Understanding Vaccines

VaccineCOVID19
Written By Sophia Shim

COVID-19 vaccine trials are taking place around the globe, and it’s time we understand what a vaccine is and how they help us prepare to fight disease. Quoted as ‘one of the most effective ways to prevent disease’, by the World Health Organization, vaccines are widely used to fight common diseases such as Varicella (chicken pox), Influenza, Measles, and more. In this post we will outline what vaccines do, the different types of vaccines, and a few of the current vaccines being tested in COVID-19 trials.

*Note: In this post we will only be speaking of the prophylactic, or preventative, types of vaccines and NOT therapeutic vaccines (such as those used in cancer treatments). We will also be speaking in terms of virus-based vaccines.*

What are Vaccines?

Sometimes misnomered as a ‘treatment’, prophylactic vaccines are a preventative measure used to prepare the body’s immune system by building adaptive or acquired immunity toward specific viruses. Adaptive immunity refers to the immune system’s specific defense against a certain pathogen, ie: a COVID-19 vaccine would fight specifically against the SARS-CoV-2 virus. The body ‘acquires’ or develops this immunity through exposure to the specific virus.

The purpose of a vaccine is to safely expose the body to the virus or an agent that resembles the virus, such as parts of the virus or the toxins the pathogen produces. This exposure gives the immune system a chance to familiarize itself with the specific virus, and learn how to generate the appropriate immune response in order to fight it off.

The above figure shows innate vs adaptive immunity. Innate immune responses are quick, automatic, and non-specific while adaptive immune responses take longer to respond but will produce specific molecules to combat the virus.

The above figure shows innate vs adaptive immunity. Innate immune responses are quick, automatic, and non-specific while adaptive immune responses take longer to respond but will produce specific molecules to combat the virus.

Vaccines can be split into 4 main types depending on what is injected into the body. These four types include: (1) live attenuated vaccines, (2) inactivated vaccines, (3) subunit, recombinant, and conjugate vaccines, and (4) toxoid vaccines.

 

Types of Vaccines

(1) Live Attenuated Vaccines

Live attenuated vaccines carry live, but weakened, copies of the specific virus, or can carry a closely-related and less dangerous relative of the virus. An attenuated or  ‘weakened version’ refers to a virus that has been modified in the laboratory to weaken their virulence; the degree of disease severity caused by a virus. These types of vaccines generate specific acquired immunity to the virus itself.

Alternatively, live attenuated viruses which use related, but less dangerous forms of the target virus, will allow the immune system to generate a broader, more generic response to viruses that look similar. 

Disclaimer: Since these vaccines inject a small amount of live virus into the body, immunocompromised individuals and those with chronic and pre-existing medical conditions should always speak to their doctors to see if it is safe to receive these vaccines. 

Varicella or the chickenpox vaccine is an example of a live attenuated virus.

Varicella or the chickenpox vaccine is an example of a live attenuated virus.

(2) Inactivated Vaccines

Inactivated vaccines contain inactive, or ‘dead’ copies, of the target virus which will NOT cause disease. These viruses, inactivated by heat, chemicals, or radiation, will allow the body’s immune system to recognize the virus and develop an immune response without endangering the patient. 

(3) Subunit, Recombinant, and Conjugate Vaccines

These vaccines contain specific parts or a combination of parts of the target virus to create an immune response that is highly sensitive to that specific part of the pathogen.

Sub-unit - Includes a viral protein, sugar, or capsid (See graphic to the left for the different parts of a virus!)
Recombinant - Includes multiple parts of a virus or a combination of genetically engineered virus parts
Conjugate - Includes a combination of pieces from the outer coating of bacteria linked to a carrier protein which will prompt an immune response

(4) Toxoid Vaccines

These vaccines contain copies of inactivated toxins produced by the pathogen instead of the pathogen itself. This is done to create an immune response to the toxin specifically, as the toxin is the disease-causing agent. These vaccines are mostly used to combat bacterial disease. 

 

Pros and Cons of Different Vaccine Types

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Some Vaccines Being Developed for COVID-19…

In this section we will be outlining only a few of the current vaccines being tested. For more information of different vaccines currently being tested, visit our Vaccine News page here.

ChAdOx1 nCoV-19 - Oxford University -
Recombinant vaccine

The Jenner Institute is in the midst of clinical trials for their vaccine which contains spike proteins from the SARS-CoV-2 virus, as well as a modified version of a chimpanzee adenovirus vector which will stimulate a strong immune response. Read more about it here.

mRNA-1273 - NIAID - Subunit vaccine

The US National Institute of Allergy and Infectious Diseases is also in the midst of testing their vaccine which contains a copy of the mRNA (cell instructions) to create the spike protein found on the outside of the SARS-CoV-2 virus. Read more about it here.

BNT162 - Pfizer and BioNTech - Subunit vaccine

In Germany, approval for clinical testing of 4 vaccine candidates has been given. All 4 of these vaccines contain different types of SARS-CoV-2 RNA sequences to examine which is the most efficient in producing an appropriate immune response. Read more about it here.

Sophia Shim
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