Side effects - Janssen COVID-19 vaccine

Like all medicines, vaccines can cause side effects. Most of these are mild to moderate and do not last long. Not everyone gets side effects.

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

Common side effects

More than 1 in 10 people may experience:

  • feeling tired
  • pain in the arm where they had the vaccine injection
  • headache
  • muscle pain
  • nausea

More than 1 in 100 people may have:

  • redness or swelling where they had the injection
  • joint pain
  • cough or a fever (temperature of 38 degrees Celsius or above)

Rare side effects

More than 1 in 1,000 people may have:

  • back pain
  • lack of energy
  • muscle weakness
  • pain in the limbs
  • pain in the throat
  • a rash
  • sneezing
  • sweating
  • a tremor

More than 1 in 10,000 people may have a red, itchy rash.

Very rare side effects

Very rare side effects of the Janssen vaccine include:

Very unusual blood clots

Very rarely, 1 in 300,000 people may develop very unusual blood clots with low platelets. 1 in 10 of these people may die.

The risk of this very rare condition is higher in younger people.

Talk to your GP, doctor or the person giving you your vaccination about the risks and benefits of getting this vaccine.

Most people who got this very rare side effect got it within 14 days of getting the Janssen vaccine.

Symptoms of very rare blood clots

Get urgent medical help and mention your recent vaccination if you:

  • are breathless
  • have pain in the chest or stomach
  • have swelling or coldness in a leg
  • get a severe or worsening headache (particularly 3 or more days after the vaccine)
  • have blurred vision
  • feel confused
  • have seizures
  • have persistent bleeding under the skin where there was no previous injury
  • have many small bruises, reddish or purplish spots, or blood blisters under the skin

Capillary leak syndrome

A very small number of people have experienced capillary leak syndrome after getting the Janssen vaccine.

Capillary leak syndrome is a very rare but serious condition. It can sometimes be fatal.

Do not get the Janssen vaccine if you have previously had capillary leak syndrome.

Get urgent medical attention if you have had a Janssen vaccine in the past few days and you notice:

  • rapid swelling of your arms or legs
  • sudden weight gain

You may also feel faint due to low blood pressure.

These are symptoms of capillary leak syndrome.

Guillain-Barré syndrome (GBS)

GBS is a rare and serious condition that affects the nerves. Symptoms can range from mild weakness to severe paralysis (being unable to move a part of your body). Most people eventually fully recover even from the most severe symptoms. Some may continue to experience weakness.

Getting GBS after having the Janssen vaccine is very rare. Only a small number of people have reported getting the condition after being vaccinated.

You should know the symptoms to look out for in the weeks after being vaccinated.

Get urgent medical help if you develop:

  • double vision or difficulty moving your eyes
  • difficulty swallowing, speaking, or chewing
  • co-ordination problems and unsteadiness
  • difficulty walking
  • tingling sensations in your hands and feet
  • weakness in your arms, legs, chest or face
  • problems with bladder control and bowel movements

Serious side effects

Serious side effects, like a severe allergic reaction, are extremely rare.

Your vaccinator is trained to treat very rare serious allergic reactions. If these happen, it's usually soon after your vaccination. This is why we ask you to wait in the vaccination centre for 15 minutes after you've been vaccinated.

Fever after the vaccine

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

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

If your fever lasts more than 48 hours or if you are worried, talk to your GP.

If you develop COVID-19 symptoms after your vaccine

COVID-19 vaccines cannot give you COVID-19.

You can get COVID-19 before you get your vaccine and not develop symptoms until after your vaccination.

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 COVID-19 test.

Read about what happens during the COVID-19 test

Updates on vaccine side effects

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

As more people get vaccinated, more information on side effects may become available.

We will update this page with any new information.

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:

Give as much information as you know. Include the vaccine batch number, if possible. 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 GP 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: 27 July 2021 at 1pm