Vaccines are being hailed as the solution to the COVID-19 pandemic, but the vaccine trials currently underway are not designed to tell us if they will save lives, reports Peter Doshi, Associate Editor at The BMJ today.
Several COVID-19 vaccine trials are now in their most advanced (phase 3) stage, but what will it mean exactly when a vaccine is declared “effective”?
Many may assume that successful phase 3 studies will mean we have a proven way of keeping people from getting very sick and dying from COVID-19. And a robust way to interrupt viral transmission.
Yet the current phase 3 trials are not actually set up to prove either, says Doshi.
“None of the trials currently underway are designed to detect a reduction in any serious outcome such as hospitalisations, intensive care use, or deaths. Nor are the vaccines being studied to determine whether they can interrupt transmission of the virus,” he writes.
He explains that all ongoing phase 3 trials for which details have been released are evaluating mild, not severe, disease – and they will be able to report final results once around 150 participants develop symptoms.
In Pfizer and Moderna’s trials, for example, individuals with only a cough and positive lab test would bring those trials one event closer to their completion.
Yet Doshi argues that vaccine manufacturers have done little to dispel the notion that severe COVID-19 was what was being assessed.
Moderna, for example, called hospitalisations a “key secondary endpoint” in statements to the media.
But Tal Zaks, Chief Medical Officer at Moderna, told The BMJ that their trial lacks adequate statistical power to assess that endpoint.
Part of the reason may be numbers, says Doshi. Because most people with symptomatic COVID-19 infections experience only mild symptoms, even trials involving 30,000 or more patients would turn up relatively few cases of severe disease.
“Hospitalisations and deaths from COVID-19 are simply too uncommon in the population being studied for an effective vaccine to demonstrate statistically significant differences in a trial of 30,000 people,” he adds.
“The same is true regarding whether it can save lives or prevent transmission: the trials are not designed to find out.”
Zaks confirms that Moderna’s trial will not demonstrate prevention of hospitalisation because the size and duration of the trial would need to be vastly increased to collect the necessary data. “Neither of these I think are acceptable in the current public need for knowing expeditiously that a vaccine works,” he told The BMJ.
Moderna’s trial is designed to find out if the vaccine can prevent COVID-19 disease, says Zaks. Like Pfizer and Johnson and Johnson, Moderna has designed its study to detect a relative risk reduction of at least 30% in participants developing lab-confirmed COVID-19, consistent with FDA and international guidance.
Zaks also points to influenza vaccines, saying they protect against severe disease better than mild disease.
“To Moderna, it’s the same for COVID-19: if their vaccine is shown to reduce symptomatic COVID-19, they will feel confident it also protects against serious outcomes,” Doshi writes.
But Doshi raises another important issue – that few or perhaps none of the current vaccine trials appear to be designed to find out whether there is a benefit in the elderly, despite their obvious vulnerability to COVID-19.
If the frail elderly are not enrolled into vaccine trials in sufficient numbers to determine whether there is a reduction in cases in this population, “there can be little basis for assuming any benefit against hospitalisation or mortality,” he warns.
Doshi says that we still have time to advocate for changes to ensure the ongoing trials address the questions that most need answering.
For example, why children, immunocompromised people, and pregnant women have largely been excluded; whether the right primary endpoint has been chosen; whether safety is being adequately evaluated; and whether gaps in our understanding of how our immune system responds to COVID-19 are being addressed.
“The COVID-19 vaccine trials may not have been designed with our input, but it is not too late to have our say and adjust their course. With stakes this high, we need all eyes on deck,” he argues.
To gain a better understanding of the clinical data, it is important to understand concepts in vaccine immunology. There is no “one size fits all” protective antiviral immune response.
Every virus is different with different routes of infection, different range of infectable cell types, and different associated pathology. Accordingly, the immune response best suited for protection against each virus will also be variable.24 Other factors such as sex, age, pregnancy, and route of infection can also influence the immune response.24,25
It is widely reported that some people become heavily infected with SARS-CoV-2, but remain asymptomatic, and that some become critically ill and succumb to the disease. This extreme variability in response to infection underscores the variability of individual immune responses to this virus, suggesting that there may not be a single perfect strategy that will achieve uniform long-lasting immunity in everyone.
The specific immune responses that elicit the most rapid and dependable viral clearance need to be understood and replicated by the vaccines. Major unanswered questions are whether humoral and/or cellular cytotoxic responses are required, what types of helper T cells are most effective (e.g., Th1 vs Th2 vs Th17) as well as what isotype of antibody response (e.g. IgG vs IgA) most effectively protects against this virus.26−28
Most of these questions are being answered through laboratory studies as well as through analysis of serum and circulating cells from recovered patients.
Given the variability of host immune responses, there is unfortunately no guarantee that a vaccine, even if it has progressed into advanced clinical trials, will protect against SARS-CoV-2. While a single vaccination can confer lifelong protection against small pox29 or poliovirus,30
HIV continues to evade protection by vaccination despite a major worldwide effort to develop an effective HIV vaccine.31 Additionally, there are indications that respiratory viruses are especially difficult to protect against with vaccines.
The respiratory syncytial virus is a prime example in which there are no approved vaccines, despite considerable efforts to develop one.32
One reason for vaccine failure against respiratory viruses is that the respiratory tract, including the lungs, is an external mucosal surface that is protected by the generation of secreted IgA antibodies; yet, the antibodies measured to determine whether an experimental subject has “responded” to a vaccine often focus on IgG, IgM, or total immunoglobulin in the blood.33,34
Most vaccines are delivered intramuscularly, and mucosal immunity and IgA secretion is thereby minimal.35 Furthermore, eliciting IgA production from conventional vaccines is difficult, and vaccines may lack the immunogenicity required to elicit necessary IgA protection.36
Regardless, there are efforts and reports on development of SARS-CoV-2 vaccine candidates that can elicit IgA responses (see Table S1). For instance, Altimmune, an adenovirus (Ad)-based nonreplicating viral vector vaccine administered intranasally, showed 29-fold IgA induction in mice.37
Other companies such as Stabilitech Biopharma Limited and Quadram Institute Biosciences are also developing mucosal vaccines.38,39 The value of IgA or other immunoglobulin isotypes in protection against SARS-CoV-2 has not been fully elucidated, but it is believed that IgA can prevent SARS-CoV-2 binding to the airway epithelium thereby helping to block both initial infection and subsequent transmission.34,37
It is important to note that it is not known what role, if any, IgA plays in protection against SARS-CoV-2 and that many of the current vaccines are not specifically looking to activate IgA responses. Of course, it is possible that IgA production will not be important for an effective vaccine, or may even be harmful, as IgA production was negatively correlated to increased severity in COVID-19 patients.40
SARS-CoV-2 is unusual for a respiratory virus in that it binds to a receptor, angiotensin converting enzyme 2 (ACE2), expressed in virtually all organs,40 but especially in the lungs,41 brain,42 and gut.43 Therefore, unlike most respiratory viruses, SARS-CoV-2 has broader biodistribution and can cause considerable damage outside the respiratory system.
It adversely affects the digestive, urogenital, central nervous, and circulatory systems, and the pervasiveness of the ACE2 receptor is why symptoms are highly variable and can range among dyspnea, diarrhea, headache, high blood pressure, venous thromboembolism, and more.40
Therefore, since much of the pathology is outside the airway due to systemic viral infection, a vaccine that elicits IgG antibodies could protect patients from systemic circulation of the virus. IgG antibodies opsonize the targeted antigens presenting the opsonized products to phagocytes while also activating the complement system.44
Another hallmark of vaccine development is T-cell involvement, and differences in T-cell responses can influence generation of high affinity and neutralizing antibodies (NAbs) as well as elimination of infected cells.45 Immune memory and generation of high affinity class-switched antibodies are highly dependent on T-cells and normally do not develop without proper T-cell involvement.46,47
Immune memory is the main driver of long-term immunoprotection, and studies have shown that immune titers from patients infected with the first SARS-CoV can have significant antibody levels for up to 3 years postinfection.48 Such antibody maintenance would be extremely beneficial in the fight against SARS-CoV-2, and this prolonged immune memory could potentially confer long-term protection by a vaccine. However, it is likely that periodic booster vaccination will be necessary in areas of rebounding cases as is done for other infectious diseases.
It is currently unclear whether any of the tested vaccines will confer protection against SARS-CoV-2. Fortunately, as noted below, there are encouraging early results from multiple vaccines that are safe and immunogenic in limited patients. This early success warrants the progression into Phase III clinical trials, and expectations are that 20,000–40,000 subjects would be involved.
There is a daunting task left in the effort to develop effective vaccine(s) against SARS-CoV-2, and no guarantee of success, but we are encouraged by the early testing success and rapid development of so many candidate vaccines.
Nanotechnology Offers Opportunities in Vaccine Design
Nanoparticles and viruses operate at the same size scale; therefore, nanoparticles have an ability to enter cells to enable expression of antigens from delivered nucleic acids (mRNA and DNA vaccines) and/or directly target immune cells for delivery of antigens (subunit vaccines).
Many vaccine technologies employ these direct benefits by encapsulating genomic material or protein/peptide antigens in nanoparticles such as lipid nanoparticles (LNPs) or other viruses such as Ads. BioNTech/Pfizer and Moderna encapsulate their mRNA vaccines within LNPs while the University of Oxford/Astrazeneca (from here on out referred to as Oxford/Astrazeneca) and CanSino incorporate antigen-encoding sequences within the DNA carried by Ads.17,19,22,23 Novavax decorates recombinant S proteins of SARS-CoV-2 onto their proprietary virus like particle (VLP) nanoparticles.49 The nanoparticles are described in further detail in the discussion below.
Beyond antigen delivery, nanoparticles can codeliver adjuvants to help prime the desired immune responses. Adjuvants are immunostimulatory molecules administered together with the vaccine to help boost immune responses mainly by activating additional molecular receptors that predominantly recognize pathogens or danger signals.
These pathways function primarily within the innate immune system, and each adjuvant generally has a different range of stimulation of these pathogen or danger receptors. While the vaccine goal is to stimulate recognition and response by lymphocytes, not innate cells, the activation of the innate immune cells is required to activate the lymphocytes to obtain both B and T-cell responses.50,51
Encapsulation and/or conjugation of both the adjuvant and antigen within the same nanoparticle enables targeted, synchronous delivery to the same antigen presenting cell (APC). Many adjuvants have previously failed in the clinic due to toxicity issues, and this codelivery can help to direct antigen and adjuvant activity only in APCs that have taken up the antigen thereby reducing off-target side effects.52
Targeted delivery of appropriate adjuvants can also reduce the necessary antigen dose for immune protection thereby producing a dose-sparing effect.52 This effect would be abundantly helpful practically and financially in the current pandemic due to the enormous number of doses needed for global vaccination.
Furthermore, when adjuvants and antigens are not codelivered they may dissociate quickly within the body, which may lead to off-target effects and/or rapid degradation of the adjuvant reducing the potency of the vaccine.53 Both Moderna and BioNTech encapsulate their mRNA vaccines within LNPs to protect the mRNA from nuclease degradation.17,19
Loss of temporal synchronization, i.e., uptake of the antigen and adjuvant by APCs at separate times, can also lead to autoimmunity against host proteins, as the adjuvant can activate APCs that are not primed against the antigen but rather primed against self-antigens.52
Therefore, nanotechnology offers an opportunity in vaccine design, and there are several strategies that enable codelivery of SARS-CoV-2 antigens and adjuvants.
The three main methods are
- (i) codelivery through encapsulation within or conjugation onto a nanoparticle,
- (ii) direct antigen-adjuvant conjugation, and
- (iii) utilizing the delivery vehicle as an adjuvant.53,54
Another benefit that nanoparticles can confer is multivalent antigen presentation and orientation of subunit antigens in their native form.55 For example, BioNTech/Pfizer, one of the frontrunner companies producing a SARS-CoV-2 vaccine, formulates their receptor binding domain (RBD) antigens onto a T4 fibritin-derived “foldon” trimerization base to better resemble the trimeric form of the spike (S) protein of SARS-CoV-2.19,56
Furthermore, display of different RBD epitopes of influenza A on multivalent ferritin nanoparticles can increase production of cross-reactive B-cells against influenza A, and produce a more diverse and effective antibody response than ferritin nanoparticles with homotypic RBD display.57
The study also found that the multivalent, heterotypic nanoparticles induced a broadly NAb response, which makes the generation of an all-encompassing, universal influenza A vaccine possible.
Lastly, due to the “nano” scale of nanomaterials as well as their composition, they can traffic in vivo differently from other materials. The lymphatic system is critical in initiating immune responses as APCs, and other lymphocytes travel from peripheral organs to nearby lymph nodes using the lymphatic system.58
Accessing the lymphatic system can be challenging, but nanomaterials can traverse the interstitial spaces and access nearby lymph nodes. For instance, inhaled radiolabeled solid lipid nanoparticles were shown to traffic from the alveoli into nearby lymph nodes via the lymphatic system, while the free radiotracers trafficked via the systemic circulation.59
Lymphatic drainage especially into lymph nodes near the lungs could be extremely beneficial in the fight against respiratory diseases such as SARS-CoV-2. Companies such as etheRNA and Intravacc are developing intranasally delivered vaccines delivered into the respiratory system that may target such nearby lymph nodes.60,61
For further reading on the opportunities of nanotechnology in SARS-CoV-2 vaccine design, we would like to refer the reader to the following review.62 This review also discusses challenges and opportunities of the manufacturing processes and delivery platforms that are necessary for global vaccination.
The Landscape of COVID-19 Vaccine Candidates
According to the WHO and the Milken Institute, as of August 11, 2020, there are 202 companies and universities worldwide working on a coronavirus vaccine (Table S1).7,63 The vaccine types vary from well-established vaccines (e.g., inactivated and live-attenuated) to vaccines that have recently gained clinical approval (e.g., subunit) to those that have not yet made the transition into the clinic (e.g., mRNA, DNA, nonreplicating viral vector, replicating viral vector) (Figure 1).
Inactivated vaccines are similar to the native pathogen but are replication deficient due to chemical or heat treatment.64 Live-attenuated vaccines are weakened forms of the virus that can replicate in a limited manner unable to cause the actual disease,65 and subunit vaccines confer immunoprotection using portions of the virus.66
Subunit vaccines are usually less immunogenic and require an adjuvant to stimulate the immune recognition of the antigens in the vaccine. Nucleic acid based vaccines can be mRNA or DNA based, and rather than directly injecting the antigen, express it within host cells using the genomic material.67
Lastly, viral vector vaccines contain engineered genomes to encode the antigen of the target pathogen. When administered in vivo, the viral vectors enter target cells and the genomic material is transcribed and translated for in vivo antigen production.68 They can, but do not always, possess the ability to replicate within the host.
Replication deficient vaccines may impart better safety, but immune memory is not as long-lasting. Currently, there are no viral vector vaccines used in the clinic for humans, but there are some that have been utilized for veterinary applications.69 Of the 202 companies, only a select few have advanced into clinical trials; as of August 11, 2020, the WHO indicates there are 29 vaccine candidates in clinical trials (FigureFigure22a,b).7
Of these select few vaccines, even fewer have released data of their initial safety and immunogenicity from completed Phase I and II studies (Table 1).
It is noteworthy that every company that has released data has reported positive results from their early Phase clinical trials, allowing advancement into wider and more broadly encompassing efficacy studies.
Table 1 – Summaries of Clinical Trials That Have Been Completed by Companies in the Vaccination Effort Against SARS-CoV-2a
In this review, we discuss the vaccine nanotechnologies employed by the companies that have released their results in publication form. Two of the companies that have released early results are Moderna and the BioNTech/Pfizer partnership. While Moderna has already published their data in the New England Journal of Medicine, BioNTech/Pfizer’s data is currently prepublished in medRxiv and is awaiting peer review.17,19
Both employ similar techniques for their vaccine, utilizing mRNA that encodes for subunits of the SARS-CoV-2 S protein (Table 2). On the other hand, Oxford/Astrazeneca and CanSino are both developing vaccines based on nonreplicating viral vectors.7,22,23
Descriptions of Vaccines That Have Moved Beyond Their Initial Safety and Immunogenicity Phase I Studiesa
In the clinic, both viral vector vaccines and mRNA vaccines have enjoyed variable successes with neither vaccine type currently approved for a specific use. mRNA vaccines and viral vector vaccines both depend on nucleic acids that encode the target antigen(s) but differ in their approach to vaccination.
Instead of mRNA, viral vectors use DNA to encode the antigen of interest.
Viral vector vaccines can impart high gene transduction capabilities due to their ability to enter into cells using the virus’ own receptor for infection, and efficient intracellular trafficking enables high production of target gene expression.70 However, immunogenicity of the viral vectors and other adverse effects bear hurdles to safe use.
The immunogenicity of the viral vector may decrease vaccine efficiency caused by NAbs against the viral vector in patients that either are developed during the course of vaccination or are pre-existing due to previous exposure to the Ad vectors they use.70
Viral vectors that humans are not commonly exposed to such as the chimpanzee Ad utilized by Oxford/Pfizer can reduce neutralization.23 Another safety concern for viral vectors is possible host genome integration, which may cause cancer if integrated into oncogenes and other regulatory sequences.70
mRNA vaccines are generally encapsulated within nanoparticles. Both BioNTech/Pfizer and Moderna encapsulate their RNA vaccines within LNPs, which may enable cytoplasmic delivery via fusogenic mechanisms.17,19 However, neither Moderna nor BioNTech/Pfizer specifically mention the use of fusogenic LNPs although BioNTech/Pfizer does mention using cationic lipids.71
Cytoplasmic delivery may improve translation efficiency, but it may also decrease RNA immunostimulation. RNA stimulates the immune system, and therefore acts as an adjuvant, by activating specific toll like receptors (TLRs), mainly TLRs 3, 7, and 8, which are all located within the cell’s endosomes.72 TLRs 7 and 8 are especially important for mRNA vaccines as they recognize single stranded RNA and engage in virus recognition.73
Encapsulation within nanoparticles improves RNA phagocytosis by APCs with subsequent localization within the endosomes. Failure of the RNA to be endocytosed can lead to nuclease degradation and weak immune stimulation.
While advances in nanoparticle design have enabled cytoplasmic delivery of mRNA, synthetic nanoparticles do not match the efficiency of the machinery evolved by the viral vectors that enables trafficking inside the cell. Once inside the cell, the mRNA is translated directly within the cytoplasm; in contrast, DNA plasmids from the viral vectors need to be translocated into the nucleus, transcribed, and exported back to the cytoplasm.74
This means that mRNA vaccines may produce greater amounts of antigen from smaller doses, but a caveat is that DNA tends to be more stable than mRNA meaning mRNA expression is generally shorter lived. The interplay between stability and translation efficiency can be a big determinant in effective antigen production.
Results–mRNA Vaccines: Moderna and BioNTech/Pfizer
As mentioned above, Moderna and BioNTech/Pfizer utilize mRNA vaccines that encode for the S protein of SARS-CoV-2. The S protein is the viral protein that binds to ACE2 on cells to mediate infection and is a frequent vaccine target since it is expected that antibodies binding to the correct epitope on the S protein could be neutralizing and therefore block intercellular viral spread.2
The S protein has two subsections: S1 and S2. The S1 subunit contains the RBD and is responsible for initial attachment to the host cell through the ACE2 receptor, while the S2 subunit promotes viral fusion with cells to initiate infection.2 Moderna’s vaccine, mRNA-1273, was codeveloped with the National Institute of Allergy and Infectious Diseases and specifically encodes the prefusion form of the S antigen (named S-2P) that includes a transmembrane anchor and an intact S1–S2 cleavage site.17
Two proline substitutions in the vaccine mRNA at amino acids 986 and 987, which are within the central helix of the S2 subunit, keep the protein stable in its prefusion conformation.56 The mRNA is encapsulated within an LNP composed of four lipids (Table 2).
The exact formulation is not provided; however, inferences can be made based on previous LNP vaccines by Moderna, which utilize formulations of ionizable lipid, 1,2-distearoyl-sn-glycero-3-phosphocholine, cholesterol, and polyethylene glycol-lipid.100
The exact lipids used are not stated. The mRNA-containing LNPs are solubilized and injected directly into the deltoid muscle. Moderna does not explicitly state the use of an adjuvant, but the LNP carrier may be an adjuvant since other lipids have been reported to have adjuvant properties (see the Discussion).101
The mRNA utilized by BioNTech/Pfizer encodes for the RBD.19 Named BNT162b1, the mRNA is modified with single nucleoside incorporations of 1-methylpseudouridine (Table 2), which not only reduces the immunogenicity of the mRNA in vivo but also increases its translation.102
The exact mechanism for increased translation has not been entirely elucidated, but one hypothesis is that the nucleoside modification improves RNA stability by decreasing rates of hydrolysis by phosphodiesterases.103 Another study proved that RNA modification improves RNA secondary structure stability due to increased RNA stacking.104
Additionally, as mentioned above, the formulated RBD antigen is constructed on a T4 fibritin-derived “foldon” trimerization base, which helps to guide antigen folding into the native trimeric state.56,105 The T4 trimerization also augments the immunogenicity mainly due to the multivalent display offered in the trimerized state.19 It is important to note that BioNTech/Pfizer is testing at least four mRNA vaccines in parallel (BNT162a1, BNT162b1, BNT162b2, and BNT162c2) although the prepublished manuscript only contains data from the BNT162b1 candidate.77
Similar to Moderna, BioNTech/Pfizer has also encapsulated the mRNA within an LNP and administers the vaccine via intramuscular injection.76 The LNP is provided from a partnership with Acuitas Therapeutics. The exact formulation is not specified, but previous publications from Acuitas Therapeutics states that their LNPs are formulated using ionizable cationic lipids, phosphatidylcholine, cholesterol, and polyethylene glycol-lipid with a ratio of 0.05 of RNA to lipid (w/w).
Similarly to Moderna, the exact lipids used were not stated.71 There is no indication that BioNTech/Pfizer utilizes an additional adjuvant with their vaccine although they do mention that the RNA acts as an adjuvant.
Both BioNTech/Pfizer and Moderna released encouraging safety and immunogenicity data.17,19 Moderna tested a higher range of mRNA (25, 100, and 250 μg), while BioNTech/Pfizer tested 10, 30, and 100 μg.
Safety evaluations noted no severe adverse events that warranted the discontinuation of either trial. Some of the more prominent adverse events in the Moderna trial included pain, headache, and chills, while BioNTech/Pfizer’s vaccine mainly caused pain, fatigue, and headache (Table 1).
Antibody response was also positive in both trials. Moderna tested antibody response through ELISA assays while BioNTech/Pfizer utilized a RBD-binding IgG assay.17,19 For Moderna, when comparing the response in vaccinated patients to convalescent serum from past SARS-CoV-2 patients, the 250 μg group generated higher S-2P geometric mean titers (GMTs) by day 15 (163,449 vs 142,140 arbitrary units (AU)), while the 25 and 100 μg groups produced higher GMTs by day 36 (391,018 and 781,399 AU, respectively), 7 days after a second boost.
BioNTech/Pfizer recorded neutralizing anti-RBD titers much higher than convalescent serum levels. By day 21 (day of the second dose, or 21 days), the 30 μg group had a higher geometric mean concentration (GMC) than convalescent sera (1,536 vs 602 U/mL), while it took until day 28 (7 days after a second dose) for the 10 μg group (4,813 U/mL).
The 100 μg group, which only used one dose, had higher GMC levels by day 21 (1,778 U/mL). Both Moderna and BioNTech/Pfizer tested T-cell responses and demonstrated TH1 skewed T-cell responses with detectable CD4+ and CD8+ response to their respective antigens.17,20 Neither developer mentioned the production of antibodies other than IgG. It is difficult to directly compare the results between the trials because measurements and data reporting are not standardized, highlighting an opportunity and need to standardize vaccine trials and reporting requirements.
The vaccination schedule in the Phase III trials by both Moderna and BioNTech/Pfizer will not deviate from their Phase II setups.106,107 Moderna will continue to boost on day 29 after an initial injection, and BioNTech/Pfizer will boost at day 21. However, Phase III trials will only evaluate one dose.
In Moderna’s case, the midlevel dose led to higher immunogenicity than the highest dose while BioNTech/Pfizer demonstrated no substantial differences between their mid- and high-level doses.17,19 Therefore, Moderna and BioNTech/Pfizer both chose to move forward with their midlevel doses (100 μg and 30 μg, respectively). For phase III, Moderna and BioNTech/Pfizer will also vaccinate much larger populations of 30,000 participants each.108,109,110
Results-Nonreplicating Viral Vector Vaccines: Oxford/Astrazeneca and CanSino
One of the most explored viral vector options is the Ad, which is currently being used by both CanSino and Oxford/Astrazenca (Table 2). Ads are common cold causing viruses that have a double-stranded DNA genome.
Specifically, CanSino is utilizing Ad type 5 (Ad5), giving the vaccine the name Ad5-nCoV.21 Oxford/Astrazeneca is employing a different viral vector, an Ad derived from the chimpanzee (the use of the chimpanzee vector minimizes possible interaction with prevalent antibodies against Ads), which was subsequently named AZD1222.23 Ad5-nCoV specifically encodes for the full-length S protein of SARS-CoV-2, unlike both Moderna and BioNTech/Pfizer, which both encoded subunits of the S protein.
The gene was derived from the Wuhan-Hu-1 sequence for SARS-CoV-2 and, along with a tissue plasminogen activator signal peptide, was cloned into an E1 and E3 deleted Ad5 vector.21 Deletion of E1 inactivates the replication potential of the vaccine while deletion of E3 allows for the insertion of larger genes up to 8 kb.113
The CanSino vectors were solubilized and administered intramuscularly into the arms of patients. Each shot contained 5 × 1010 viral particles per dose, and patients being tested with higher doses would receive multiple shots allowing for administration of multiples of 5 × 1010 particles.
AZD1222 is designed similarly to Ad5-nCoV, with deletion of E1 to block replication and deletion of E3 to enable incorporation of larger genetic cargo into the viral vector.23,113 The added sequence encodes for the full-length S protein with a tissue plasminogen activator leader sequence, and the S protein sequence is codon-optimized.
Key differences between the two viral vector platforms is discussed in more detail within the discussion section. Neither vaccine manufacturer mentions the use of an adjuvant, so these vectors most likely depend on immune recognition of the nonreplicating virus, perhaps through the DNA they carry which can activate TLR9 within endosomes.114 Another possibility is recognition of the viral capsid, which can occur through both TLR-dependent and TLR-independent mechanisms.115 The intracellular adaptor protein MyD88 has been shown to play a prominent role in invoking TLR-mediated immunogenicity with viral vectors being able to engage multiple MyD88 signaling pathways.115
The data from the phase I/II trials are summarized in Table 1. CanSino has already completed and published results for both their Phase I and II vaccines; the following will only present data from their Phase II trials. The biggest difference between the two trials is that CanSino removed the highest dose from their Phase I trials due to greater adverse events such as severe fever, fatigue, and muscle and joint pain.21
Therefore, CanSino’s Phase II trial only tested the low and medium concentrations at 1 × 1011 and 5 × 1010 viral particles per dose in 253 and 129 patients, respectively.22 By day 28, the 1 × 1011 group saw RBD-specific antibody GMT levels peak around 656.5 AU while the 5 × 1010 group peaked at around 571 AU.
RBD-specific antibody seroconversion occurred in 96% and 97% of patients within the 1 × 1011 and 5 × 1010 groups, respectively.
Seroconversion of NAbs occurred in fewer patients at 59 and 47% in the 1 × 1011 and 5 × 1010 groups, respectively. Antibodies were measured using ELISA assays. Neutralization was measured in vitro using a live SARS-CoV-2 virus as well as a pseudovirus.
Oxford/Astrazeneca only tested one dose at 5 × 1010 viral particles per dose (n = 533) but in a few patients (n = 10) also tested a booster of the same dose 28 days after the first.23 Some of the patients were also prophylactically given a common anti-inflammatory drug, acetaminophen, and in those patients, there were fewer adverse events.
Antibody concentrations against the SARS-CoV-2 S protein peaked at day 28 in the single-dose patients (157 AU), while the extra dose further improved response (639 AU within 56 days). The acetaminophen did not block generation of the antibody response.
Depending on the neutralization assay used, the vaccine induced neutralizing titers from 62 to 100% of patients in the single-dose group and 100% in the double-dose patients. Both trials also documented moderate increases in T-cell responses as measured by ELISpot assays.22,23
Adverse events from both the Oxford/Astrazeneca and CanSino vaccines were varied from mild to moderate and did not warrant termination of either trial. The most prominent adverse effects in both the Oxford/Astrazeneca and CanSino trials were pain, fatigue, and headache (Table 1).
Oxford/Astrazeneca did not test for other antibodies outside of IgG.23 CanSino did test for IgM antibodies, but only against the nucleocapsid of SARS-CoV-2 to ensure that participants did not have previous exposure to the virus.21 For advancement into Phase III clinical trials, CanSino noted that their lowest dose (5 × 1010 particles) demonstrated similar immunogenicity compared to their middle dose (1 × 1011 particles) while also reducing adverse events.22 Thus, their Phase III trial will move forward at the lowest dose.
They are set to begin testing in countries outside of China such as Saudi Arabia, where they already have an agreement to vaccinate 5,000 participants.116 The company is looking to increase the number of participants by setting up clinical trials in other countries such as Russia, Brazil, and Chile.116 Oxford/Astrazeneca will continue to test at its dose of 5 × 10,10 but in its Brazil trials, will not test a prime boost.117
This is most likely due to the similar immunogenicity and neutralization capability observed between the single dosed and double dosed groups.23 They will also test in multiple countries with Phase III trials ongoing in Brazil, South Africa, and the United Kingdom.23 Oxford/Astrazeneca will continue providing prophylactic acetaminophen for pain management.117
reference link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7553041/
More information: Will covid-19 vaccines save lives? Current trials are not designed to tell us, BMJ (2020). DOI: 10.1136/bmj.m4037