Flu Jab Side Effects: What Is Normal and When to Get Help

Worry about feeling rough afterwards is one of the most common reasons people give for skipping the flu jab. The reality, for most people, is a sore arm and perhaps a day of feeling a little flat. Serious reactions are very rare. Knowing what is normal, how long it lasts and what is not normal makes the decision easier.

This page sets out the side effects of the flu vaccines used in the UK in 2026/27, how often they occur, how to manage them, and when to seek help. It is part of our complete guide to flu vaccines for 2026/27.

What most people notice after a flu jab

National guidance lists the reactions people most often report after an injected flu vaccine. Where the needle went in, they are soreness, swelling and redness. Elsewhere in the body, they are a low temperature, a general sense of being unwell, shivering, tiredness, headache and aching muscles or joints. Some people also notice induration, a firm, painless lump under the skin at the injection site.

Each of these reflects the immune system reacting to the vaccine rather than any infection, and they normally fade by themselves within 48 hours. Many people notice nothing more than an arm that feels bruised when they lie on it that night.

How common are flu jab side effects?

The clearest figures come from the clinical trials summarised in each vaccine’s product information. For the cell-based trivalent vaccine, which is used for most adults aged 18 to 64 in Scotland this season, the most commonly reported reactions in adults were:

For the adjuvanted vaccine offered to people aged 65 and over, injection site pain was reported by between 16% and 32% of older adults across two trials, with tiredness and headache each reported by roughly 10% to 16%. In adults aged 50 to 64 the same vaccine caused more reactions, with injection site pain in almost half and tiredness in nearly a third, probably reflecting a stronger immune response at that age. Bruising at the site is less common.

These figures describe how often people reported a symptom at all, most of them mild. They are not a measure of how unwell people felt.

Side effects of the children’s nasal spray

The nasal spray used for children works differently, so its side effects are different. A blocked or runny nose is the reaction parents notice most, along with a poorer appetite, tiredness or weakness, and headache. These usually last a few days at most. Because the vaccine viruses are weakened and cold-adapted, they cannot cause flu in the lungs.

In Scotland the children’s flu vaccine is given through the NHS, at school or after an invitation from NHS Scotland. If your child is due a vaccine, the information sent home with the consent form explains what to expect.

Can the flu jab give you flu?

No. Every injected flu vaccine in UK use is inactivated, so there is nothing in it that can replicate and cause infection. Feeling achy, tired or slightly feverish for a day is the immune response at work.

There are two other reasons people sometimes feel the vaccine made them ill. The vaccine needs around a fortnight to take effect, so an infection picked up in the days just before or after the jab can still develop. And autumn and winter bring many other respiratory viruses, including Covid-19, RSV and common colds, which the flu vaccine does not protect against. A heavy cold the week after a flu jab is usually a coincidence of timing.

How to manage side effects at home

If you are booking your flu vaccine around a busy week, it makes sense to avoid the day before a long drive, a big event or an important exam, just in case you are one of the people who feels tired for a day.

Rare and serious reactions

Any vaccine can very occasionally trigger an immediate allergic reaction, from hives or facial swelling to wheezing and anaphylaxis. These develop within minutes of the injection, so anyone giving vaccines has to be trained to manage anaphylaxis and keep the emergency medicines to hand.

Over 30 years of use, a handful of other events have been reported very rarely after flu vaccination, including nerve pain, tingling, a temporary drop in platelets and inflammation of blood vessels, without a causal link being established. Guillain-Barré syndrome, a rare nerve condition, has been reported in around one case per million people vaccinated in one US study. Other research, including a large UK analysis, found no such link with the vaccine, but a considerably stronger one with having a flu-like illness.

Side effects when two vaccines are given together

Flu vaccine is often given at the same appointment as another vaccine, such as Covid-19 or shingles. The side effects of each are expected to be similar to when they are given alone, but you may notice them in both arms, and if both vaccines contain an adjuvant the effects can add together. We tell you what to expect for each vaccine and roughly when it tends to appear. Our guide to having the flu and Covid vaccines together covers this in more detail.

When to get medical advice

If you are not sure whether something is related to your vaccine, ask. A pharmacist can help you work out whether it is a normal reaction, something unrelated or something that needs to be seen.

Reporting side effects: the Yellow Card scheme

In the UK, suspected side effects of medicines and vaccines can be reported to the Medicines and Healthcare products Regulatory Agency (MHRA) through the Yellow Card scheme, online or through the Yellow Card app. Patients and carers can report as well as healthcare professionals. Reports help the MHRA spot rare or unexpected effects that clinical trials are too small to detect.

Some of this season’s flu vaccines, including the cell-based vaccine, carry a black triangle symbol, which means they are under additional monitoring. For these, the regulator asks for all suspected reactions to be reported, even minor ones.

How to get started

1. Book online. Pick a slot in the booking calendar on this page, or phone the clinic on 0131 563 6824.
2. Answer a few health questions. Tell us about allergies and any reaction you have had to a vaccine in the past, so we can check the vaccine is suitable.
3. Attend your appointment. The vaccine is injected into the muscle of your upper arm.
4. Leave knowing what to expect. We explain which side effects are normal, how long they usually last and when to get help.

Flu jab side effects: your questions answered

How long does a sore arm last after the flu jab?

Usually one to two days. Keeping the arm moving helps. If it is getting worse after a few days, or the area becomes hot, red and swollen, get it checked.

Is it normal to feel tired or achy after the flu jab?

Yes. Tiredness, headache and aching muscles are among the commonest reactions. They are the immune system responding and usually pass within a day or two.

Can I take paracetamol after my flu jab?

Yes, if you need it for fever or aches, following the dose on the pack. There is no need to take it beforehand just in case.

Can I exercise after my flu jab?

Most people carry on as normal. If your arm is sore or you feel tired, it is sensible to take it easy for the rest of the day.

Can I drink alcohol after my flu jab?

There is no specific rule against it. If you feel feverish or tired, rest and fluids will help more.

I have had a reaction to a flu jab before. Can I have another?

It depends on the reaction. A sore arm or a day of feeling unwell is not a reason to avoid vaccination. A severe allergic reaction is, at least until you have had specialist advice. Tell us about it when you book.

Why do I feel worse after the flu jab than my friend did?

People respond differently. Younger adults tend to have more noticeable reactions than older adults, probably because their immune response is stronger. Vaccine type also plays a part.

For everything else about flu vaccination this season, see our complete guide to flu vaccines for 2026/27 and our explanation of the different types of flu vaccine.

This page is for general information and does not replace an individual clinical assessment. Flu vaccines are prescription-only medicines. If you have side effects, contact your pharmacist or GP. In an emergency, call 999.

Sources: UKHSA Green Book, chapter 19, Influenza, 25 August 2026 (adverse reactions); Summaries of Product Characteristics for the cell-based and adjuvanted trivalent influenza vaccines, section 4.8 (electronic medicines compendium); MHRA Yellow Card scheme; NHS inform.