What is the difference between mRNA and viral vector-based vaccines?

TORONTO – The National Advisory Committee on Immunization recently doubled down on its recommendation that mRNA vaccines are “preferred” over their viral vector-based counterparts in the fight against CORNA-19, raising questions about vaccine differences. .

So far, Health Canada has currently authorized the use of four different COVID-19 vaccines made by Pfizer-BioNTech, Moderna, AstraZeneca and Janssen (the vaccine division of Johnson & Johnson).

Pfizer-BioNTech and Moderna vaccines use mRNA technology, while AstraZeneca and single-dose J&J shots are considered viral vector-based vaccines.

So what is the difference between mRNA and viral vector-based vaccines and why is NACI recommending one type over another?

What are viral vectors-based doses?

Viral vector-based vaccines, such as those developed by AstraZeneca and Johnson & Johnson, use a harmless virus or adenovirus as a delivery mechanism to create antibodies to trigger the immune system to fight an infection. For SARS-CoV-2, the virus that causes COVID-19.

Adenovirus is not SARS-CoV-2 itself, but is a separate, harmless virus that has been manipulated, so it is unable to replicate and cause the disease.

There are adenovirus viruses that cause the common cold and there are many different types, which have been used for decades to direct proteins, Health Canada explains On its website

In the case of COVID-19, the vector virus gives specific genetic instructions to cells in the body to produce a harmless piece of SARS-COV-2 called a spike protein. The cells then display this spike protein and the immune system triggers a response.

As a result, the immune system produces antibodies to specific spike proteins to think that it is an infection. If the immune system encounters the actual SARS-CoV-2 virus and its spike protein, it will already be ready to launch a defense against it.

Viral vector-based vaccines are genetically modified, so they are unable to replicate, meaning that once antibodies are formed, the viral vector clears up for good.

According to the American Centers for Disease Control and Prevention (CDC), the advantage of viral vector-based vaccines is that they provide protection against SARS-CoV-2, without risking the serious consequences of becoming ill with COVID-19.

The CDC also emphasized that these types of vaccines may not cause infection with COVID-19 or that adenoviruses can be used as a vaccine vector.


For their COVID-19 vaccines, Pfizer-Bayonet and Modern use a novel technique that has never been approved for widespread use before the epidemic.

This technique uses messenger ribonucleic acid (mRNA), a molecule that provides genetic instructions to make proteins necessary for many cellular functions in the body, including energy and immune defense.

In a lab, scientists develop synthetic mRNAs that are able to direct the body’s cells that the SARS-CoV-2 virus develops the same specific spike protein that also targets viral vector-based vaccines.

After a piece of protein is formed, the cell breaks down genetic instructions and gets rid of them. Both Health Canada and the CDC asserted that mRNA never enters the central part of the cell where a person’s DNA content is located, meaning that the vaccine does not affect or interact with DNA in any way.

Like viral vector-based vaccines, the immune system identifies foreign spike proteins produced by cells and initiates an immune response by building antibodies against them. If the immune system encounters the real SARS-CoV-2 virus, it will be ready to shut it down.

While both mRNA and viral vector-based vaccines have similarities in that SARS-COV-2 spikes direct cells to form proteins, mRNA vaccines differ in that they do not contain any living viruses.

Which is a better type of vaccine?

The viral vector-based COVID-19 vaccine developed by AstraZeneca and Johnson & Johnson has been linked to an extremely rare and potentially life-threatening blood clotting syndrome called vaccine-induced thrombotic thrombocytopenia (VITT) – a low platelet count. Is a combination. With blood clots.

The risk of developing this syndrome is estimated to be anywhere from one case in 100,000 to one case in 250,000.

In Canada, there are only seven reported VITT cases out of AstraZeneca’s nearly 1.7 million doses that have been administered so far in the country.

Due to the very low risk of developing VITT after vaccination with a viral vector-based vaccine, however, NACI recently confirmed that mRNA vaccines are “preferred” over others and may be in the best interest of some Canadians. Have a lower risk of exposure to COVID-19 to wait for an mRNA dose.

Despite this guidance, both Health Canada and NACI have emphasized that vaccines by AstraZeneca and Johnson & Johnson are safe and effective for the majority of the population and that contracting COVID-19 can have many bad consequences.


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