Background
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In December 2019, the World Health Organization (WHO) Country Office in China was first alerted to an unknown outbreak of contagious and often severe lower respiratory illnesses originating from the city of Wuhan, the biggest city in and capital of China’s Hubei province.(1) The cause of the respiratory illness is a virus of the betacoronavirus class now termed coronavirus infectious disease-19 (COVID-19). The virus was named SARS-CoV-2 due to its genetic and structural similarity with SARS-CoV.(1,2) On March 11, 2020, the WHO officially identified SARS-CoV-2 as a pandemic due to its quick global spread.(1) As of August 11, 2020, there are 19,936,210 confirmed cases worldwide and 732,499 deaths due to SARS-CoV-2.(3) The continued rise of both cases and deaths necessitates the rapid development of an effective SARS-CoV-2 vaccine. The second wave happening in some countries that have reopened their economies further accentuates this need.(4,5) While masking, social distancing, and contact tracing can slow the spread of this virus, it appears too infectious to be eliminated by these strategies, and a vaccine is essential to enable a return to normal human social interaction.
Fortunately, in the relatively few months since SARS CoV-2 was identified as the cause of COVID-19, over two hundred academic laboratories and companies have undertaken vaccine development, and many are making record time in advancing to clinical trials (Table S1).(6,7) Moderna reached clinical trials 63 days after their sequence selection.(8) It is striking that an unestablished nanotechnology formulation reached clinical testing almost a full month before established approaches (i.e., inactivated and live-attenuated vaccines) entered clinical trials.(9,10) This highlights the opportunity for less developed technology platforms in vaccine development and, if proven successful, may enable a more rapid response to future emergent infectious diseases. It is also of note that in previous severe coronavirus outbreaks of SARS-CoV and MERS-CoV clinical trials were not reached until 25 and 22 months after the outbreaks began.(11) Older severe infectious disease outbreaks such as Dengue and Chikungunya did not reach clinical trials until 52 and 19 years after the outbreak.(11) The improved speed into clinical trials is hopeful, but despite the rapid progress, there are still reasons for concern.
Vaccine development takes time as the vaccines must not only be protective but also safe. Unlike other drugs that are delivered into sick patients, vaccines are administered into healthy patients and require very high safety margins.(12) Therefore, the population should be carefully monitored if vaccine candidates are widely administered based on Emergency Use Authorization. This is especially vital as for past respiratory diseases such as SARS-CoV, MERS-CoV, respiratory syncytial virus, and measles it had been shown that antibodies can exacerbate disease severity through antibody-dependent enhancement.(13) Many of the vaccines that are frontrunners are preclinical nanotechnologies and have not been proven in clinical settings. For instance, mRNA vaccines have been in development and clinical testing for the past 30 years, but the technology has not been previously approved.(14) The platform technology offers speed and adaptability, so these vaccine candidates can be rapidly developed by repurposing previously developed nanostructures as shown by Moderna.(15) Likewise, Novavax’s vaccine is also modeled off of their previous vaccine against influenza.(16) Even so, the vaccines must be rigorously tested for safety before widespread vaccination can occur, which Moderna and Novavax have accomplished through their Phase I studies.(17,18) Beyond Moderna and Novavax, several other companies have moved beyond their safety and immunogenicity Phase I and II clinical studies and have released pertinent data corresponding to these trials.(17,19−23) This review analyzes these posted results and highlights the nanotechnological aspects of the vaccines from these leading companies as well as summarizes the potential of other rapidly developed vaccines in clinical trials.
Vaccination Immunology
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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 pox(29) 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
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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.