Flu Symptoms: How Long Flu Lasts and When to Worry

Flu gets used loosely in everyday speech. People say they have flu when they have a heavy cold, then say it was only flu when they have been genuinely floored by it. The distinction matters more than it sounds, because real influenza follows a recognisable pattern, carries a real risk of complications for some groups and has a vaccine offered free on the NHS to the people most likely to end up in hospital.

What flu actually feels like

The single most useful clue is how fast it arrives. A cold builds up over a day or two, starting in the nose and throat and staying fairly mild. Flu tends to hit suddenly, often within a few hours and the dominant sensation is whole-body rather than nasal. People describe being unable to get off the sofa, aching in their limbs and back, with a fever, a dry cough, a pounding headache and a complete loss of appetite. A runny nose and sneezing can happen, but they are rarely the main event.

Fever is the other strong signal. Adults with flu usually run a genuine temperature, often coming on quickly and sometimes with shivering and sweats. Colds occasionally produce a slightly raised temperature, though a high fever with severe aches points much more towards influenza. Our comparison of COVID-19 and the common cold covers the same distinction from the other direction and the honest conclusion in both cases is that symptoms overlap enough that you cannot always tell from feel alone.

The usual timeline

Most otherwise healthy adults follow a broadly similar course. Symptoms appear one to four days after exposure. The first two or three days are the worst, with fever and aching at their peak. By day four or five the fever generally settles and the aches ease, leaving a cough and a heavy fatigue behind. Those two symptoms are the stragglers. A flu cough can rumble on for two or three weeks after you otherwise feel well and the tiredness often takes longer to shift than people expect, which catches out anyone who assumes they will be back to normal the moment the fever breaks.

Children can run higher temperatures than adults and may have stomach symptoms such as being sick or having diarrhoea, which is less typical in adults. Older adults sometimes present less dramatically, without a marked fever, which is one reason flu is easy to underestimate in that group.

How long you are infectious

Adults are generally infectious from roughly a day before symptoms start until about five to seven days after, with the greatest risk in the first three or four days when viral shedding peaks. Children can shed the virus for longer, as can anyone with a weakened immune system. The practical implication is uncomfortable but simple: by the time you feel unwell you have already been contagious, which is why flu moves through households, offices and schools so efficiently. The same logic drives the seasonal pattern we covered in why respiratory viruses spread more in winter.

There is no legal requirement to stay off work with flu in the UK and there is no testing regime for the general public. NHS advice is to stay at home and avoid contact with other people until you feel better, which for flu usually means several days rather than one.

Looking after yourself at home

Most flu needs rest rather than treatment. Drinking enough fluids matters, particularly with a fever, since dehydration is what tips a lot of people from unwell into genuinely unwell. Paracetamol or ibuprofen will bring a temperature down and take the edge off the aching. Sleep as much as the illness demands. There is no evidence that pushing through helps and a good deal of practical experience that it lengthens the tail of fatigue.

Antibiotics do nothing against flu, because flu is a virus. They are only relevant if a bacterial complication such as pneumonia develops on top of it. Antiviral medicines such as oseltamivir do exist and can shorten the illness slightly, though they need to be started within the first day or two and in the UK they are generally reserved for people at higher risk of complications rather than offered routinely.

Who is most at risk of complications

Flu is unpleasant for almost everyone and dangerous for a minority. The groups at higher risk of serious illness are broadly the ones offered the free vaccine: adults over 65, people with long-term heart, lung, liver, kidney or neurological conditions, people with diabetes, pregnant women, anyone with a weakened immune system, people with a very high body mass index and young children. Asthma is a particular one to watch, since flu is a common trigger for a serious flare. Our guide to respiratory conditions and viral infection covers how to plan for that.

The main complications are chest infections and pneumonia, worsening of an existing condition and, less commonly, inflammation of the heart or brain. Hospital admissions for flu in the UK run into the tens of thousands in a typical winter and in a bad season flu accounts for a substantial share of the pressure on emergency departments.

The flu vaccine

The flu jab is reformulated every year, because influenza viruses change enough between seasons that last year's vaccine is not a good match for this year's strains. This is why it is an annual offer rather than a one-off course and why having had it before does not carry over. The vaccine is offered free on the NHS each autumn to the clinical risk groups above, to school-age children as a nasal spray and to frontline health and social care staff. Everyone else can pay for it at most pharmacies.

It is not a perfect vaccine and it is worth being straightforward about that. Effectiveness varies by season and by how well the chosen strains match what actually circulates and in a poorly matched year it may reduce your chance of catching flu only modestly. What it does more reliably is reduce the severity of illness if you do catch it, which is the outcome that matters most for the people offered it. You can check the current eligibility on the NHS flu vaccine pages, which are updated for each campaign.

Two points come up constantly. The injected vaccine contains no live virus, so it cannot give you flu. Feeling a bit rough or achy for a day afterwards is an immune response to the vaccine, not an infection. Having it at the same time as a COVID-19 booster is both permitted and common, often in the same appointment, which is covered in our guide to booster eligibility in the UK. Our overview of every winter vaccine in the UK and who is eligible sets the flu jab alongside the others you may be due.

Flu and COVID-19 together

Both viruses now circulate in the same months, symptoms overlap heavily and it is entirely possible to have both at once, which tends to make for a worse illness than either alone. Since free universal COVID-19 testing ended, most people have no practical way of telling which they have and for a healthy adult the answer does not change what they should do: rest, fluids, stay away from other people, seek help if warning signs appear. It matters more if you are in a risk group where antivirals might be considered, since the treatments differ. We go into this in catching COVID-19 and flu at the same time and the difference between COVID-19 and flu.

When to get medical help

Contact NHS 111 or your GP if you are in a higher-risk group and develop flu symptoms, if you are not starting to improve after about a week, if your symptoms get suddenly worse after appearing to settle, or if you are worried about a child or an older relative. Call 999 or go to A&E for difficulty breathing or breathlessness at rest, chest pain, coughing up blood, confusion or drowsiness, blue or grey lips or skin, or a child who is floppy, unresponsive or having difficulty breathing. Our list of emergency warning signs in respiratory illness applies closely here and if you are unsure where to turn, choosing between 111, a pharmacy and A&E sets out the options.

A quick recap

Flu arrives suddenly with fever and body aches rather than building up in the nose. The worst is usually over in three to five days, while the cough and fatigue can last weeks. You are infectious from just before symptoms until roughly five to seven days after. Most people need rest and fluids rather than treatment. The annual vaccine is reformulated each year and matters most for the risk groups it is offered to free.

Frequently asked questions

How can I tell flu from a bad cold?

Speed of onset and whole-body symptoms are the best guide. Flu tends to arrive within hours with fever, severe aching and exhaustion, while a cold builds gradually and stays centred on the nose and throat. Neither is a certain test.

How long should I stay off work with flu?

Until you feel better and your fever has gone, which for most people is several days. There is no fixed rule in the UK, though you are most infectious in the first three or four days.

Can the flu jab give me flu?

No. The injected vaccine contains no live virus. Mild aching or tiredness for a day afterwards is a normal immune response rather than an infection.

Why do I need a flu vaccine every year?

Influenza viruses change between seasons, so the vaccine is reformulated each year to match the strains expected to circulate. Protection from a previous year's dose also wanes.

Do antibiotics help with flu?

No. Flu is viral and antibiotics have no effect on it. They are only used if a bacterial complication such as pneumonia develops.

Can I have the flu jab and a COVID-19 booster together?

Yes. They are frequently given at the same appointment, often in different arms and doing so does not reduce how well either works.

Is it possible to have flu without a fever?

It is less typical but it happens, particularly in older adults. A lack of fever does not rule flu out, so judge by the overall picture rather than temperature alone.