Open Outbreak

Seasonal influenza

Also known as: flu, influenza A, influenza B, H3N2, H1N1pdm09

Seasonal influenza, usually called flu, is a viral infection of the nose, throat, and lungs. It is caused by influenza viruses that circulate around the world every year. WHO describes it as an acute respiratory infection that can range from mild illness to severe disease, including hospitalization and death. In late 2025, WHO reported rising global activity with influenza A viruses predominant, especially A(H3N2), but still within expected seasonal ranges overall. Influenza viruses keep changing over time, which is why vaccine strains are reviewed regularly and why protection from prior infection or vaccination is not complete. Annual vaccination remains one of the main public-health tools, especially for people at higher risk of complications. Antiviral drugs such as neuraminidase inhibitors exist, but a major review found they shorten symptoms only modestly in otherwise healthy adults and children. Seasonal influenza occurs globally and year-round, with clearer winter peaks in temperate regions.

Key facts

TransmissionPerson-to-person respiratory spread is established, but a precise numeric secondary attack rate is unknown from the sources retrieved here.
Incubationunknown
Case fatalityunknown
Reproduction numberunknown
VaccineYes. Seasonal influenza vaccines are updated and WHO reported in 2025 that vaccination remained expected to protect against severe illness, especially in high-risk groups.
TreatmentSupportive care plus antiviral drugs in some cases. A Cochrane review found neuraminidase inhibitors modestly reduced time to first symptom relief in uncomplicated influenza.
ReservoirHumans for seasonal strains that circulate annually; influenza viruses also evolve and related influenza viruses circulate in animals.
Endemic regionsGlobal, year-round circulation, with seasonal peaks that vary by region and hemisphere.

Transmission

Seasonal influenza spreads between people. WHO describes it as an acute respiratory infection caused by influenza viruses that circulate globally and year-round. The sources retrieved for this page do not give a single agreed number for how easily seasonal flu spreads, so a basic reproduction number is best left as unknown here. Influenza activity rises and falls with the season, especially in temperate climates during winter, but the timing is not identical everywhere. The encyclopedia source notes that symptoms usually appear about two days after infection and that local epidemics often build over several weeks, but that is a general description rather than a formal estimate for all settings. For a public reader, the main point is simple: seasonal flu is a regularly recurring human respiratory infection, and transmission increases when influenza is circulating widely in the community.

Symptoms and severity

Seasonal influenza can cause illness ranging from mild to severe. WHO’s 2025 global update says it can sometimes lead to hospitalization or death. The sources retrieved here do not provide a single overall case-fatality ratio for ordinary seasonal influenza, so that figure is listed as unknown. Severity is not constant from year to year. WHO reported that influenza A(H3N2) viruses were predominant in late 2025 and that some places were seeing earlier or higher activity than usual for that time of year. At the same time, WHO said the overall global pattern was still within expected seasonal ranges and that available data did not show increased severity from the newly rising A(H3N2) J.2.4.1, or subclade K, viruses. In plain terms, flu is often self-limited, but it is not trivial: some seasons place real pressure on hospitals, and some infections become dangerous, especially in vulnerable people.

Treatment and vaccines

Vaccines and antiviral medicines both exist, but they do different jobs. WHO says seasonal influenza vaccination remains one of the most effective public-health measures and is especially important for people at high risk of complications and for their caregivers. In the 2025 global update, WHO said early estimates suggested the vaccine was still providing protection against hospital attendance in children and adults, even though effectiveness against milder clinical disease for that season was still uncertain. For treatment, supportive care matters, and antiviral drugs may be used in some patients. A Cochrane review of neuraminidase inhibitors found that, in otherwise healthy adults, these drugs reduced the time to first alleviation of symptoms by about 16.8 hours, and in healthy children by about 29 hours, with no effect shown in children with asthma. That means treatment can help, but it is not a cure-all.

Where it occurs

Seasonal influenza occurs worldwide. WHO states that influenza viruses circulate globally and year-round. What changes is not whether flu exists, but when activity is highest and which virus types are dominant. In the northern hemisphere, activity usually rises during the colder months; in the southern hemisphere, peaks occur during that hemisphere’s winter; and tropical areas can have more sustained circulation. WHO’s 2025 report described increases from October 2025 onward, with influenza A viruses predominant globally. A(H3N2) was especially common in many places, while A(H1N1)pdm09 and influenza B/Victoria lineage viruses were also still circulating at lower levels. The report also noted that some southern hemisphere countries had unusually long seasons and some northern hemisphere countries had early starts. So the practical message is that seasonal flu is not confined to one region. It is a global infection whose timing and intensity shift with place, climate, and the strains in circulation.

Recent history

On this site’s archive, seasonal influenza has appeared in 13 WHO notices or major pandemic-related records since 1996. The archived items include routine seasonal updates, such as vaccine-season notices and reports of early or unusual influenza activity, as well as a 2025 WHO Disease Outbreak News report on the global seasonal influenza situation. That 2025 WHO update said global influenza activity had increased in recent months, with influenza A(H3N2) making up a larger share of detections. WHO also highlighted the rapid rise of A(H3N2) J.2.4.1, also called subclade K, but said available epidemiological data did not indicate increased disease severity. The archive also includes occasional notices about unusual human infections involving reassortant or variant influenza viruses, such as a seasonal reassortant A(H1N2) report from the Netherlands in 2018. Taken together, the history shows that seasonal influenza is monitored constantly because the viruses evolve and seasonal patterns can change quickly.

What to watch

The main warning signs are changes that make seasonal influenza less predictable or harder to control. WHO’s 2025 report gives a useful checklist. One signal is spread outside the usual seasonal pattern, such as early starts, unusually long seasons, or sustained high activity in new settings. Another is the appearance and rapid growth of genetically changed viruses, such as the A(H3N2) subclade K viruses noted by WHO, especially if later data show more severe disease. A third is reduced vaccine protection: WHO said early estimates still suggested protection against hospital attendance in 2025, but effectiveness against clinical disease was uncertain, so any further drop would matter. Another concern is treatment performance. The retrieved review suggests current antivirals offer only modest symptom benefit in uncomplicated disease, so evidence of antiviral resistance or poorer outcomes would be important. Finally, pressure on healthcare systems is itself a key signal, because WHO warns that seasonal epidemics can strain hospitals even when activity remains within expected ranges.

Updated