As the COVID-19 pandemic expands, interests in the progress of vaccine development are intensifying. Despite an unprecedented rate of progress, it is still uncertain when a safe, effective vaccine will be available for wide distribution to the public. Successful vaccine development goes through a series of stages, from animal studies for the evaluation of its protective immunogenicity to phase 1 (safety and antibody production), phase 2 (safety and immunogenicity by including a placebo group), and phase 3 (verification of safety, and efficacy in, a large population group) clinical trials. This is obviously a long, drawn out process, yet it is necessary to ensure the efficacy and safety of vaccines. In addition, it takes very high numbers of participants to generate meaningful and significant statistics to prove vaccine protection. This makes these trials expensive and their enrollment process lengthy, but phase 3 trials are clearly necessary, and are the most important step for its approval.
Immunogenicity studies are especially critical because there is not yet a clear understanding of what constitutes a protective immune response. Neutralizing or IgG antibody titers against the spike (S) protein do not seem to correlate inversely with disease severity, although it may be that rapid expression of such antibodies would be protective. Another issue is that neutralizing antibodies may only last a few months. However, immune memory cells may facilitate rapid production of such antibodies after infection. Even less is known about the role or relevance of T cell responses in protection. With all these caveats in mind, we will discuss nine candidate vaccines that are in the most advanced stages of development. Most, but not all, are focused exclusively on the S protein, in large part because it is the target of neutralizing antibodies.
There are currently two vaccines in the late stage of development that depend upon injection of mRNA encoding the spike protein or portions including the receptor binding domain (RBD) of the S protein. These have an advantage of being easy to produce but have the disadvantage of needing to be stored at 4°C, requiring a cold chain supply, thus presenting difficulties for use in low-income countries. These two vaccines are produced by Moderna and Pfizer/BioNTech. Its limitations associated with the intracellular instability and inefficient delivery of mRNA have been addressed by chemically modifying the RNA and encapsulating it in lipid nanoparticles.
The Moderna vaccine (mRNA-1273) and one of the two Pfizer vaccines (BNT162b2) encode prefusion conformation of the S proteins. The other Pfizer vaccine (BNT162b1) encodes trimerized soluble S protein receptor binding domains on a peptide linker scaffold. The Moderna vaccine was protective in rhesus macaques. Human phase 1 trial results were reported in June by demonstrating its safety and immunogenicity with induction of binding and neutralizing antibodies equivalent to the levels that are seen in natural infection. The antibody levels persisted until at least day 43 post-vaccination. A phase 2 trial with 600 participants was begun in June. Phase 3 trials to determine efficacy and safety were initiated in August and will have 30,000 participants.
Pfizer decided to concentrate on BNT162b2, as it is equally immunogenic to BNT162b1 but generates fewer side effects. There do not appear to be any reports of trials with non-human primates. Three phase 1 trials showed that both vaccines elicited binding and neutralizing antibodies, but lesser side effects led to the selection of BNT162b2 for phase 3 trials (Publication 1, Publication 2). Trials began in August and, as with the Moderna trials, aim to enroll 30,000 participants.
The other nucleic acid-based vaccine, developed by Inovio (INO-4800), is comprised of DNA encoding the S protein. The DNA vaccine, unlike the mRNA vaccines, is stable at room temperature. The DNA is injected intramuscularly and then electroporated into cells by a hand-held device delivering a brief electric pulse. The vaccine was partially protective in rhesus monkeys against a viral challenge three months after vaccination as judged by a reduction in viral titers. Inovio claims that antibody and/or T cell responses were induced after two doses of the vaccine in 94% of the 40 participants in Phase 1 trials, but they have not yet published the results. They are scheduling Phase 3 trials for September.
The Novavax vaccine candidate, NVX-CoV2373, is a full-length stabilized spike protein produced in insect cells and formulated into a lipid nanoparticle. Reports from a phase 1-2 trial showed that binding and neutralizing antibodies were elicited(1). Antibody levels were greatly increased, and T cell activities (especially Th1) were induced when NVX-CoV2373 was combined with a saponin-based adjuvant. Phase 3 trials are planned for late 2020.
There are currently three late stage vaccines that use adenoviral vectors to deliver their payloads to express the S protein. These include AstraZeneca/University of Oxford, which uses a chimpanzee adenovirus originally isolated from a chimp stool sample, Johnson and Johnson/Janssen (adeno26) and Cansino/Beijing Institute of Biotechnology (adeno5), the two latter of which are human adenoviruses. All three adenoviral vectors have been genetically modified to render them incapable of self-replication. The reasoning behind the use of a chimp adenovirus was to avoid the possibility that vaccinees previously infected by human adenoviruses would mount in an immune response against the vector, thus diminishing the efficacy of the vaccine.
The AstraZeneca vaccine, ChAdOx1 nCoV-19, was shown to partially protect rhesus macaques from viral challenge(2). Out of 6 vaccinated animals, none showed signs of pneumonia or lung pathology, while 3 of 6 controls developed interstitial pneumonia. The vaccine elicited binding and neutralizing antibodies against the S protein as well as Th1 and Th2 responses. Protection was not, however, sterilizing. Vaccinated animals had reduced viral loads in their lower respiratory tracts compared to controls, but viral loads in the nasopharynx were equivalent in both groups. A randomized phase1/2 trial with >1,000 subjects was injected with either ChAdOx1 nCoV-19 or the same vector with an unrelated antigen(3). The vaccine elicited binding and neutralizing anti-S antibodies as well as a T cell response without exhibiting serious adverse events. ChAdOx1 nCoV-19, currently in phase 3 trials, has recently been in the news because of a potential serious adverse reaction that temporarily halted the trials. A vaccinated women developed a severe spinal inflammation (transverse myelitis), which can occasionally develop following viral infections. She has since recovered, and it is not clear whether this is related to the vaccine. Trials have since resumed in Britain, but the Food and Drug Administration (FDA) has not yet approved resumption in the US. There are two ways to view this event. It could be considered to reflect the speed with which these vaccines are being developed and might be a cause for apprehension.
The Johnson and Johnson vaccine, Ad26.COV2.S, expresses a prefusion conformation of S protein (proline-stabilized S protein) in a human adeno 26 vector. In rhesus macaques, vaccinated animals developed high levels of binding and neutralizing anti-S protein antibodies and a Th1 biased T cell response(4). The authors suggested that neutralizing antibodies, but not cell-mediated immune activities, were correlative on protection. All 20 controls were infected and developed minimal disease after intratracheal and intranasal challenge. Five of six vaccinated animals were protected from detectable infection, and the sixth had a 3-4 log reduction in virus loads. Ad26.COV2.S is currently in phase 1/2 trials with 11,000 subjects that was started in June. Phase three trials are scheduled for September with 30,000 participants.
The CanSino vaccine, Ad5-S-nb2, contains a codon-optimized gene expressing the S protein. In rhesus macaques, a single dose elicited neutralizing and S protein binding antibodies and activated cell mediated immune responses after intramuscular inoculation(5). Intranasal inoculation induced antibody production but only weak cellular immunity. In an open label non-randomized trial, the vaccine was immunogenic in humans and generally well tolerated with the main adverse effect of being pain(6). A phase 2 trial with ~600 participants confirmed immunogenicity and safety(7).
There are three vaccines, developed by Sinovac, Beijing Institute of Biological Products, and Sinopharm, that are based upon chemically inactivated whole SARS-CoV-2. These vaccines, unlike the others, contain all the viral structural proteins, and thus, might be expected to induce a wider T cell response than the other vaccines, which contain only the S protein. The Sinovac candidate, Coronavac, elicited neutralizing and binding antibodies against the S protein(8). The highest vaccine dose protected animals completely against an intratracheal challenge, and lower doses prevented severe interstitial pneumonia and resulted in greatly reduced vial loads. In a phase 1/2 trial, Sinovac claimed that 90% of the volunteers developed neutralizing antibodies and had no serious adverse effects. There was no sign of antibody-dependent enhancement within the time frame reported. Sinovac initiated phase 3 trials in Indonesia and Brazil in August and is planning another trial in Bangladesh. Another inactivated virus vaccine (BBIBP-CorV), developed by the Beijing Institute of Biological Products, induced anti-S protein binding and neutralizing antibodies in rhesus macaques and cynomolgus monkeys and protected rhesus macaques from intratracheal challenge(9). BBIBP-CorV will soon be entering human trials. Two other similarly inactivated whole vaccines, produced by Sinopharm, induced neutralizing antibodies in phase 1 trials and had no serious adverse effects(10). Phase 3 trials with this vaccine were started in July in the UAE.
The speed of vaccine development with which this has happened is remarkable. The general take home message gleaned from an overview of these vaccines is that they induce neutralizing antibodies, stimulate T cell-mediated activity, and partially or completely protect non-human primates from infection and/or serious disease. None appear to cause an undue level of adverse events. The most pressing question is of course when one or more will be available. However, many uncertainties remain given the lack of robust clinical data. We still need to wait for finalization of phase III trials to confirm the safety and efficacy of the vaccine candidates. In particular, potential induction of antibody-dependent enhancement could be a concern. Immunogenicity of vaccine candidates are focused on the induction of neutralizing antibodies. Furthermore, they are mostly administrated by using the intramuscular route, thus limiting the induction of mucosal immunity. Intranasal immunization approach also needs to be considered. In addition, most vaccine candidates might require two doses (prime and boost vaccinations) to enhance their protective efficacy. For a global vaccination, this poses challenges financially and logistically. Therefore, we also need to consider the non-specific protective effects of live vaccines based on stimulation of innate immunity and trained innate immunity (i.e. epigenetic changes induced by live vaccines).
- C. Keech et al., Phase 1-2 Trial of a SARS-CoV-2 Recombinant Spike Protein Nanoparticle Vaccine. N Engl J Med, (2020).
- N. van Doremalen et al., ChAdOx1 nCoV-19 vaccine prevents SARS-CoV-2 pneumonia in rhesus macaques. Nature, (2020).
- P. M. Folegatti et al., 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. Lancet 396, 467-478 (2020).
- N. B. Mercado et al., Single-shot Ad26 vaccine protects against SARS-CoV-2 in rhesus macaques. Nature, (2020).
- L. Feng et al., An adenovirus-vectored COVID-19 vaccine confers protection from SARS-COV-2 challenge in rhesus macaques. Nat Commun 11, 4207 (2020).
- F. C. Zhu et al., Safety, tolerability, and immunogenicity of a recombinant adenovirus type-5 vectored COVID-19 vaccine: a dose-escalation, open-label, non-randomised, first-in-human trial. Lancet 395, 1845-1854 (2020).
- F. C. Zhu et al., 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. Lancet 396, 479-488 (2020).
- Q. Gao et al., Development of an inactivated vaccine candidate for SARS-CoV-2. Science 369, 77-81 (2020).
- H. Wang et al., Development of an Inactivated Vaccine Candidate, BBIBP-CorV, with Potent Protection against SARS-CoV-2. Cell 182, 713-721 e719 (2020).
- S. Xia et al., Effect of an Inactivated Vaccine Against SARS-CoV-2 on Safety and Immunogenicity Outcomes: Interim Analysis of 2 Randomized Clinical Trials. JAMA, (2020).