At present, four types of COVID-19 vaccines have been developed or are in development. They include viral vector vaccines, nucleic acid vaccines, subunit vaccines, and whole virus vaccines. They each have two subtypes, so there are eight subtypes of vaccines.
1.) Nucleic acid vaccines
Nucleic acid vaccines use the SARS-CoV-2 virus’s genetic materials (either RNA or DNA and usually the COVID-19 spike protein) to instruct the cells to make the antigen. They are relatively easy and inexpensive to manufacture. However, this is a new technology, so the overall safety of the vaccines has to be assessed, especially over the long term.
Also, nucleic acid vaccines need to be stored at ultra-low temperatures (i.e. below negative 70 degrees Celsius), which can make them unavailable in low-income countries that lack storage facilities.
a.) DNA-based
The genetic materials used are the virus’s DNA. So far, the World Health Organization hasn’t approved any DNA-based COVID-19 vaccines.
b.) RNA-based
The mRNA vaccines are delivered with a lipid nanoparticle formulation, to protect the RNA strands and help cells absorb them. However, lipid nanoparticles have been suggested to be highly inflammatory, which might have contributed to the increased cases of pericarditis and myocarditis among vaccinated individuals.
Examples: The two mRNA-based vaccines approved by the WHO are mRNA-1273 (produced by Moderna) and BNT162b2 (produced by Pfizer/BioNTech).
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2.) Viral vector vaccines
Viral vector vaccines are designed to use a harmless virus (i.e. vector) that’s different from the virus, for which the vaccines are supposed to prevent, to give cells genetic instructions to produce antigens. These vaccines are supposed to mimic natural viral infections.
However, in the case of the COVID-19 vaccines, the vectors used such as the human adenovirus Ad26 and the chimpanzee adenovirus ChAdOx1 may not be as harmless as the vaccine developers have hoped. For instance, Ad26 can potentially cause minor illnesses. And ChAdOx1 is said to have a safe profile, according to a study published in The Lancet.
a.) Replicating viral vector
In replicating viral vector vaccines, the vectors can replicate.
b.) Non-replicating viral vector
In non-replicating viral vector vaccines, the vectors cannot replicate.
Examples: Ad26.COV2.S (produced by Johnson & Johnson), AZD1222 (produced by Oxford/AstraZeneca), and Covishield (produced with Oxford/AstraZeneca formulation) are viral vector vaccines.
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3.) Recombinant subunit vaccines
Also known as subunit vaccines. Recombinant subunit vaccines use pieces of the pathogen (e.g. proteins) to trigger an immune response. The use of these vaccines minimizes the risk of side effects but at the expense of vaccine effectiveness. As a result, these vaccines often require adjuvants. Subunit vaccine production uses well-established technology and is suitable for immunocompromised individuals.
a.) Virus-like particles (VLP)
These subunit vaccines use virus-like particles, which are proteins assembled into the natural shape of the virus’s outer cell.
b.) Protein subunit
Protein subunit vaccines for COVID-19 use fragments of spike antigens and are efficient for mass production.
Examples: Nuvaxovid and Covovax vaccines are both protein subunit vaccines. Medicago Plant-based VLP vaccines are VLP subunit vaccines.
4.) Whole virus vaccines
The production policy of whole virus vaccines is well-established.
a.) Inactivated: Inactivated vaccines use virus particles that have been killed (to prevent them from replicating themselves) to trigger an immune response.
b.) Live-attenuated: Live-attenuated vaccines use weakened live viruses (that can still replicate) to trigger an immune response without causing illness. However, this may not be the case for people with weakened immunity.
Examples: Whole virus vaccine products include BBIBP-CorV produced by Sinopharm (China) and CoronaVac produced by Sinovac (China). Both products are inactivated vaccines.
According to the CDC, these vaccines can all cause various types of side effects and adverse events, though rare in overall cases. And according to Reuters, possible vaccine injuries have also been reported.
According to a study published in the journal Frontiers in Cell and Developmental Biology, COVID-19 vaccines can also potentially weaken the vaccine recipients’ immunity, as they might contain N1-methylpseudouridine. In addition, according to an article published in the journal Neurobiology of Disease, the spike proteins contained in the vaccines can trigger a pro-inflammatory response in brain endothelial cells and damage vascular endothelial cells.
In addition, according to the CDC, COVID-19 vaccine boosters’ effectiveness would wane quickly after inoculation. These are usually the third doses. However, as per the British Medical Journal protection from a fourth dose has been proven to wane quickly as well.