Side effects - Moderna COVID-19 vaccine

Like all medicines, vaccines can cause side effects. Most of these are mild to moderate and short-term. Not everyone gets side effects.

If you are concerned about side effects, phone your GP for advice.

Common side effects

The common side effects after all the COVID-19 vaccines happen less often in people aged 65 and older. 

You are more likely to experience common side effects after the second dose.

The official name for Moderna's COVID-19 vaccine is 'Spikevax.' But most people call it 'Moderna'.

After the Moderna COVID-19 vaccine, more than 1 in 10 people may experience:

  • feeling tired
  • tenderness, swelling, redness or itching in your arm where you had the vaccine injection
  • headache
  • swollen lymph glands under the arm where you had the injection
  • muscle pain
  • joint pain
  • nausea or vomiting
  • fever (temperature of 38 degrees Celsius or above)

Rare side effects

After the Moderna COVID-19 vaccine more than 1 in 10,000 people may develop:

  • Bell's palsy
  • swelling of their face - this can happen in people who have had facial fillers

Very rare side effects

Myocarditis and pericarditis

Very rarely, people may develop myocarditis and pericarditis after getting the Moderna vaccine.

Myocarditis and pericarditis are inflammatory heart conditions.

The risk of these very rare conditions is higher in younger men.

These conditions are more likely to occur after the second dose and mostly happen within 14 days of getting the vaccine.

Two European studies have estimated the risk of myocarditis, after the second dose of the vaccine as:

  • one additional case for every 7,600 men aged 12 to 29 (within 7 days)
  • one additional case for every 5,320 men aged 16 to 24 (within 28 days)

The risk of myocarditis or other rare side effects after a booster dose is not yet known.

Symptoms of myocarditis and pericarditis

Get urgent medical help if you get any of these symptoms in the weeks after the Moderna COVID-19 vaccine:

  • breathlessness
  • palpitations (a forceful heartbeat that may be irregular)
  • chest pain

Extremely rare side effects

'Erythema multiforme' is an extremely rare side effect. This is a skin reaction that causes red spots or patches on the skin. It may look like a target or 'bulls-eye' with a dark red centre surrounded by paler red rings.

Serious side effects, like a severe allergic reaction, are extremely rare. Your vaccinator is trained to treat any serious allergic reactions.

You should not get another mRNA vaccine if you have had a severe allergic reaction to the Moderna vaccine, any of its ingredients or have been told by your doctor not to have the Moderna vaccine.

mRNA vaccines offered in Ireland are the Moderna vaccine and the Pfizer/BioNTech vaccine.

Fever after COVID-19 vaccines

It’s common to develop a fever (temperature of 38 degrees Celsius or above) after any vaccination. This usually happens within 2 days (48 hours) of getting the vaccine. It usually goes away within 2 days.

You are more likely to get a fever after the second dose of your vaccine.

If you feel uncomfortable, take paracetamol or ibuprofen following the instructions on the box or leaflet. 

If your fever starts more than 2 days after you are vaccinated, or lasts longer than 2 days, you should self-isolate (stay in your room). Phone your GP to arrange a COVID-19 test.

Updates on COVID-19 vaccine side effects

The COVID-19 vaccines have gone through the same clinical trials and safety checks as all other licensed vaccines. But these vaccines are new and long-term side effect information is limited.

As more people in Ireland and around the world get vaccinated, more information on side effects may become available.

We will update this page with any new information.

If you develop COVID-19 symptoms after your vaccine

COVID-19 vaccines cannot give you COVID-19.

It is possible to have caught COVID-19 before getting your vaccine and not realise you have the symptoms until after your vaccination.

If you have symptoms after the first dose of your COVID-19 vaccine, you still need to have the second dose. While you may get some protection from the first dose, having the second dose will give you the best protection against the virus.

Self-isolate and phone a GP to arrange a COVID-19 test if you have a fever which:

  • starts more than 2 days after you get your vaccine
  • lasts longer than 2 days after you get your vaccine

If you have any common symptoms of COVID-19, it's important to self-isolate (stay in your room) and arrange a test to find out if you have COVID-19.

Read about what happens during the COVID-19 test

Reporting side effects of COVID-19 vaccines

As with all vaccines, you can report suspected side effects to the Health Products Regulatory Authority (HPRA).

There are 2 ways you can report a suspected adverse reaction to a COVID-19 vaccine:

As much information as is known should be provided, and where possible, the vaccine batch number should be included. You'll find this on your vaccine record card.

You can also ask your doctor or a family member to report side effects for you.

The HPRA cannot provide clinical advice on individual cases. Talk to your doctor or pharmacist about any medical concerns you may have.

Read more about reporting vaccine side effects to the HPRA


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

Last updated: 17 December 2021 at 10.45am

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