Abstract: As the epidemic of COVID-19 is still threatening global health, scientists all over the world are accelerating their research on drugs and vaccines, and the research on several types of vaccines has made a great progress.
As we all know, vaccines contain weakened or inactive parts of a particular organism (antigen) that triggers an immune response within the body, so if a person is infected with a pathogen, his or her immune system can quickly prevent the infection from spreading in the body and causing disease. In this way, vaccines can mimic natural infections, but they don’t actually make people sick.
Now, to fight with COVID-19, global scientists are focusing on vaccine development. So what are the different types of vaccines that are currently being developed against SARS-CoV-2? This article will introduce several types in detail.
Killed Vaccine
Several killed vaccines (or inactivated vaccines) for the novel coronavirus have been developed, including those developed by Sinovac Biotech, Sinopharm, Wuhan Institute of Biological Products, and Bharat Biotech.
Virus inactivation is a mature vaccine production method. The inactivated virus stimulates the body’s immune system to produce antibodies. Therefore, when people are exposed to natural viruses, antibodies work to fight the virus. Among the many types of vaccines, live attenuated and inactivated vaccines constitute the vast majority of vaccines currently in use, and the production procedures are also complete and relatively simple. Like all vaccines, the immunogenicity of new killed vaccines must be strictly tested to ensure safety and effectiveness.
Protein Vaccine
Many vaccines that are being developed against SARS-CoV-2 contain only viral proteins, not genetic material, including vaccines from companies such as Novavax, Sanofi, GlaxoSmithKline, and SpyBiotech.
Protein immunogens may be delivered as viral subunit proteins, virus-like particles, or as inactivated virus. They are produced in a laboratory; and are often administered with an adjuvant to increase response magnitude and durability, for protein vaccines sometimes fail to induce strong CD8 T cell responses, which can destroy virus-infected cells, and adjuvants can help improve this situation.
Viral Vector Vaccine
The current SARS-CoV-2 vector vaccines use non-replicating human or chimpanzee adenoviruses, including vaccines developed by AstraZeneca in cooperation with Oxford University, Johnson & Johnson, Cansino Bio, and the Gamaleya Research Institute.
Viral vector vaccines use another non-replicating virus to deliver the genes of SARS-CoV-2 into human cells in the form of DNA. In the process, viral proteins are produced to induce a protective immune response. The research of viral vectors in gene therapy, cancer therapy, molecular biology research and vaccines has a history of decades. In July 2020, the European Commission approved the use of the AD26.ZEBOV (derived from human adenovirus serotype 26) produced by Johnson & Johnson. This is the first adenovirus vector vaccine approved for use in humans, and it is also the viral vector used by Johnson & Johnson for its new vaccine for SARS-CoV-2.
Gene Vaccine
Genes can be directly administered as DNA or RNA, instead of using viral vectors to deliver virus genes to human cells. The several SARS-CoV-2 vaccines that have made the most progress in Phase III trials are messenger RNA (mRNA) vaccines that can deliver spike protein genes, including vaccines produced by Moderna, BioNTec in collaboration with Pfizer, CureVac, and Imperial College.
After obtaining the genetic sequence of SARS-CoV-2 in January 2020, candidate genetic vaccines were produced quickly. Compared with other types of vaccines, the development of mRNA vaccines has subversive advantages in terms of immunogenicity, safety and industrial production. This is why they are one of the first vaccines to enter human trials. However, no mRNA vaccine has been licensed and approved for use in humans before, and most of the human experience using this technology is to treat cancer.
“Early phase I and phase II studies of the mRNA vaccine indicate that the immune response induced by these vaccines may have a protective effect, including in the elderly. However, before the completion of phase III clinical trials, the safety, effectiveness and duration of mRNA vaccines are not yet known, and at least two administrations are required,” introduced by a scientist at Creative Biolabs.
Until all phase III clinical trials are completed, we cannot know exactly the safety, effectiveness, and their relative advantages and disadvantages of different types of SARS-CoV-2 vaccines. It is important to not only monitor short-term vaccine safety (such as pain and fever), but also monitor the risk of long-term adverse events (such as exacerbation of disease after exposure to natural infections and autoimmune diseases). Of particular concern is the effectiveness of the vaccine in vulnerable populations, such as the elderly and people with underlying diseases, including diabetes, HIV infection, chronic heart, kidney, and lung diseases. The battle with the novel coronavirus still goes on but we may anticipate a success in a near future.