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Unlike many other vaccines that contain an infectious pathogen or part of it, viral vector vaccines use a harmless virus to deliver a piece of genetic code to our cells, allowing them to make the protein for a pathogen. . This causes our immune system to respond to future infections.
When we have a bacterial or viral infection, our immune system reacts to the molecules of the pathogen. If this is our first encounter with the invader, a finely tuned cascade of processes comes together to combat the pathogen and boost immunity for future encounters.
Many traditional vaccines deliver an infectious pathogen or part of it to our body to train our immune system to fight against future exposures to the pathogen.
Viral vector vaccines work differently. They use a harmless virus to transmit a piece of genetic code from a pathogen to our cells to mimic an infection. The harmless virus acts as a delivery system, or vector, for the genetic sequence.
Our cells then make the viral or bacterial protein that the vector delivers and present it to our immune system.
This allows us to develop a specific immune response against a pathogen without the need to have an infection.
However, the viral vector itself plays an additional role in stimulating our immune response. This leads to a more robust reaction than if the genetic sequence of the pathogen were delivered alone.
The Oxford-AstraZeneca COVID-19 vaccine uses a chimpanzee cold viral vector known as ChAdOx1, which delivers the code that allows our cells to make the SARS-CoV-2 spike protein.
Scientists have studied many different types of viral vectors, including adenoviral vectors. Adenoviruses can cause the common cold, and there are many different types of these viruses.
Originally, researchers worked with adenoviruses modified for gene therapy. However, because they are able to stimulate our immune system, adenoviral vectors are good candidates for vaccine development.
The Oxford-AstraZeneca COVID-19 vaccine uses a chimpanzee adenoviral vector. It delivers the gene that encodes the SARS-CoV-2 spike protein to our cells.
Our cells then transcribe this gene into messenger RNA, or mRNA, which in turn prompts our cellular machine to make the spike protein in the main body, or cytoplasm, of the cell.
Then our cells present the spike protein, along with small parts of it, to the cell surface, which prompts our immune system to make antibodies and mount T cell responses.
Researchers have shown that this vaccine is safe and can effectively prevent COVID-19 in most people.
The ChAdOx1 viral vector in the Oxford-AstraZeneca COVID-19 vaccine has been genetically modified so that it cannot replicate. Therefore, it is unable to cause adenovirus infection in people who have received the vaccine.
It also cannot cause COVID-19 because it does not contain enough SARS-CoV-2 genetic material for our cells to assemble all of the SARS-CoV-2 virus. It only carries the code to make the spike protein.
The vaccine does not cause any permanent changes in our cells and the genetic code for the spike protein is not part of our own DNA.
With all viral vectors, one issue to consider is pre-existing immunity. If a person has encountered the virus that serves as a vector in the past, they may have antibodies against the virus. This means that their body will try to fight and destroy the viral vector, potentially making a vaccine less effective.
The University of Oxford research team behind the Oxford-AstraZeneca COVID-19 vaccine previously reported that the levels of pre-existing antibodies to the ChAdOx1 viral vector were low when evaluated in adult samples from the UK and The Gambia.
Write in Nature medicine as of December 2020, researchers saw no correlation between vector immunity and how the COVID-19 vaccine worked or whether volunteers who received it had side effects in a phase 1/2 clinical trial.
Other COVID-19 vaccines that use viral vectors include the Russian vaccine Sputnik V and the single-dose candidate vaccine Janssen.
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