04/09/2021
THE FOUR MAIN TYPES OF COVID-19 VACCINE
There are four categories of vaccines in clinical trials: WHOLE
VIRUS, PROTEIN SUBUNIT, VIRAL VECTOR and NUCLEIC ACID (RNA
AND DNA). Some of them try to smuggle the antigen into the body, others
use the body’s own cells to make the viral antigen.
1.WHOLE VIRUS
Many conventional vaccines use whole viruses to trigger an immune
response. There are two main approaches. Live attenuated vaccines use a
weakened form of the virus that can still replicate without causing illness.
Inactivated vaccines use viruses whose genetic material has been
destroyed so they cannot replicate, but can still trigger an immune
response. Both types use well-established technology and pathways for
regulatory approval, but live attenuated ones may risk causing disease in
people with weak immune systems and often require careful cold storage,
making their use more challenging in low-resource countries. Inactivated
virus vaccines can be given to people with compromised immune systems
but might also need cold storage.
2.PROTEIN SUBUNIT
Subunit vaccines use pieces of the pathogen - often fragments of protein
- to trigger an immune response. Doing so minimises the risk of side
effects, but it also means the immune response may be weaker. This is
why they often require adjuvants, to help boost the immune response. An
example of an existing subunit vaccine is the hepatitis B vaccine.
3.NUCLEIC ACID
Nucleic acid vaccines use genetic material – either RNA or DNA – to
provide cells with the instructions to make the antigen. In the case of
COVID-19, this is usually the viral spike protein. Once this genetic material
gets into human cells, it uses our cells' protein factories to make the
antigen that will trigger an immune response. The advantages of such
vaccines are that they are easy to make, and cheap. Since the antigen is
produced inside our own cells and in large quantities, the immune reaction
should be strong. A downside, however, is that so far, no DNA or RNA
vaccines have been licensed for human use, which may cause more
hurdles with regulatory approval. In addition, RNA vaccines need to be kept
at ultra-cold temperatures, -70C or lower, which could prove challenging for countries that don’t have specialised cold storage equipment, particularly
low- and middle-income countries.
4.VIRAL VECTOR
Viral vector vaccines also work by giving cells genetic instructions to
produce antigens. But they differ from nucleic acid vaccines in that they use
a harmless virus, different from the one the vaccine is targeting, to deliver
these instructions into the cell. One type of virus that has often been used
as a vector is adenovirus, which causes the common cold. As with nucleic
acid vaccines, our own cellular machinery is hijacked to produce the
antigen from those instructions, in order to trigger an immune response.
Viral vector vaccines can mimic natural viral infection and should therefore
trigger a strong immune response. However, since there is a chance that
many people may have already been exposed to the viruses being used as
vectors, some may be immune to it, making the vaccine less effective.
