COVID-19 Vaccine Side Effects: What Is Normal, What Is Rare and When to Get Help

A sore arm and a day or two of feeling tired are the most common things people notice after a COVID-19 vaccine. Most reactions are a sign of the immune system responding, and they pass within a few days. A handful of rarer effects are worth knowing about in advance, mainly so that you can tell the difference between what is expected and what needs a call to a doctor.

This guide sets out how often each side effect happens, how long it usually lasts, what helps and when to get help. It is part of our guide to the 2026/27 COVID-19 booster, which covers what is in this season’s vaccine and how well it protects.

How common each reaction is

The figures below come from the official product information for the mRNA vaccine we use, which draws on clinical trials and on reports collected since the vaccine came into wide use. The frequency bands are the standard ones used for every medicine in the UK.

Source: Summary of Product Characteristics for the XFG-adapted 30 microgram mRNA vaccine, section 4.8 (electronic medicines compendium).

In the original trials of the mRNA vaccines in adults, more than 80% had a sore arm, more than 60% felt tired and more than half had a headache, and a raised temperature affected one in ten to one in five, mostly after the second dose. Reactions bad enough to stop people going about their day were much less common: severe tiredness affected around 4% and a severe headache around 2%.

After this season’s single-dose booster

Most people having a booster now have had several doses and at least one infection, and the reactions they report are similar or milder. In a study of a recent single-dose updated booster, more than 60% had a sore arm, more than 30% felt tired and more than 20% had a headache. Reactions also become less frequent with age. Changing from one vaccine product to another can bring a stronger reaction than staying with the same one, and our guide to the types of COVID-19 vaccine explains how the products differ.

What helps in the first few days

Most reactions need nothing more than time. Rest if you feel tired, drink plenty of fluids and keep the vaccinated arm moving gently. If you need to, take a painkiller such as paracetamol at the dose on the packet. If you reacted strongly last time, avoid booking the day before something you cannot miss.

Some side effects can affect your ability to drive or use machinery for a short time. If you feel dizzy, faint or unusually tired, wait until you feel normal again before you drive.

Feeling faint, light-headed or shaky around the time of the injection is usually a stress reaction to the vaccination itself, and it passes on its own. If you have fainted with injections before, tell whoever is vaccinating you.

Rarer effects to know about

Heart inflammation

Myocarditis is inflammation of the heart muscle and pericarditis is inflammation of the lining around the heart. Both are recognised, rare effects of the mRNA vaccines. They have been seen most often in young men, usually within a few days of vaccination, and more often after a second dose than a first. Large European studies estimated around 3 to 6 extra cases for every 100,000 second doses given to young men in their teens and twenties, and reports after booster doses have been few. Most people recover, although a small number of cases have been severe.

Severe allergic reactions

Anaphylaxis, a sudden and severe allergic reaction, is very rare. It usually comes on soon after the injection, which is why every vaccination site has the training and equipment to treat it. Milder allergic reactions, such as an itchy rash, are uncommon, and hives are rare.

Other rare reports

Temporary drooping on one side of the face, known as facial palsy, is listed as rare. Heavier periods have been reported, in most cases temporary and not serious. Erythema multiforme, a rash of red, target-shaped patches, has been reported uncommonly and is generally mild and settles on its own. Facial swelling has been reported in people with dermal fillers. Guillain-Barré syndrome, a rare nerve condition, has been reported in very small numbers, but no more often than expected by chance.

When to get help

If you are unsure whether a symptom is related to the vaccine, ask, and mention the date of your vaccine to whoever you speak to.

Reporting a side effect

The black triangle printed on COVID-19 vaccine information marks a medicine under additional monitoring by the MHRA, the UK medicines regulator. It wants to hear about any suspected side effect, minor as well as serious. You can report through the Yellow Card scheme online or in the Yellow Card app. A suspicion is enough; you do not need proof that the vaccine caused the problem.

What a reaction means for your next dose

A reaction this time rarely rules out future doses. National guidance sorts reactions broadly as follows.

Tell us about any previous reaction when you book. Our guide to COVID-19 vaccination in pregnancy, with a weakened immune system or with allergies covers allergy in more detail.

How to get started

1. Book online. Use the booking calendar on this page or call 0131 563 6824. See our COVID-19 vaccination page and vaccine prices page for details.
2. Tell us about past reactions. Your health check covers allergies, earlier vaccine reactions, heart inflammation, blood thinners and your immune system.
3. Attend your appointment. If the vaccine is right for you, it is given as a single injection into the upper arm.
4. Leave with aftercare advice. We explain what is normal and which symptoms mean you should get help.

COVID-19 vaccine side effects: your questions answered

How long do side effects from the COVID-19 vaccine usually last?

Most are gone within a day or two, and nearly all within a few days. If a reaction is building rather than fading after a few days, or something worries you, get advice from us, your GP or NHS 24 on 111.

Is a stronger reaction than last time a bad sign?

Not usually. Reactions vary from dose to dose and from person to person, and changing from one vaccine product to another can bring a stronger reaction than staying with the same one. A heavier reaction that settles within a few days is not a sign that anything is wrong.

Can I take a painkiller after my vaccine?

Yes, if you need one. Paracetamol at the dose on the packet is a sensible choice for a sore arm, a headache or a raised temperature. There is no need to take anything before your appointment.

My armpit is swollen on the injected side. Is that normal?

Yes. Swollen glands in the armpit, or sometimes the neck, on the same side as the injection are a common sign of the immune system responding, and they usually settle on their own. If the swelling does not go down or you are worried about it, speak to your GP.

If I have no side effects, has the vaccine still worked?

Yes, it can have. Older adults tend to have fewer reactions, and many people in the trials had little or no reaction at all. Having no side effects is not a sign that the vaccine has failed.

Is a heavier period after the vaccine anything to worry about?

Some people notice a heavier period after vaccination, and in most cases it is temporary and not serious. Speak to your GP if bleeding is very heavy or keeps happening, or if you have any bleeding after the menopause.

For the wider picture, read our guide to the 2026/27 COVID-19 booster.

This page gives general information and is not a substitute for a personal clinical assessment. COVID-19 vaccines are prescription-only medicines, so we check that vaccination suits you before giving one. For side effects, speak to your pharmacist or GP, or call NHS 24 on 111. In an emergency, call 999.

Sources: UKHSA, Immunisation against infectious disease (the Green Book), chapter 14a, COVID-19, 10 September 2026; Summary of Product Characteristics for the XFG-adapted 30 microgram mRNA vaccine (electronic medicines compendium); MHRA Yellow Card scheme.