Analysis of COVID‐19 vaccines: Types, thoughts, and application

Abstract Objective To deal with COVID‐19, various countries have made many efforts, including the research and development of vaccines. The purpose of this manuscript was to summarize the development, application, and problems of COVID‐19 vaccines. Methods This article reviewed the existing literature to see the development of the COVID‐19 vaccine. Results We found that different types of vaccines had their own advantages and disadvantages. At the same time, the side effects of the vaccine, the dose of vaccination, the evaluation of the efficacy, and the application of the vaccine were all things worth studying. Conclusion The successful development of the COVID‐19 vaccine concerns almost all countries and people in the world. We must do an excellent job of researching the immunogenicity and immune reactivity of the vaccines. We hope this review can help colleagues at home and abroad.


| MAIN MECHANIS M
The design of the COVID-19 vaccines must take into account both humoral and cellular immunity. In addition, COVID-19 is mainly spread through the respiratory tract and contact, so the role of mucosal immunity in preventing viral infections should be paid more attention. The virus contains four structural proteins. They are Spike S protein, Envelope E protein, Membrane/matrix protein, and Nucleocapsid N protein. The S protein has two subsections, S1 and S2. The S protein binds to specific receptors, causing the virus to infect cells. [5][6][7] The neutralizing antibody against the S protein can block this process and prevent the virus from invading. 8

S protein
can also effectively stimulate T-cell immune response, so it is the most important target antigen for vaccine design. N and M proteins have also been shown to induce the body to produce an efficient cellular immune response. [9][10][11] SARS-CoV-2 is unusual for a respiratory virus that binds to a receptor, angiotensin-converting enzyme 2 (ACE2). ACE2 can be expressed in virtually all organs, but especially in the lungs, 12 gut, 13 and brain. 14 Therefore, unlike most respiratory viruses, SARS-CoV-2 has a wider biological distribution and may cause considerable damage outside the respiratory system. It adversely affects the genitourinary system, digestive system, circulatory system, and central nervous system. The universality of the distribution of ACE2 receptors leads to multiple changes in symptoms, such as dyspnea, headache, diarrhea, venous thromboembolism, and high blood pressure. 15 The S protein binds to ACE2 on cells to mediate infection. The S1 subunit contains the receptor-binding domain (RBD) and is responsible for initial attachment to the host cells through the ACE2 receptor, while the S2 subunit promotes viral fusion with cells to initiate infection. 16 The S protein is a frequent vaccine target as it is expected that antibodies binding to the correct epitope on the S protein may be neutralizing and block intercellular viral spread. 16

| T YPE S OF VACCINE S
The vaccines currently under study can be roughly divided into the following categories. Different types of vaccines have their characteristics (Table 1).

| DNA vaccines
DNA vaccines can enter cells like viral infections and use the host protein translation system to generate target antigens. As an endogenous immunogen, it can induce humoral and cellular immune responses at the same time. Given the advantages of nucleic acid vaccines, DNA vaccines do not require live viruses, so safety is improved. DNA vaccines insert genes encoding foreign antigens into plasmids containing eukaryotic expression elements and then directly introduce the plasmids into humans or animals, allowing them to express antigen proteins in host cells and induce immune responses to prevent diseases. 17 The manufacturing process of plasmid DNA is relatively straightforward, and the double-strand DNA molecules are more stable than the virus and can be freeze-dried for long-term storage. DNA vaccine vaccination method limits its application. Since the vaccine is mainly distributed in the intercellular space after vaccination, only a very small amount can enter the cell to produce protein immunogen, so the immune effect is greatly reduced. The plasmid DNA vaccine's main prohibitory factor is the low transfection efficacy, which requires transfection modalities. For example, Inovio's COVID-19 vaccine candidate, INO-4800, uses a handheld electroporation device, CELLECTRA. 18 The vaccine will be injected intradermally along  and an entire S1−S2 cleavage site. 21

| Non-replicating viral vector vaccines
One of the most explored viral vector options is the Adenovirus (Ad), currently being used by both CanSino and Oxford/ AstraZeneca.

DNA vaccines DNA vaccines can enter cells like viral infections
and use the host protein translation system to generate target antigens. It can induce humoral and cellular immune responses at the same time.
DNA vaccines do not require live viruses. The manufacturing process of plasmid DNA is relatively straightforward, and the doublestrand DNA molecules are more stable than the virus and can be freeze-dried for longterm storage.

INO−4800
mRNA vaccines mRNA vaccines need to enter the cytoplasm to achieve target antigens' expression theoretically safer The immune effect is not satisfactory, and a relatively high proportion of side effects.

mRNA−1273
Non-replicating viral vector vaccines It can encode for the full-length S protein of SARS-CoV−2.
The effectiveness of this vaccine is relatively high.
It may not effective for people with recessive infectious viruses.

Ad5-nCoV
Inactivated vaccines Inactivated vaccines are mainly obtained through three inactivation methods, which make it lose its infectivity and toxicity while maintaining immunogenicity. They are easy to prepare and can efficiently cause humoral immune responses.
Vaccine production requires the operation of high concentrations of live viruses, which poses a certain biological safety risk. The T-cell immune response caused by inactivated vaccines is generally weak.
The inactivated SARS-CoV−2 vaccine (Vero cells) Live attenuated vaccines Live attenuated vaccine reduces virus virulence through point mutation or deletion of crucial virus protein but does not affect its immunogenicity and replication ability.
This vaccine program has very good immunogenicity and can induce systemic immunity and mucosal immune response, and the immunity is lasting.
It is possible to restore virulence in the body due to retrograde mutations Polio vaccine

Subunit vaccines
The antigen protein of the pathogen is expressed and purified through genetic engineering to induce an immune response.
Subunit vaccines are composed of purified recombinant proteins and are considered to be the safest vaccines. It has good safety and immunogenicity.
As a non-endogenous antigen, subunit vaccines cannot be presented through MHC-I and cannot effectively produce sensitized cytotoxic T cells (CTL).

Recombinant subunit SARS-CoV−2 vaccine (CHO cells)
Trained immunitybased vaccines Trained immunity-based vaccines can activate the adaptive immune system and provide pathogen-specific protection / The production standards of the BCG vaccine will be different.

BCG vaccine
Adenovirus is common cold viruses with a double-stranded DNA genome. CanSino is using Ad type 5 (Ad5) and named the vaccine Ad5-nCoV. 22 Ad5-nCoV can encode for the full-length S protein of SARS-CoV-2. This gene is derived from the Wuhan-Hu-1 sequence of SARS-CoV-2 and is cloned into the E1-and E3-deleted Ad5 vector together with the tissue plasminogen activator signal peptide. 16 The effectiveness of this vaccine is relatively high, but the disadvantage is that it may not effective for people with recessive infectious viruses.

| Live attenuated vaccines
Live attenuated vaccine reduces virus virulence through point mutation or deletion of crucial virus protein but does not affect its immunogenicity and replication ability. This vaccine program has very good immunogenicity and can induce systemic immunity and mucosal immune response, and the immunity is lasting. Several live attenuated vaccines have been on the market, including yellow fever, smallpox, measles, polio, mumps, rubella, and chickenpox. The SARS live attenuated vaccine will recover its virulence after continuous passage in cells or mice, suggesting that the vaccine scheme has a greater biological safety risk. 27 Without sufficient evidence to ensure that live attenuated vaccines will not regain strength, this strategy is not currently recommended for COVID-19 vaccine development.

| Subunit vaccines
Subunit vaccines are composed of purified recombinant proteins and are considered to be the safest vaccines. There are currently several subunit vaccines on the market, including hepatitis B, hepatitis E, and human papillomavirus vaccines. SARS and MERS subunit vaccines can produce high-titer neutralizing antibodies in mice, and nasal or oral vaccination can also induce a mucosal immune response, thereby more effectively blocking the virus transmission through the respiratory tract. The data also prove the protective efficacy of mucosal vaccination better than intramuscular inoculation. [28][29][30][31] However, as a non-endogenous antigen, subunit vaccines cannot be presented through MHC-I and cannot effectively produce sensitized cytotoxic T cells (CTL). Considering the key role of cellular immunity in clearing coronavirus infections, the subunit vaccine of COVID-19 is best used in conjunction with other platform vaccines. It is recommended to include nasal and oral mucosal vaccination routes to activate mucosal immune responses.

| Trained immunity-based vaccines
Trained immunity-based vaccines can activate the adaptive immune system and provide pathogen-specific protection. 32,33 Currently, Bacille Calmette-Guerin (BCG), a vaccine against tuberculosis, can induce trained immunity against COVID-19 and is currently undergoing clinical evaluation, which will take time to prove. 34 Even if the BCG vaccine is effective against COVID-19, it also faces unique challenges. That is, the production standards of the BCG vaccine will vary from country to country, and it is not clear whether certain quality standards are required to provide protection against COVID-19. 35

| QUE S TIONS AND THOUG HTS
There are always various problems in the development of vaccines.
Among them, the later application and evaluation of vaccines are particularly prominent.

| Dosage problems
Many phase III studies failed because of incorrect identification of the dose that best balances safety and efficacy. 36

| Adverse reactions of vaccines
The vaccine also has adverse reactions such as redness, swelling, muscle pain, and fever. 38,39 The strategic objectives of the COVID-19 vaccine roadmap formulated by WHO include a series of preferred and most basic requirements such as vaccine safety and effectiveness. 40 The preferred requirements for safety/reactogenicity include "safety and reactogenicity sufficient to provide a highly favorable benefit/risk profile in the context of the observed vaccine efficacy and only mild, transient related to adverse vaccination events without serious adverse events". The most basic requirements for safety/reactogenicity include the benefits of vaccines outweigh the potential safety hazards. The long-term results were a safety that is sufficient to provide highly favorable benefits/risk characteristics in the context of the observed vaccine efficacy and immunogenicity. There were no serious adverse events related to vaccination. The preferred requirements for effectiveness include the protection effectiveness in the population is at least 70%, and the same is true for the elderly. If it is an outbreak treatment, the protective effect must appear within two weeks and last for at least one year. The most basic requirements include the population's protective effect is at least about 50% for at least six months.

| Clinical trials and Efficacy evaluation (endpoint observation)
Key Symptoms, 43 trialists have the latitude in selecting specific symptoms and severities to trigger virologic testing. It is important to define a common COVID-19 endpoint that can be used consistently across trials, not only to interpret results but also to facilitate trials' meta-analyses.
It is very important to use a standard set of clinical endpoints for vaccine efficacy evaluation in all trials to conduct a unified and comprehensive assessment of benefits and risks support aggregated data to analyze the immunosurrogate endpoint. In any case, COVID-19 and severe COVID-19 should be important independent clinical endpoints to be evaluated in each vaccine efficacy trial. All participants should be followed up sufficiently to collect meaningful data to evaluate long-term protection against these two endpoints, especially the severe COVID-19. More endpoint counts are needed to quantify vaccine efficacy reliably. Considering that vaccine-induced decrease in the incidence of symptomatic SARS-CoV-2 infections may be accompanied by a shift toward more asymptomatic infections, the ability to evaluate vaccine efficacy against the asymptomatic infection endpoint in trial designs should be included.

| Vaccine application
Preventive vaccines will control COVID-19 is justified by the impact of vaccines on preventing disability and death from infectious diseases. 44

| Travel immunization
If the epidemic situation is well controlled, and the future epidemic situation is mainly imported, entry and exit personnel should be the target of implementing the immunization strategy, and close contacts of entry personnel should be used as vaccinations.

| Immunization after exposure
If it is confirmed that the COVID-19 vaccine has the effect of preventing or alleviating the symptoms of the disease on the exposed subjects, it is possible to consider adopting a post-exposure immunization strategy for close contacts of confirmed COVID-19 cases. Therefore, it is necessary to evaluate the protective effect of COVID-19 vaccines, especially vaccines developed by new technologies, to determine post-exposure vaccination's scientific nature.

| Pre-exposure immunization
For subjects who may be exposed to COVID-19 patients or highrisk infections, such as medical staff in fever clinics, COVID-19 pathogen testing personnel, contact persons from COVID-19 endemic countries, etc., should take exposure pre-immune prevention strategies.

| Emergency immunization
In the event of a COVID-19 epidemic, under the premise of confirming the emergency immunization effect of the COVID-19 vaccine, an emergency immunization strategy can be considered for the population in the epidemic area. Therefore, in the early stage of vaccine marketing, it is necessary to carry out the effect evaluation of emergency immunization, especially the effect evaluation of the ring immunization strategy's implementation.
In addition to the above four immunization strategies for pandemic immunization, because COVID-19 is susceptible to the entire population, this feature is different from the H1N1 pandemic. 49 In the case of vaccine supply in batches, comprehensive consideration of protection priorities, reducing deaths and cluster outbreaks determine the targets and immunization procedures for mass pandemic vaccination.

| CON CLUS ION
In short, the successful development of the COVID-19 vaccine concerns almost all countries and people in the world. We must do an excellent job of researching the immunogenicity and immune reactivity of the vaccines.

ACK N OWLED G EM ENTS
Not applicable.

CO N FLI C T S O F I NTE R E S T
The authors declare no conflicts of interest.

AUTH O R S CO NTR I B UTI O N S
XCH designed the current study and was major contributors in writing the manuscript. PFX and QY were responsible for the modification and giving final approval of the manuscript. All authors read and approved the final manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analyzed in this study.