of the and methods are standard at this 1 developing an mRNA vaccine against SARS-CoV-2 has that it be as an measure to as early as Vaccine the of the of SARS-CoV-2, the development of a vaccine the (+)ssRNA genome of SARS-CoV-2, can be These can be into or that for antigen (Fig. can replicate using host is into host it can viral antigens that are as the host immune to generate an antibody and the response. This of vaccine development advantages similar to those for mRNA vaccines nucleic are that the with protein which can recombinant have demonstrated that can enter host and can in a immune This technology has been shown to be and in to there have been no vaccines for use in Recombinant Vaccines Recombinant viral have been as vaccine over the few for their ability to antigens and their (Fig. are of recombinant vaccines is that they can be made and highly effective through recombinant The of recombinant vaccines is by the virion replication This is achieved by the antigen in the of the genome for viral replication, thereby its ability to most vaccines are administered either or vaccines provide the possibility of This is a more administration than For these many different recombinant vaccines targeting SARS-CoV-2 are currently under development. Immunization immunization may be achieved by administration of to a or recently infected protection against pathogens within the immunized The protective of the are eventually down or this the protective effect is because not or the immune to provide immunization. are with and patients the at a of infection. Currently, there are a number of for the treatment of patients with Immunity to COVID-19 and should a to a vaccine against SARS-CoV-2. health measures will need to the development, and administration of a vaccine against SARS-CoV-2 to a large proportion of the population. with these morbidity and mortality will The rate increase from to between and of the population while the the herd immunity threshold (Pcrit) may be to A number of immunity, immunity, and vaccine the of a COVID-19 vaccine TABLE - and the of COVID-19 Vaccine Immunization on COVID-19 Vaccine Immunization immunity are of Immunity in individuals may vaccination individuals may not be fully immune Vaccine is only effective May vaccination or vaccination of more individuals than Pcrit Vaccine or use of vaccines to achieve Pcrit for development for vaccine development and is the development can only if necessary the first Vaccine takes to similar efforts have of vaccine development, we still not have a vaccine Vaccine takes to techniques more effective of recombinant Development Vaccines or manufacturing of vaccine development to from and worldwide on vaccine Development Vaccine to by the is especially A by the for that of not and were if they when a vaccine becomes A of found that were at and an were in a COVID-19 is an that Pcrit or lead to the of COVID-19 The COVID-19 vaccine will not be the This was with polio and both of which through in the and types that were used when or were in the early The vaccines which to they are administered and toward and their methods will with limiting vaccine is immunity. illness are of the This that SARS-CoV-2 may exposed individuals. may be to either antibody and cell or The need for vaccination the of Pcrit. possibility is that only some individuals have full immunity. immunity is still it the numbers of infected individuals and those requiring care on the level of an care The of developed vaccines will most be by of blocking Vaccines that have entered into have already been for blocking antibodies in their The administration of different vaccines has been in achieving immunity. and polio vaccines administered have a for viruses with substantial as help to neutralizing antibody for is a complex of and on immune In antibody to vaccines are in while both humoral immune to novel that are related to affect the time for immunity to a is 2 to 3 following the Problems that are of in developing a vaccine to RNA viruses are the of and is the of the viral disease rather than its This has led to and in immunized has been with a number of those against is the of infection. of infection when viruses are by via their or This has been shown to in human with It is not that vaccine administration will have a musculoskeletal However, the time to testing, and will have for the and of care It is standard methods or will be during the production and of viable as many of the currently being used have resulted in doses at that vaccines and their will be to more and effective and administration to the Immunization with candidate vaccines should not lead to should be to allow the of an immune response and of severe morbidity and mortality associated with The for the development of effective vaccines should be with the that the to achieve will be will be many and we must to this The protective measures that we have social protective and the use of testing, will be a of COVID-19 has morbidity and Pcrit will be in to a of social Pcrit will be through immunization. an early and response toward developing a COVID-19 remain in Recombinant techniques may this A between the community and should the need for vaccine development with Appendix by the is with the of this article as a at