How COVID-19 vaccines work - mRNA and viral vector

Vaccines are the most effective way to prevent infectious diseases. They prepare your immune system (your body's natural defences) to recognise and defend itself against a specific virus.

All vaccines do this. But different types of vaccine work in different ways.

Some types of vaccine contain a live virus. For example, the flu vaccine contains a small amount of the flu virus. But none of the COVID-19 vaccines in use have the COVID-19 virus in them. They work differently.

There are currently 2 types of COVID-19 vaccine in use:

All vaccines are tested for safety and effectiveness before they can be used. The HSE only uses a vaccine if it meets the required standards of safety and effectiveness.

mRNA vaccines

The Pfizer/BioNTech and Moderna COVID-19 vaccines are both mRNA vaccines.

mRNA vaccines teach your body how to make a protein that will trigger an immune response, without using a live virus.

After you get an mRNA vaccine, your body makes antibodies that help fight the infection if a virus enters your body in the future.

Researchers have been studying and working with mRNA vaccines for decades.

How COVID-19 mRNA vaccines work

The surface of the virus that causes COVID-19 is studded with proteins known as "spike proteins". The virus uses these spikes to enter human cells, infecting you with COVID-19.

mRNA COVID-19 vaccines contain the instructions for making this spike protein.

After you get your vaccine, your immune system recognises that the protein doesn't belong there. Your body then begins building an immune response to fight off what it thinks is an infection. This immune response makes antibodies.

The antibodies offer you protection from COVID-19. It is much safer for your immune system to learn how to protect you from COVID-19 through vaccination than by catching the virus.

Your cells then break down the instructions for making the spike protein and gets rid of them from your body.

Viral vector vaccines

The AstraZeneca and Janssen COVID-19 vaccines are both viral vector vaccines.

Viral vector vaccines use a harmless virus as a delivery system. They teach your body how to make a protein that will trigger an immune response.

Hundreds of scientific studies of viral vector vaccines have been done and published around the world. Some vaccines recently used for Ebola outbreaks have used viral vector technology. Other studies have focused on viral vector vaccines against Zika, flu, and HIV.

How COVID-19 viral vector vaccines work

Viral vector vaccines are like messengers. They use a weakened version of a different virus (the vector) to deliver instructions to cells in your body.

COVID-19 vaccines use the adenovirus as the vector. Adenovirus is the virus that causes the common cold.

The surface of the virus that causes COVID-19 is studded with proteins known as "spike proteins". The virus uses these spikes to enter human cells, infecting you with COVID-19.

When you get a viral vector vaccine, the vector (adenovirus) enters a cell in your body. It then teaches the cell how to produce the COVID-19 spike protein.

After you get your vaccine, your immune system recognises that the protein doesn't belong there. Your body then begins building an immune response to fight off what it thinks is an infection. This immune response makes antibodies.

The antibodies offer you protection from COVID-19. It is much safer for your immune system to learn how to protect you from COVID-19 through vaccination than by catching the virus.


This content was fact checked by vaccine experts working in Ireland.

Last updated: 5 July at 11.35am