Flu symptoms in 2026 are the classic ones: a fever or chills that hit suddenly, body aches, a dry cough, sore throat, headache and a tiredness that keeps you in bed. What’s new this season is the vaccine, which has been updated to target the H3N2 strain known as “subclade K” that spread widely last winter.
What flu looks like in the 2026-27 season
Flu viruses change a little every year, but the illness they cause looks much the same. According to the CDC’s flu symptoms page, flu usually comes on suddenly and can cause:
- Fever or chills (not everyone with flu gets a fever)
- Cough
- Sore throat
- Runny or stuffy nose
- Muscle or body aches
- Headache
- Fatigue
- Vomiting and diarrhoea, which are more common in children than adults
The NHS adds difficulty sleeping, loss of appetite and, in children, ear pain or seeming less active than usual.
The reason this season gets extra attention is subclade K. The World Health Organization described it as a notably different H3N2 variant, classified as J.2.4.1, that emerged in August 2025 and spread rapidly. The CDC’s 2026-2027 season page says the new vaccine includes a change to protect against it after it spread widely during the 2025-2026 season.
Flu, cold or COVID? How to tell them apart
All three can cause a cough, sore throat and blocked nose. Speed of onset and how wiped out you feel are good clues, but the CDC is clear that you can’t tell flu from COVID-19 by symptoms alone. A test is the only way to know, and some tests check for both at once.
| Feature | Flu | Common cold | COVID-19 |
|---|---|---|---|
| How it starts | Suddenly, within a few hours; 1 to 4 days after infection | Gradually, over 1 to 2 days | Often more slowly than flu; 2 to 5 days after infection, up to 14 |
| Where you feel it | Whole body | Mainly nose and throat | Shares most flu symptoms; a test is needed to tell them apart |
| Energy | Exhausting; hard to carry on normally | Unwell, but usually able to manage daily life | Can’t be judged from symptoms alone |
| Loss of taste or smell | Possible but less common | Not on the NHS list of cold symptoms | More common than with flu |
| Most contagious | First 3 days of illness | Until all symptoms have gone, often 1 to 2 weeks | On average about 8 days after symptoms begin |
| Usual length | Most people recover within a week; cough can last 2 weeks or more | About 1 to 2 weeks | Varies; some people develop long COVID |
Sources: NHS (flu and common cold pages), CDC and WHO. Our guide to when flu season starts and how to protect yourself covers the seasonal pattern in more detail.
How flu usually plays out, day by day
Based on CDC and WHO information, the timeline tends to look like this:
- Before you feel ill: Symptoms typically begin about two days after you catch the virus, though it can be anywhere from one to four days. You can already pass flu on from about one day before symptoms start.
- Days 1 to 3: Symptoms arrive fast. Fever, aches and exhaustion are usually at their worst. This is also when you’re most contagious, and it’s the window when antiviral medicine works best.
- Days 4 to 7: Most people start to turn the corner. WHO says most people recover on their own within a week.
- Week 2 and beyond: A cough and tiredness can hang around. WHO notes the cough can be severe and can last two weeks or more.
One pattern to watch closely: if you start to feel better and then your fever or cough comes back or gets worse, the CDC lists that as an emergency warning sign. It can mean a complication such as pneumonia. Our piece on early signs of pneumonia in older adults explains what to look for in older relatives.
How long are you contagious with flu?
The CDC’s page on how flu spreads says people are most contagious during the first three days of their illness. The virus can be detected from one day before symptoms start and for up to five to seven days after you become sick. Young children and people with weakened immune systems may be contagious for longer.
The NHS says you’re most likely to pass flu on in the first five days. Its advice is to stay at home and avoid other people while you have a high temperature, feel hot, cold or shivery, or can’t do your normal activities.
Who is at higher risk of serious flu?
For some groups, flu can lead to hospital. The CDC’s list of people at higher risk includes:
- Adults 65 and older
- Children under 5, and especially those under 2
- Pregnant women, and up to 2 weeks after the end of pregnancy
- People living in nursing homes and other long-term care facilities
- People with asthma or chronic lung disease
- People with heart disease, endocrine or metabolic conditions, kidney, liver or blood disorders, or neurological conditions
- People with a body mass index of 40 or higher
- People with weakened immune systems from illness or treatment, and people who have had a stroke
- Anyone under 19 on long-term aspirin or salicylate medicines
If your child has asthma, flu can set off a flare. Our guide to asthma warning signs in children explains what laboured breathing looks like.
Antiviral treatment: why the first 48 hours matter
The CDC’s antiviral guidance says they work best when started within one to two days of symptoms beginning, and that they can lessen symptoms and shorten illness by about a day.
Four antivirals are recommended by the CDC this season: oseltamivir (Tamiflu), zanamivir (Relenza), peramivir (Rapivab) and baloxavir marboxil (Xofluza). They usually need a prescription, and which ones are available depends on where you live.
The CDC says antivirals should be started as soon as possible for people in hospital with flu, people who are very sick, and anyone at higher risk of complications. If you’ve missed the 48-hour window, don’t assume it’s too late: starting later can still help, particularly for higher-risk people. Antibiotics, on the other hand, don’t work against flu because it’s a virus.
Practical tip: if you’re in a higher-risk group and flu symptoms start, contact your doctor or pharmacist the same day rather than waiting to see if it passes.
The 2026-27 flu vaccine: what it covers
In February 2026, WHO experts chose the strains for northern hemisphere vaccines. The WHO recommendation for the 2026-2027 season covers three viruses:
| Component | Egg-based vaccines | Cell, recombinant or nucleic acid vaccines |
|---|---|---|
| Influenza A (H1N1) | A/Missouri/11/2025-like | A/Missouri/11/2025-like |
| Influenza A (H3N2) | A/Darwin/1454/2025-like | A/Darwin/1415/2025-like |
| Influenza B (Victoria lineage) | B/Tokyo/EIS13-175/2025-like | B/Pennsylvania/14/2025-like |
What that means in practice:
- The vaccine is trivalent (three strains). The CDC says 100% of the projected US supply for 2026-2027 is trivalent.
- There’s no B/Yamagata strain. WHO notes that no B/Yamagata lineage viruses have been recorded since March 2020.
- The H3N2 part was changed to protect against subclade K.
- Flu vaccines are available for people aged 6 months and older, according to the CDC.
Programmes and eligible groups differ between countries, so ask your doctor or pharmacist what’s offered where you live.
Looking after yourself at home
Most people get through flu at home. The NHS flu guidance recommends:
- Rest and sleep as much as you need.
- Drink plenty of fluids. Pale yellow or clear urine is a sign you’re drinking enough.
- Paracetamol or ibuprofen to bring down a fever and ease aches. Check the label for children’s doses.
- Don’t double up on paracetamol. Many cold and flu remedies already contain it, so check before combining them.
- No aspirin for under-16s.
- Ask a pharmacist about which products suit you, especially if you take other medicines.
For a cough that lingers after the fever has gone, our guide to cough types and relief covers what helps.
When to see a doctor or go to hospital
Call your local emergency number or go to hospital straight away if an adult has any of these, based on the CDC’s emergency warning signs and NHS advice:
- Difficulty breathing or shortness of breath
- Persistent chest or abdominal pain or pressure, or sudden chest pain
- Coughing up blood
- Persistent dizziness, confusion or being hard to wake
- Seizures
- Not passing urine
- Severe muscle pain, severe weakness or unsteadiness
- Fever or cough that improves, then returns or gets worse
- A long-term condition that’s getting worse
Get emergency help for a child who has:
- Fast or difficult breathing, or ribs pulling in with each breath
- Bluish lips or face
- Chest pain
- Muscle pain so severe they refuse to walk
- Signs of dehydration: no urine for 8 hours, a dry mouth, no tears when crying
- Not alert or not responding when awake
- Seizures
- A fever above 40°C (104°F) that medicine doesn’t bring down
- Any fever in a baby under 12 weeks old
- Fever or cough that improves, then comes back or gets worse
Book an urgent appointment with your doctor if you’re worried about a baby or child, you’re 65 or over, pregnant or recently gave birth, you have a long-term condition or weakened immune system, you feel very unwell or short of breath, or your symptoms haven’t improved after 7 days.
Frequently asked questions
What are the flu symptoms for 2026?
The usual ones: sudden fever or chills, body aches, cough, sore throat, runny nose, headache and exhaustion. Children may also vomit or have diarrhoea.
How long is the flu contagious?
From about one day before symptoms start to five to seven days after, with the first three days being the most contagious. Young children and people with weakened immune systems can spread it for longer.
How long does the flu last?
Most people recover within a week, according to WHO, though a cough can last two weeks or more. See a doctor if you’re not improving after 7 days.
Does the 2026-27 flu shot cover subclade K?
Yes, its H3N2 component was updated to protect against subclade K, the variant that spread widely in the 2025-26 season.
Can I take Tamiflu after 48 hours?
Antivirals work best within one to two days of symptoms starting, but the CDC says starting later can still help, especially for people at higher risk of complications. Ask your doctor.
This article is for general information and is not a substitute for advice from a doctor. If you have symptoms or a health condition, please consult a qualified healthcare professional.
