Flu Symptoms 2026/27: How to Recognize Influenza Early

Flu (influenza) usually starts suddenly with fever, cough or sore throat and muscle aches and/or headache, and it most often lasts 5 to 7 days, according to the Robert Koch Institute (RKI, January 2026). A positive rapid test confirms influenza, a negative one does not rule it out.

Key takeaways

• A sudden start with fever, cough or sore throat and muscle aches and/or headache is typical; fever is often missing (RKI, January 2026).
• The flu usually lasts 5 to 7 days (RKI, January 2026); cough and fatigue can persist for more than 2 weeks (CDC, February 2026).
• A positive rapid antigen test confirms influenza; a negative one does not rule it out (pooled sensitivity 62.3% in a meta-analysis; Chartrand et al., 2012).
• Test as early as possible, ideally less than 4 days after illness starts (CDC, June 2026).
• Trouble breathing, chest pain, confusion or symptoms that improve and then return need urgent medical help (CDC, August 2024).

What is the flu (influenza)?

Influenza is a contagious respiratory infection caused by influenza viruses, mainly types A and B (RKI FAQ, September 15, 2026). The incubation period is short: about 1 to 3 days (RKI) or 1 to 4 days (CDC, last updated September 17, 2024). Infected people can already spread the virus before the first symptoms appear.

In temperate climates the season in the Northern Hemisphere usually runs from calendar week 40 (early October) to week 20 (mid-May), according to the RKI (FAQ, September 15, 2026). The wave usually begins around the turn of the year and lasts 3 to 4 months. In the United States influenza spreads all year, with activity peaking most often between December and February (CDC, last updated September 26, 2025). Simplified, the RKI considers a flu wave to have started when influenza A or B is found in at least 1 in 5 patient samples (RKI FAQ, September 15, 2026).

Influenza A and B

Influenza A(H1N1)pdm09, A(H3N2) and influenza B cause the seasonal epidemics in Germany. In the 2025/26 season, A(H3N2) made up 71% of sentinel detections and A(H1N1)pdm09 28%, while influenza B was found only very rarely (RKI weekly report, week 13 of 2026, data as of March 31, 2026).

Evidence on clinical differences between the types is limited. A study of 343 hospitalized children (260 with influenza A, 83 with B) found similar illness for both, with headache and abdominal pain more common in older children with influenza B (Eşki et al., 2024). In challenge studies with healthy adult volunteers, fever occurred in 37.0% to 40.6% of influenza A infections but only 7.5% of influenza B infections (Carrat et al., 2008).

The A(H3N2) subclade K variant

Subclade K (formerly J.2.4.1) appeared in August 2025 and spread rapidly worldwide, according to the WHO (February 27, 2026). By November 2025 it was detected on all continents and accounted for close to half of A(H3N2) sequences in the EU/EEA (ECDC, November 20, 2025).

Compared with the 2025/26 vaccine virus, it carries several hemagglutinin changes (ECDC, November 20, 2025). In laboratory tests, antiserum against the 2025/26 vaccine strain reacted 4 to 5 log2 steps less well with subclade K viruses (RKI weekly report, week 13 of 2026). The ECDC did not confirm more severe illness: countries in East Asia with fading waves reported no unusually high severity, and the 2025/26 vaccine was still expected to protect against severe illness (ECDC, November 20, 2025). Rapid antigen tests and standard PCR detect influenza A but do not distinguish subclades; only genome sequencing does (Hariyanto, January 2026). The 2026/27 vaccines include subclade K strains (WHO, February 27, 2026).

Main flu symptoms

Typical flu starts suddenly with fever, cough or sore throat and muscle aches and/or headache, according to the RKI (January 2026). Fever is missing in many cases: as a rule of thumb the RKI sees about one third of infections as febrile, one third as mild and one third without symptoms.

Symptoms build up within a few hours. Typical signs are:

• sudden onset, often with fever
• dry cough or sore throat
• muscle, limb, back or headache
• general weakness and sweating
• runny nose

The RKI describes nausea, vomiting and diarrhea as rare (RKI, January 2026), while the CDC notes they are more common in children than in adults (CDC, last updated August 26, 2024). Many other respiratory pathogens cause similar complaints. In an analysis of 56 challenge studies with 1,280 healthy volunteers, fever occurred in 37.0% of A(H1N1), 40.6% of A(H3N2) and 7.5% of influenza B infections, and 66.9% of infections were symptomatic (Carrat et al., 2008).

Flu symptoms in children

Children usually fall ill suddenly, and vomiting and diarrhea are more common than in adults (CDC, last updated August 26, 2024). According to the NHS (last reviewed February 3, 2026), children may also have ear pain, irritated eyes and swollen lymph nodes, and they often seem less active.

Older children with influenza B more often had headache and abdominal pain (Eşki et al., 2024).

Flu symptoms in older adults

In older people the signs are often less pronounced and resemble a cold (BIÖG, April 2018). Many older and immunocompromised people have no fever, which can make the flu easy to miss (CDC, February 2026). A worsening of a chronic condition is one of the CDC emergency warning signs (CDC, August 26, 2024). The RKI advises people over 60 to seek medical advice early (RKI FAQ, September 29, 2025).

Summer flu: is it influenza?

What is called "summer flu" is usually not influenza but an infection with typical cold viruses such as rhinoviruses or enteroviruses, according to the RKI (FAQ, September 15, 2026). From week 24 to 27 of 2026 (June 8 to July 5, 2026) no influenza viruses were detected in the practice sentinel; rhinoviruses predominated (36%), followed at a distance by parainfluenza viruses (13%) (RKI monthly report, weeks 24 to 27/2026).

From week 26 to 31 of 2026 fewer than 100 influenza cases per week were reported to the RKI (monthly report, August 5, 2026), and in weeks 32 to 35 influenza A and B were found in only 2% of sentinel samples, while rhinoviruses led with 24% (RKI monthly report, data as of September 1, 2026). At the end of August, the RKI saw first signs of rising influenza activity in some surveillance systems, at an overall low level (RKI monthly report, data as of September 1, 2026). Other respiratory viruses also circulate outside flu season; in September 2026 the RKI sentinel also found adenoviruses (RKI ARE report, September 23, 2026). See adenovirus symptoms, course and testing.

Influenza is nevertheless possible at any time of year. The RKI states that influenza and SARS-CoV-2 are detected all year round (September 15, 2026), and the WHO notes that influenza occurs year-round in tropical regions (fact sheet, February 28, 2025). So a summer infection can be influenza, but the numbers make a cold virus far more likely.

Sudden vomiting and diarrhea without much fever usually have another cause. A norovirus illness is sometimes called "stomach flu," but it has nothing to do with influenza, and most people recover within 1 to 3 days (CDC, last updated April 24, 2024). Symptoms last about 12 to 48 hours (RKI guide to norovirus, November 29, 2019).

Flu or cold: how to tell the difference

Flu begins within hours and affects more than nose and throat, while a cold builds up over 1 to 2 days, according to the NHS (last reviewed February 3, 2026). The speed of onset is a useful clue, but symptoms alone cannot confirm the flu; a positive test confirms it, while a negative rapid test does not rule it out (CDC, last updated September 17, 2024).

For RSV and COVID-19, see flu vs. RSV vs. COVID-19: symptoms compared. Flu and COVID-19 cannot be told apart by symptoms alone; a test is needed (CDC, last updated September 17, 2024).

Flu course day by day and how long it lasts

The illness usually lasts 5 to 7 days (RKI, January 2026), and an uncomplicated flu settles after 3 to 7 days (CDC, February 2026). Most people recover from fever and other symptoms within 1 week (WHO, February 28, 2025), while cough and fatigue can persist for more than 2 weeks (CDC, February 2026).

You can pass on the virus from about 1 day before symptoms start. According to the RKI, people shed virus for an average of 4 to 5 days after onset, and children often longer (RKI, January 2026); the CDC gives up to 5 to 7 days and names the first 3 days as the most contagious (CDC, September 17, 2024).

Day-by-day timeline

Symptoms rise from day 1 and peak on days 2 to 3 in challenge studies with healthy adults; there are no such data for children or older people (Carrat et al., 2008).

This timeline summarizes challenge study findings and the courses described by the RKI and CDC; it is not an official day-by-day scheme, and it does not apply unchanged to children or older adults.

When symptoms should get better

If you are not noticeably better after a few days, or you get worse after improving, see a doctor, advises the RKI (FAQ, September 29, 2025). In severe illness, deterioration often appears 3 to 10 days after symptoms start, mostly with lung complications (RKI, January 2026). The NHS advises getting urgent medical advice if there is no improvement after 7 days (February 3, 2026). A cough that drags on should also be checked, including for atypical pneumonia as a cause of prolonged cough.

Emergency warning signs

Seek urgent medical help for trouble breathing, chest pain, confusion or symptoms that improve and then return, according to the CDC (last updated August 26, 2024). In severe illness, deterioration 3 to 10 days after onset can signal lung complications (RKI, January 2026), so do not simply wait it out.

In Germany, the RKI also advises people over 60 or under 2 years, chronically ill people and pregnant women to seek medical advice early (FAQ, September 29, 2025).

Warning signs in adults

The CDC lists the following emergency warning signs in adults (last updated August 26, 2024):

• trouble breathing or shortness of breath
• persistent pain or pressure in chest or abdomen
• persistent dizziness, confusion or inability to wake up
• seizures
• not urinating
• severe muscle pain, severe weakness or unsteadiness when walking or standing
• fever or cough that improve and then return or get worse
• worsening of chronic conditions

The NHS (February 3, 2026) advises calling emergency services for sudden chest pain, severe shortness of breath or coughing up blood, and seeking urgent medical advice for shortness of breath, no improvement after 7 days, age 65 and over, pregnancy, a chronic condition or a weakened immune system.

Warning signs in children

In children, the CDC (last updated August 26, 2024) lists these emergency warning signs:

• fast or difficult breathing, or ribs pulling inward with each breath
• bluish lips or face
• chest pain
• severe muscle aches
• dehydration, for example no urine for 8 hours
• seizures
• not waking up or not interacting
• fever or cough that improve and then return or get worse
• fever above 40 °C (104 °F) that does not come down with medication
• any fever in infants under 12 weeks of age

Who is most at risk

Older people, pregnant women and people with chronic conditions have a higher risk of severe illness (RKI, January 2026). In the 2025/26 season, 96% of the 2,248 people in Germany who died with laboratory-confirmed influenza were aged 60 or older (RKI, preliminary data as of September 1, 2026), and adults 65 and over accounted for most hospital admissions in the EU/EEA that season (ECDC, February 20, 2026).

Risk groups named by the RKI include people over 60 and children under 2 years, people with chronic heart or lung disease, diabetes, immune deficiencies, neurological disease or severe obesity, and pregnant women. Pregnancy, especially later in pregnancy, raises the risk of severe illness (RKI, January 2026).

In the United States, the CDC rated the 2025/26 season as moderate overall and of high severity for children (CDC FluView, week 16 of 2026); 195 pediatric deaths had been reported as of week 37 of 2026 (CDC FluView, September 25, 2026). For people in risk groups, the RKI advises seeing a doctor early (FAQ, September 29, 2025).

Flu season 2026/27: the current situation

In week 38 of 2026 (September 14 to 20), influenza activity in Germany was very low and rhinoviruses clearly dominated, according to the RKI (report of September 23, 2026). Acute respiratory illness reached a moderate level overall; according to the GrippeWeb estimate, about 5.5 million people (6,600 per 100,000) newly fell ill that week, while influenza incidence and wastewater levels stayed very low. In the EU/EEA, influenza activity was low overall in week 37 of 2026, with detections in only a few countries (ECDC, September 18, 2026).

For comparison: the 2025/26 wave in Germany ran 15 weeks, from week 48 of 2025 to week 10 of 2026, and peaked in week 5 of 2026 (RKI weekly reports, weeks 11 and 12 of 2026). The 2026/27 season begins in week 40 (early October 2026), when the RKI resumes its detailed weekly reports (RKI ARE report, September 23, 2026). For current figures, see current flu season 2026/27: developments and testing options.

When to test for the flu

Test as early as possible, ideally less than 4 days after illness starts; a positive rapid test confirms influenza, a negative one does not rule it out (CDC, last updated June 22, 2026). The chance of a positive laboratory test falls steadily after the first 2 days of illness (RKI, January 2026).

Nasal swabs are more sensitive than throat samples (RKI, January 2026), and false-negative self-tests were more common when testing happened later than 72 hours after symptoms started (Geyer et al., 2022). If you belong to a risk group or feel worse, contact a doctor early (RKI FAQ, September 29, 2025).

Rapid antigen test vs. PCR

A classic rapid antigen test detects far fewer influenza cases than a laboratory RT-PCR, which is the reference method (Merckx et al., 2017).

In industry-funded studies, pooled sensitivity was 6.2 to 34.0 percentage points higher than in other studies (Merckx et al., 2017), so read manufacturer figures with caution. Always read the result at the time stated in your test's instructions. For more on the numbers, see sensitivity and specificity of medical tests.

What a negative result means

A negative rapid test does not rule out influenza (CDC, June 22, 2026; RKI, January 2026). Sensitivity is lower in adults than in children and lower for influenza B than for A (Chartrand et al., 2012).

If your symptoms continue or get worse, see a doctor (RKI FAQ, September 29, 2025). The authors of an independent evaluation recommend confirming a negative rapid test in people with symptoms by PCR (Meyer et al., 2025). When influenza is widespread in the population and your suspicion remains, the CDC advises considering confirmation by RT-PCR or another molecular test (June 22, 2026).

Combination tests for flu, COVID-19 and RSV

A combination antigen test checks several respiratory viruses at once, but a negative result gives you no certainty (CDC, June 22, 2026). In an independent evaluation at a university hospital, one combination test detected influenza A/B with a sensitivity of 54.3% at Ct ≤ 35, and 100% only at very high viral load (Meyer et al., 2025). The test was performed in a laboratory by trained staff using nasopharyngeal swabs, that is, under best conditions rather than as a home self-test.

You can browse combo self-tests for home use and respiratory and flu tests. Before buying, the combo test buying guide explains what to look for, and how to perform a self-test correctly covers sample collection. Under the IVDR, self-tests that are not explicitly assigned to class B fall into class C, so antigen self-tests for respiratory pathogens need a notified body (Paul-Ehrlich-Institut FAQ, November 3, 2025).

Treatment: what helps with the flu

Most people manage the flu at home with rest and fluids; antiviral drugs are prescription-only and work best when started within 48 hours (CDC, last updated September 2, 2025). Children and teenagers with flu or suspected flu should not be given aspirin or any salicylate-containing products; in rare cases this can cause Reye's syndrome (CDC, last updated August 30, 2024).

If you belong to a risk group or your condition is worsening, contact a doctor early (RKI FAQ, September 29, 2025); the CDC recommends starting treatment as early as possible for suspected or confirmed flu, without waiting for test results (March 10, 2026).

Home care

For an uncomplicated flu, recover at home and contact a doctor in good time if needed (RKI guide, January 14, 2026).

• rest and sleep as much as you need
• drink enough fluids
• do not give aspirin or salicylate-containing products to children or teenagers (CDC, August 30, 2024)
• avoid close contact with other people, especially with people at higher risk (RKI FAQ, September 15, 2026)

If symptoms do not improve within a few days or get worse again after improving, see a doctor (RKI FAQ, September 29, 2025).

Antiviral medication

Antiviral drugs shorten the illness by about 1 day and work best when started within 48 hours of symptom onset (CDC, last updated September 2, 2025). They are considered when severe illness is threatening or the risk is elevated; without risk factors, treating the symptoms is usually enough (RKI, January 2026). The CDC recommends early treatment for people who are hospitalized, have severe, complicated or progressive illness, or are at higher risk (CDC, March 10, 2026); the IDSA names, among others, children under 2, adults 65 and older, pregnant women and women up to 2 weeks after giving birth (IDSA, December 2018).

In Germany almost only oseltamivir and zanamivir are used (RKI, January 2026). Baloxavir is approved in the EU from 3 weeks of age for uncomplicated influenza and post-exposure prophylaxis, is taken once orally and, according to the RKI, was not marketed in Germany (RKI guide, therapy chapter updated November 2025). In the United States, peramivir (intravenous) and baloxavir from 5 years are also approved (CDC, March 10, 2026).

Starting early matters. In a US study of 26,233 adults hospitalized with influenza-associated pneumonia, 30-day mortality was 7.5%, 8.5% and 10.2% when treatment started on day 0, day 1 and days 2 to 5 (adjusted odds ratios 1.14 and 1.40 compared with day 0; Tenforde et al., February 15, 2025). In severe illness, treatment can still be tried later (RKI, January 2026), and a later start may still help people at high risk (CDC, September 2, 2025).

Prevention measures

When you are sick, cleaner air and ventilation, hand hygiene, a well-fitting mask, keeping your distance and testing lower the risk of passing the virus on (CDC, last updated August 18, 2025).

• let in fresh air
• wash your hands often
• wear a well-fitting mask around others
• keep your distance from others
• avoid close contact with people at higher risk (RKI FAQ, September 15, 2026)

Flu vaccination 2026/27

The STIKO recommends annual flu vaccination for everyone aged 60 and over and for defined risk groups, according to the RKI (FAQ, September 10, 2026). For people 60 and over, a high-dose or an MF-59-adjuvanted vaccine is recommended, and both are considered equivalent (RKI/STIKO FAQ, September 8, 2025).

Indication-based vaccination applies, among others, to all pregnant women from the second trimester, or from the first trimester when an underlying condition raises the risk, and to people aged 6 months and over with an increased health risk due to an underlying condition. Also named are residents of care homes, close contacts of risk groups and people with occupational exposure (RKI/STIKO FAQ, July 17, 2025; Epidemiological Bulletin 4/2026, January 22, 2026).

For 2026/27 all three vaccine strains were adjusted, including A/Darwin strains from subclade K (WHO, February 27, 2026). The Paul-Ehrlich-Institut released more than 5 million doses for the market by August 23, 2026 (August 27, 2026).

Timing: to be protected in time, get vaccinated from October to mid-December; full protection takes 10 to 14 days to build up (RKI/STIKO FAQ, September 10, 2026). If you missed the window, catching up at the start or during the wave is still worthwhile.

For the previous season, early estimates were 52% (95% CI 29 to 69) against A(H3N2) illness requiring medical attention in EU primary care (ECDC, December 19, 2025), and 72% to 75% in people under 18 and 32% to 39% in adults against emergency visits and hospitalization in England (Kirsebom et al., November 20, 2025).

Summary

Flu starts suddenly and usually lasts 5 to 7 days (RKI, January 2026), but fever is often missing. Test early: a positive rapid test confirms influenza, a negative one does not rule it out (pooled sensitivity 62.3% in a meta-analysis; Chartrand et al., 2012). Seek urgent help for trouble breathing, chest pain or confusion (CDC, August 26, 2024).

Frequently asked questions

What are the typical flu symptoms in 2026/27?

Typical is a sudden start with fever, cough or sore throat and muscle aches and/or headache (RKI, January 2026). Fever is missing in many cases, especially in older and immunocompromised people (CDC, February 2026). For subclade K, the variant behind the 2025/26 wave, the ECDC reported no unusually high severity in East Asian countries (ECDC, November 20, 2025). For the current situation, see the current flu season 2026/27: developments and testing options.

What is a "summer flu"?

It is usually not influenza but an infection with typical cold viruses such as rhinoviruses or enteroviruses (RKI, September 15, 2026). In weeks 24 to 27 of 2026 no influenza viruses were found in the practice sentinel, and rhinoviruses predominated (36%) (RKI monthly report, weeks 24 to 27/2026, July 2026). Influenza is nevertheless detected all year round, so timing alone does not rule it out (RKI FAQ, September 15, 2026).

How long does the flu last?

Most people are ill for 5 to 7 days (RKI, January 2026), and an uncomplicated flu settles after 3 to 7 days (CDC, February 2026). Most recover from fever and other symptoms within 1 week (WHO, February 2025). Cough and fatigue can persist for more than 2 weeks, especially in older people and those with chronic lung disease (CDC).

How long is the flu contagious?

You can pass on the virus from about 1 day before symptoms start. According to the RKI, people shed virus for an average of 4 to 5 days after onset, and children often longer (RKI, January 2026); the CDC gives up to 5 to 7 days after onset and names the first 3 days of illness as the most contagious (CDC, September 17, 2024). Our detailed guide explains how long you stay contagious with a cold, flu, COVID-19 or RSV and when you can return to work or school.

Flu or cold: how can I tell the difference?

The speed of onset is a useful clue. Flu comes on within hours and affects more than nose and throat, while a cold builds up over 1 to 2 days (NHS, February 3, 2026). Symptoms alone cannot tell flu from COVID-19, so a test is needed (CDC, September 17, 2024), and a negative rapid test does not rule out influenza.

Can the flu cause nausea, vomiting or diarrhea?

Nausea, vomiting and diarrhea are rare with influenza (RKI, January 2026). In children, vomiting and diarrhea occur more often than in adults (CDC, August 26, 2024), and older children with influenza B more often had abdominal pain (Eşki et al., 2024). What is called "stomach flu" is usually caused by noroviruses, not influenza (CDC, April 24, 2024).

When should you test for the flu?

Test as early as possible, ideally less than 4 days after symptoms start (CDC, June 22, 2026). The chance of a positive laboratory test falls steadily after the first 2 illness days (RKI, January 2026), and self-tests more often return false negatives after 72 hours (Geyer et al., 2022). Nasal swabs are more sensitive than throat swabs (RKI).

What does a negative rapid test mean?

It does not rule out influenza (CDC, June 22, 2026; RKI, January 2026). If symptoms persist or get worse, see a doctor (RKI FAQ, September 29, 2025). If influenza is widespread and your suspicion remains, consider confirmation by RT-PCR or another molecular test (CDC, June 22, 2026).

When should I see a doctor with flu symptoms?

Seek medical help early if you are over 60 or under 2 years, chronically ill, pregnant, or if symptoms do not improve within a few days (RKI FAQ, September 29, 2025). Emergency signs include trouble breathing, persistent chest or abdominal pain, confusion, seizures or symptoms that return after improving (CDC, August 26, 2024).

Can I have the flu and COVID-19 at the same time?

A simultaneous infection with flu and COVID-19 is possible but rare (CDC, September 17, 2024). Symptoms alone do not tell the illnesses apart, so a test is needed. For a direct comparison of symptoms with RSV and COVID-19, see flu vs. RSV vs. COVID-19: symptoms compared.

Is there such a thing as a 24-hour flu?

Not as a form of influenza. "24-hour flu" is an everyday term, not a diagnosis. Short bouts of sudden vomiting and diarrhea are often due to norovirus, which is sometimes called "stomach flu" but is unrelated to influenza (CDC, April 24, 2024); symptoms last about 12 to 48 hours (RKI, November 29, 2019), and most people recover within 1 to 3 days (CDC, April 24, 2024).

When do antiviral drugs help?

Antiviral drugs are prescription-only and work best within 48 hours after symptoms start, shortening the illness by about 1 day (CDC, September 2, 2025). They are considered when severe illness is threatening or the risk is elevated (RKI, January 2026). Without risk factors, treating the symptoms is usually enough (RKI).

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Last reviewed: September 2026.

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