The common side effects of the flu jab are a sore arm where the injection went in, a slightly raised temperature, tiredness, a headache and aching muscles. They’re usually mild and pass within a day or two. The injected vaccine can’t give you flu. A severe allergic reaction is very rare; call 999 if one happens. If you have not had yours yet, you can book it on our flu jab page.
What’s normal afterwards
A sore arm
The most common one. The arm can be sore, and sometimes a little red or swollen, where the injection went in. It usually settles within a day or two.
A slight temperature
A slightly raised temperature for a day or so is your immune system responding to the vaccine, not an infection.
Tired and achy
Feeling tired, a headache or aching muscles, usually gone within a day or two.
What helps
Rest, and a painkiller such as paracetamol if you need one, taking the dose on the pack. You can drive and go back to work straight away.
When it’s more than a sore arm
Side effects that are mild and settling need nothing more than rest. These are the times to get help.
Call 999
If anyone has swelling of the face, lips or throat, difficulty breathing, or collapses after a vaccination. A severe allergic reaction is very rare, and it happens soon after the injection.
Call us or NHS 111
If a high temperature lasts more than a couple of days, or you’re worried about how you feel after the vaccination.
Before your next one
If you’ve ever had a severe allergic reaction to a flu vaccine or one of its ingredients, tell us when you book, and the pharmacist will talk it through before anything is given.
Call us on 020 8393 1000.
Why you feel it, and why it isn’t flu
It can’t give you flu
The injected flu vaccines contain no live virus, so they can’t cause flu. Feeling a little off-colour afterwards is the immune system responding to the vaccine.
How the jab works
The vaccine teaches your immune system to recognise the flu viruses expected this winter, so it can fight them off if you catch one. Protection takes about two weeks to build.
A cold afterwards
Colds are caused by other viruses, which the flu jab doesn’t protect against, so a cold in the weeks after it is a coincidence rather than the vaccine.
Why it’s every year
The viruses change from one winter to the next, so the vaccine is updated every year to match, and a dose is needed every year.
Questions about flu jab side effects
How long do flu jab side effects last?
Usually a day or two. A sore arm can take a couple of days to settle. If a high temperature or feeling unwell goes on for longer, call us or NHS 111.
Can the flu jab give you flu?
No. The injected vaccines contain no live virus, so they can’t cause flu. Feeling off-colour for a day is the immune system responding to the vaccine.
Does the flu jab work?
It’s the best protection there is against flu. It isn’t perfect, because the viruses going round don’t always match the vaccine exactly, and if you do catch flu after it, it’s likely to be milder.
How does the flu vaccine work?
It teaches your immune system to recognise the flu viruses expected this winter, so it can fight them off if you catch one. Protection builds over about two weeks after the injection.
Can I take paracetamol after the flu jab?
Yes, if you need it for a sore arm or a slight temperature. Take the dose on the pack.
I had side effects last year. Should I still have it?
Mild side effects last year aren’t a reason to miss it this year. If you had a severe allergic reaction, tell us when you book and the pharmacist will talk it through before anything is given.
Sources
- NHS: flu vaccine, checked 17 September 2026
