Open Outbreak

Seasonal influenza

Also known as: flu, seasonal flu, influenza A, influenza B

Seasonal influenza, usually called flu, is an acute infection of the nose, throat, and lungs caused by influenza viruses. WHO says these viruses circulate globally and year-round, with activity often rising in the colder months of each hemisphere. Most people recover within about a week, but some develop severe illness, need hospital care, or die. Risk is higher for older adults, very young children, pregnant people, and people with some long-term health conditions. Influenza viruses keep changing over time, so last year’s infection or vaccine does not give perfect protection this year. Prevention relies mainly on updated seasonal vaccination, especially for people at higher risk and their caregivers. Treatment is mostly supportive, but antiviral drugs can be used, especially early in illness or for people at high risk of complications. WHO’s 2025 global update said activity had increased within expected seasonal ranges, with influenza A(H3N2) predominant in many places and early evidence that vaccine protection against hospital attendance was still present.

Key facts

TransmissionRespiratory spread, including virus-laden droplets and aerosols released when infected people breathe, talk, cough, or sneeze; close contact helps spread. Exact dominant route varies by setting.
IncubationAbout 2 days; sources here do not provide a fuller range with certainty.
Case fatalityunknown
Reproduction numberunknown
VaccineYes. Seasonal influenza vaccines are updated and WHO said in 2025 they remained expected to protect against severe illness, even when circulating viruses had some genetic differences from vaccine strains.
TreatmentSupportive care for most people; antiviral treatment is available and clinical guidelines recommend it particularly for people at high risk or with severe disease.
ReservoirHumans are the main reservoir for seasonal influenza viruses circulating in people.
Endemic regionsGlobal; influenza viruses circulate year-round, with seasonal peaks that differ by region and hemisphere.

Transmission

Seasonal influenza spreads from person to person through the air and at close range. A broad review of respiratory viruses describes transmission through droplets and smaller aerosols, meaning tiny virus-containing particles released when people breathe, speak, cough, or sneeze. WHO describes seasonal influenza as an acute respiratory infection that circulates globally and year-round, which fits this pattern of ongoing human spread. In everyday life, risk is higher in crowded indoor spaces, households, schools, workplaces, and healthcare settings, especially during the local flu season. The viruses that cause seasonal influenza change over time, so people can get flu more than once across different years. The sources used here do not give a single, settled reproduction number for all settings, so this page lists that figure as unknown rather than forcing one number that may not fit every season or country.

Symptoms and severity

Flu usually starts suddenly. A 2022 Lancet review says most people have an abrupt onset of respiratory symptoms and muscle aches, with or without fever, and recover within 1 week. Even so, influenza can be more than a bad cold. The same review notes that some people develop severe or fatal complications. WHO also says seasonal influenza can range from mild to severe and can lead to hospitalization or death. Severity is not the same for everyone. The greatest concern is usually for people at higher risk of complications, such as older adults, very young children, pregnant people, and those with certain chronic illnesses or weaker immune systems. This page does not give a single case-fatality figure because the sources in this session do not provide one reliable number for all countries and all seasons. Severity changes from season to season, by age group, and by access to care.

Treatment and vaccines

There are two main tools: vaccination to prevent severe illness, and antiviral medicines to treat some infections. The 2022 Lancet review says prevention is primarily by annual influenza vaccination. WHO’s 2025 global update says vaccines remain essential, especially for people at high risk of complications and their caregivers, and that even when viruses drift genetically, vaccination may still protect against severe illness. This matters because influenza viruses change often. For treatment, most people need rest, fluids, and symptom relief, but antivirals are part of standard care. The 2019 IDSA clinical guideline is an update on diagnosis, treatment, and chemoprophylaxis for seasonal influenza and supports the use of antiviral medications, especially for severe illness and higher-risk patients. Vaccine performance varies by season, and treatment works best when started early, so local public-health advice and timely testing can matter for people at greater risk.

Where it occurs

Seasonal influenza occurs worldwide. WHO states clearly that influenza viruses circulate globally and year-round. What changes is the timing and intensity. In many temperate countries, activity rises during the colder months, while tropical areas may have more sustained circulation. WHO’s 2025 global update described increasing activity since October 2025, with different patterns in the northern hemisphere, southern hemisphere, and tropical regions. Influenza A viruses were predominant globally during that period, especially A(H3N2), though A(H1N1)pdm09 and influenza B/Victoria lineage viruses were also still circulating at lower levels. This means there is no single “flu country” or narrow endemic zone. Instead, seasonal influenza is part of the normal global pattern of respiratory infections, with local peaks shaped by weather, population immunity, virus evolution, and how closely people mix indoors. Travel can also help new seasonal strains move between regions.

Recent history

On this site, the archive count provided for this pathogen key is 0, so there are no archived WHO notice entries filed here specifically as “seasonal influenza” since 1996. The site does, however, have a broader respiratory-season watch record that groups influenza with COVID-19 and RSV. In WHO’s own outbreak news, a key recent item is the 2025-12-10 report “Seasonal influenza - Global situation.” WHO said global influenza activity had increased in recent months but remained within expected seasonal ranges overall. Some regions saw earlier increases or longer-than-usual seasons. Influenza A(H3N2) was predominant in many places, and WHO noted a rapid rise in an A(H3N2) subclade called J.2.4.1, also called subclade K. Importantly, WHO said current epidemiological data did not indicate increased disease severity from that subclade. That combination—routine seasonal spread, virus evolution, and continued surveillance—is the main recent pattern rather than a single unusual outbreak.

What to watch

Several signals would make seasonal influenza more concerning than an ordinary flu season. One is a marked rise in severe disease, hospital admissions, or deaths compared with usual seasonal patterns. Another is the appearance of a new influenza variant or subclade that spreads widely and also shows evidence of worse illness, not just genetic change. WHO’s 2025 update noted rapid growth of an A(H3N2) subclade but said there was no sign of increased severity at that time. A third signal is reduced vaccine protection, especially against severe disease, or problems with vaccine supply before a seasonal peak. A fourth is antiviral resistance, because treatment options are limited compared with some other infections. Finally, public-health teams watch for unusual epidemiology: very early seasons, unusually long seasons, abrupt increases in new regions, or signs that a virus with animal origins is adapting to spread efficiently between people. Those are the kinds of changes that could shift concern from routine seasonal management to a broader emergency response.

Related outbreaks

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