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How COVID-19 vaccines work

Vaccines teach your immune system how to protect you from diseases. It is much safer for your immune system to learn how to protect you through vaccination than by getting COVID-19.

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

COVID-19 vaccines do not contain the live virus that causes COVID-19. The vaccines cannot cause COVID-19.

The different types of COVID-19 vaccines used in Ireland are:

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.

Viral vector vaccines

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

Viral vector vaccines are like messengers. They use a weakened version of a different virus to deliver instructions to cells in your body. Your body then makes antibodies that help fight the infection if the COVID-19 virus enters your body in the future.

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.

Protein subunit vaccines

The Novavax and Vidprevtyn Beta COVID-19 vaccines are protein subunit vaccines.

The vaccine contains a version of a protein found on the surface of the virus that causes COVID-19. It also contains an adjuvant (ingredient) to help strengthen the immune responses to the vaccine.

By getting this vaccine, your body makes antibodies that help fight the infection if the COVID-19 virus enters your body in the future.

It works in a very similar way to the hepatitis B vaccine.

Which vaccine type is best

All COVID-19 vaccines reduce your risk of getting COVID-19 and protect you from serious complications of the illness.

The mRNA vaccines have been given to millions of people around the world during the pandemic including pregnant women and children. mRNA vaccines are the recommended vaccines for the first round of vaccination, for the additional dose in people with a weak immune system and for a booster dose.

But some people may not be able to get an mRNA vaccine, or may not want to. For example, if you are allergic to polyethylene glycol (PEG), an ingredient in the mRNA vaccines. In these cases, you may want to get another type of COVID-19 vaccine that does not contain PEG. This includes the Novavax vaccine.

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

Page last reviewed: 30 May 2023