Updated: August 11,2021
The SARS-CoV-2 pandemic has caused significant loss of life, profound disruption to lives and livelihood, and widespread economic, sociological, and psychological damage. The high transmissibility, presence of asymptomatic carriers, and new variants have had a prolonged effect on the global population for the past year.
Vaccination constitutes the most promising path back toward a normal life. As of August 2021, Over 4.31 billion doses of vaccine have been administered worldwide. Vaccines are the key component in the fight against this deadly virus which has taken the lives of more than 4.26 million people around the world.
Since the identification of the SARS-CoV-2 virus, more than 300 vaccine projects started. More than 90 are undergoing clinical trials, almost eleven of them approved as far as August 2021 for emergency use globally. The WHO’s EUL assesses the quality, safety, and efficacy of COVID-19 vaccines. It allows countries to use their own regulatory approval to import and administer COVID-19 vaccines.
SARS-CoV-2 vaccines are targeting the whole SARS CoV-2 or its fragments by using different technologies. Every vaccine has the peculiarities that make it unique regarding the efficacy, safety, or duration of the induced protection.
Different platforms are employed for preparing vaccines. Here is a brief introduction to how these vaccines work:
- The whole virus vaccine’s technology is well established and relatively simple to make. Immunity is directed against other surface antigens of SARS-CoV-2 too. Whole virus Vaccines are vaccines based on attenuated SARS-CoV-2 and Vaccines based on inactivated SARS-CoV-2.
- Nucleic Acid Vaccine e.g. Naked DNA-based Vaccine and mRNA-based Vaccine.
- Vaccines based on viral vectors e.g. Non replicating viral vectors and Replicating viral vector vaccine
- Proteins and peptides-based Vaccine
- Other Nanoparticles and virus-like particle vaccines
Vaccines based on attenuated SARS- CoV-2 viruses
This is the most traditional technique. The history of vaccines begins with vaccines based on a living microbe, that has been weakened by growing in unfavorable conditions or by generating a genetically weakened version of the virus, so it can not cause disease. Since attenuated viruses retain the ability to replicate, they are very effective in inducing a strong and persistent immune memory. The attenuation of trillions of viruses is complex and delicate and can be associated with major biosafety risks especially in immunocompromised. The attenuated viruses can cause disease, even if this is a minor one. Their storage and handling also require careful monitoring.
Licensed vaccines of this type are the Oral Polio vaccine, Meningococcal vaccine, Pneumococcal vaccine, and vaccine for Hepatitis B.
Vaccines based on inactivated SARS-CoV-2 viruses
A vaccine based on killed microorganisms belongs to a very traditional technological platform. The vaccines produced by this method are more stable than live attenuated vaccines but in order to produce a longer duration of memory higher amount or an adjuvant is required. The immunity produced is lesser than that by attenuated virus vaccine, but the inactivated vaccine is cheaper, easy to handle, and much safer.
Other licensed vaccines that use this technology are Hepatitis A Vaccine, Injectable Polio Vaccine, Rabies Vaccine, and Influenza Virus Vaccine.
Vaccines based on SARS-CoV-2 proteins
This is also a well-established technology especially, advantageous for immunocompromised. Initially, these proteins were purified from the microbes while today, in most cases they are produced by using recombinant DNA technology. Relatively complex to manufacture. There are several human vaccines based on proteins present on the surface of microbes already in use. Spike proteins in their trimeric form or component of it such as the RBD region are the targets of all these vaccines. An adjuvant is combined to increase immune response either of the bacterial origin or synthetic.
Naked DNA-based Vaccines
The DNA and mRNA-based platforms offer great flexibility in terms of manipulation of the coded antigen and great potential for speed. Currently, there are no DNA vaccines registered for human use; however, DNA vaccines are commonly used in veterinary medicine. These vaccines are stable and can be produced in large amounts in bacteria. Once injected, DNA plasmids enter human cells, and their ability to enter may be enhanced by a very short local electrical pulse. Once entered, plasmid DNA induces the cell to produce temporarily the target proteins.
Messenger RNA-based Vaccines
These vaccines were the first to be developed and tested on volunteers based on novel technology. Several vaccine projects are using this technology. Unlike DNA, RNA must be transported in various ways to enter the human cell. Once entered, the mRNA vaccine temporarily induces the cell to produce the antigen protein. In most of these vaccines, the mRNA is carried by lipid microvesicles. The target antigen coded by the mRNA mostly is Spike protein, its variant, or its fragments. These vaccines have to be kept at -30 to -80 °C. Some of the vaccines using mRNA technology are:
- Pfizer, US -BioNTech, Germany
- Moderna, US
- Cure Vac, Germany
- Abogn, China
Vaccines based on viral vectors
The DNA coding for the spike proteins can be conveyed into the cells by viral vectors. By inserting the DNA into a virus, it is possible to exploit the virus’s great ability to infect and deliver the mRNA into the human cells. The virus in which DNA is inserted may lose its ability to replicate. Since a pre-existing immunity against the virus vector may affect vaccine efficacy so primates viruses are often used as vectors.
In other cases DNA is inserted into replicating active viruses: as these viruses can propagate to some extent, they may induce a stronger immune response. A well-established technology but relatively complex to manufacture. A licensed vaccine of this type is the Ebola virus vaccine.
There are numerous vaccine projects based on viral vectors that are already in clinical trials, some are approved for emergency use.
Engineered non-replicating virus vector
- Johnson and Johnson, USA.
- Sputnik V, The Gamaleya Research Institute, Russia
- CanSino Biological, China
- Covishield, India
Engineered replicating virus vectors
- Measles virus, and vesicular stomatitis virus
- Influenza virus administered by nasal spray
List of Approved Vaccines