Flu jab side effects are usually mild and short-lived: a sore arm, a slight temperature, aching muscles or tiredness for a day or two. The injected vaccine cannot give you flu, and most people carry on as normal. Serious reactions are very rare.
This guide explains which flu jab side effects are normal, how long they last and when to get advice. It is written by the team at Burnham Health Pharmacy for people in Burnham, Slough and the surrounding area who are planning their flu vaccination this autumn.
Common flu jab side effects and how long they last
Most people have no side effects or only mild ones, and they usually fade within one to two days.
Your body is building protection against the flu, and that immune response can cause a few temporary symptoms. They show the vaccine is doing its job. They are not a sign you have caught the flu.
Sore arm
A sore, tender or slightly swollen arm at the injection site is the most common flu vaccine side effect. It often feels like a bruise and is usually worse on the first day. If your arm is still sore after three days, that is not unusual, but it should be improving.
Keep using your arm normally. Gentle movement tends to ease stiffness faster than resting it.
Temperature, aches and tiredness
Some people notice a slight temperature, muscle aches, a mild headache or tiredness. These usually start within a day of the jab and settle within one to two days. They are generally milder than real flu, which tends to hit hard and last longer.
Normal vs call us
| Usually normal | Get advice |
| Sore, tender or slightly swollen arm | Redness, heat or swelling that keeps spreading |
| Low-grade temperature for a day or two | High fever, or a temperature that lasts more than a few days |
| Mild aches, headache or tiredness | Symptoms that are severe or getting worse |
| Symptoms improving after 24 to 48 hours | Pus or discharge at the injection site |
| Feeling a bit “off” | Breathing difficulty, swelling of the face or lips, or collapse (call 999) |
Can the flu jab give you the flu?
- The injected flu vaccine cannot give you the flu.
The flu jabs used in the UK do not contain a live virus that can cause the illness. Depending on your age, you may be offered an adjuvanted vaccine (for people aged 65 and over), or a cell-based or egg-grown inactivated vaccine. The vaccine your pharmacist gives you will be the one recommended for your age and health.
So why do some people feel unwell afterwards? There are two main reasons:
- Your immune response. Mild aches and tiredness are a normal reaction as your body learns to recognise the virus.
- Bad timing. Flu vaccine protection takes around 10 to 14 days to build. If you pick up flu, a cold or COVID around the time of your jab, it can look like the jab caused it. If you are not sure what you have, our guide to telling cold, flu and COVID symptoms apart can help.
This is also why timing matters. Our post on the best time to get a flu vaccination in the UK explains how to line up your jab before flu season peaks.
Rare but serious reactions and what to do
serious reactions to the flu jab are very rare, and they usually happen within minutes.
A severe allergic reaction (anaphylaxis) is the main concern. Signs include:
- Difficulty breathing or wheezing
- Swelling of the face, lips, tongue or throat
- A widespread itchy rash or hives
- Dizziness, faintness or collapse
Our vaccinators are trained to recognise and treat this, which is one reason we ask you to wait briefly after your jab. If symptoms start after you have left, call 999 straight away.
If you have ever had a severe reaction to a flu vaccine or one of its ingredients, tell us before you book. We will talk through whether it is safe to go ahead or whether you should be assessed elsewhere. For general information on how side effects compare across vaccines, see our post on the side effects of the pneumonia vaccine.
Who should speak to a pharmacist before the jab
most people can simply book and attend, but a quick conversation helps if you have had a reaction before, have allergies or are unwell.
Speak to us first if any of these apply:
- A previous reaction. You have had a serious reaction to a flu vaccine before.
- Allergies. This includes egg allergy. Most people with egg allergy can still have the flu jab, and egg-free and low-egg options exist for people who need them. Tell us about your allergy and we will advise on the right vaccine.
- A current fever. If you have a high temperature or feel properly unwell, it is best to wait until you have recovered. A mild cold alone is usually not a reason to delay.
- Medicines or health conditions. Let us know if you take blood thinners, have a bleeding disorder or have had Guillain-Barré syndrome.
- Pregnancy. The flu jab is recommended in pregnancy and protects both you and your baby.
Not sure whether you qualify for a free jab? Our guide on who is eligible for the NHS flu vaccine lists the groups covered this year.
Children’s nasal spray side effects
Many children are offered a nasal spray flu vaccine instead of an injection, usually through school or their GP. It is given as a spray in each nostril and contains a weakened virus that cannot cause flu.
Common side effects are a runny or blocked nose, a headache, tiredness and a reduced appetite. These usually pass within a few days. Tell the vaccinator if your child has asthma or has been wheezing, as this can affect which vaccine is suitable.
Easing flu jab side effects at home
Short answer: rest, fluids and gentle arm movement are enough for most people.
- Keep your arm moving. Light movement helps with stiffness. A cool, clean cloth on the area can ease soreness.
- Take a simple painkiller if needed. Paracetamol or ibuprofen can help with aches or a temperature, if they are suitable for you. Always follow the packet instructions and check with us if you take other medicines. Do not give aspirin to children under 16.
- Drink plenty of fluids and get some extra rest if you feel tired.
- Take it easy, but you do not have to stop. Light exercise is fine if you feel well. If you feel achy or tired, rest until you feel better.
If you are having your flu and COVID vaccines together, you may notice side effects a little more. They are still usually mild and short-lived.
Flu jab side effects: your questions answered
Is it normal to feel unwell after a flu jab?
Yes. A mild temperature, aches or tiredness for a day or two is a normal response to the vaccine. It does not mean you have the flu. If symptoms are severe, get worse or last longer than a few days, speak to a pharmacist or your GP.
Can I exercise after a flu jab?
Yes, if you feel well. Light activity can actually help a stiff arm. If you have aches, a temperature or feel tired, rest until you feel better.
Is it safe to have the flu and COVID vaccines together?
Yes. The two vaccines can be given on the same day, and you may notice side effects slightly more, such as a sore arm or tiredness. They are still usually mild and short-lived.
Can I have the flu jab with an egg allergy?
Usually yes. Most people with egg allergy can have the standard flu jab, and egg-free or low-egg options are available for those who need them. If you have had a severe reaction to egg, tell us before you book.
Why is my arm still sore after three days?
A sore arm usually improves within a few days but can linger a little. Keep moving it. If the area is getting more red, hot or swollen, or you develop a fever, get it checked.
Ready for your jab?
If you have questions about flu jab side effects or are not sure which vaccine is right for you, our team is happy to talk it through. You can book your appointment online, see how our flu vaccination service works, or call us on 01628 603 170. We are at 30 High Street, Burnham SL1 7JP, serving Burnham, Slough and nearby areas.