Overview - Pfizer/BioNTech COVID-19 vaccine

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.

The Pfizer/BioNTech COVID-19 vaccine is safe and effective.


You will need 2 doses of the Pfizer/BioNTech vaccine for your first round of vaccination.

You should get your second dose 21 to 28 days after your first dose.

It takes 7 days after the second dose for the Pfizer/BioNTech COVID-19 vaccine to work. This is when you have completed your first round of COVID-19 vaccination.

If you have a weak immune system you may need an additional dose of COVID-19 vaccine, at least 2 months after your second dose. This should be followed by a booster if you are over 16.


If you are 16 or older, it's important to get your booster when it is due. Without a booster, you are more at risk of serious illness if you get COVID-19.

You need to wait at least 3 months after your second vaccine dose before you can get a booster.

If you have a weak immune system, you should get your booster at least 3 months after your additional dose.

Read about getting your COVID-19 booster dose


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 from COVID-19 through vaccination than by catching the virus.

Read more about immunity after the Pfizer/BioNTech COVID-19 vaccine


The Pfizer/BioNTech COVID-19 vaccine was developed in line with international standards of safety.

Millions of people have been given the Pfizer/BioNTech COVID-19 vaccine.

Read more about the safety of the Pfizer/BioNTech COVID-19 vaccine

Side effects

Like all medicines, the Pfizer/BioNTech vaccine can cause side effects. Most of these are mild to moderate and short-term.

Not everyone gets side effects.

Read more about side effects after the Pfizer/BioNTech COVID-19 vaccine

Which vaccine is best

All COVID-19 vaccines give you good protection from severe illness from COVID-19 when you are fully vaccinated. The HSE only uses a vaccine if it meets the required standards of safety and effectiveness.

There is a greater risk of infection from newer variants of COVID-19 so it's more important than ever to get vaccinated.

Pfizer/BioNTech or Moderna are the recommended vaccines for people under the age of 50. This is because of the very rare risk of unusual blood clots with low platelets in people under 50 who get the AstraZeneca or Janssen vaccines.

The Moderna COVID-19 vaccine is only used in adults aged 30 and over in Ireland as per NIAC recommendations.

People under the age of 30 who received a previous dose of Moderna COVID-19 vaccine should receive the Pfizer/BioNTech COVID-19 vaccine as a second or subsequent dose.

If you are pregnant

Being vaccinated against COVID-19 will reduce the chance of you becoming seriously ill with COVID-19.

The information we have so far shows that the Pfizer/BioNTech COVID-19 vaccine does not have any negative effect on babies in the womb.

Read more about COVID-19 vaccines in pregnancy

If you are breastfeeding

There is no known reason to avoid breastfeeding if you are vaccinated.

If very small parts of the vaccine get into your breast milk, they will be digested in your baby’s stomach.

Read more about COVID-19 vaccines if you are breastfeeding

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

Last updated: 23 December 2021 at 3.30pm

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