How COVID-19 vaccines trigger an immune response

ABC Health & Wellbeing / 

By health reporter Olivia WillisPosted Wed 18 Aug 2021 at 5:00amWednesday 18 Aug 2021 at 5:00am, updated Wed 18 Aug 2021 at 10:13am

How do vaccines mimic a virus?

In order to train our bodies to recognise pathogens (and fight them off down the track), vaccines introduce our immune system to part of a pathogen — known as an “antigen” — which triggers an immune response.

This antigen might be a weakened or inactivated virus, or it might be just one part of a pathogen — for example, the spike protein found on the surface of SARS-CoV-2 (used by the virus to latch onto and enter human cells).

Traditional vaccines, including some COVID-19 jabs, deliver antigens directly to the body.

But other COVID-19 vaccines, such as the Pfizer, Moderna and AstraZeneca jabs, use different technology.

Instead of delivering the antigen itself, the vaccines contain a genetic blueprint (or set of instructions) that tell the body to make the SARS-CoV-2 spike protein using the body’s own cells.

To do this, the Pfizer and Moderna jabs contain a single strand of genetic material — that’s the mRNA or messenger RNA — which is encapsulated in a protective fatty coating.

The AstraZeneca vaccine, on the other hand, contains double-stranded DNA, which is carried into the body via a weakened version of a common cold virus, engineered so it doesn’t replicate.

Single glass vial of AstraZeneca vaccine sits in front of multiple packages of the same vaccine.
The genetic instructions in the AstraZeneca vaccine come in the form of DNA, which is much more stable than mRNA.(Pixabay)

“The DNA gets taken up by your cells, that DNA then encodes the mRNA, and then it turns into a protein … which is what your body is going to respond to,” says Stuart Tangye, an immunologist from the Garvan Institute of Medical Research.

“The mRNA vaccine just skips that first step.”

Read more about the spread of COVID-19 in Australia:

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