10 December 2025
Situation at a glance
Seasonal influenza (‘the flu’) is an acute respiratory infection caused by influenza viruses that circulate globally and year-round.
It can cause illness ranging from mild to severe, sometimes resulting in hospitalization or death.
Seasonal influenza activity has increased globally in recent months, with an increased proportion of seasonal influenza A(H3N2) viruses being detected.
This rise coincides with the onset of winter in the northern hemisphere and an increase in acute respiratory infections caused by influenza and other respiratory viruses typically observed at this time of year.
Although global activity remains within expected seasonal ranges, early increases and higher activity than typical at this time of year have been observed in some regions.
Seasonal influenza viruses, including A(H3N2) viruses, continually evolve over time.
Since August 2025, there has been a rapid increase of A(H3N2) J.2.4.1 alias K subclade viruses detected from several countries based on available genetic sequence data.
These subclade K viruses have several changes from related A(H3N2) viruses.
Current epidemiological data do not indicate an increase in disease severity, although this subclade marks a notable evolution in influenza A(H3N2) viruses.
Early estimates suggest that the influenza vaccine continues to provide protection against hospital attendance in both children and adults, even though its effectiveness against clinical disease during the current season remains uncertain.
Vaccines remain essential, especially for people at high risk of influenza complications and their care givers.
Even if there are some genetic differences between the circulating influenza viruses and the strains included in the vaccines, the seasonal influenza vaccine may still provide protection against drifted viruses and the other virus strains included in the vaccine.
Vaccination is still expected to protect against severe illness and remains one of the most effective public health measures.
WHO continues to monitor global influenza activity and influenza viruses, supports countries in surveillance capacity and updates guidance as needed.
Description of the situation
Globally, influenza activity has increased since October 2025 with influenza A viruses predominant among the viruses detected globally.
In many northern hemisphere countries, acute respiratory infection levels increase at this time of year.
These increases are typically caused by seasonal epidemics of respiratory pathogens such as influenza, respiratory syncytial virus (RSV) and other common respiratory viruses.
The exact timing of the onset, the duration, magnitude and the severity of each epidemic might vary by location, influenced by multiple factors such as type of circulating viruses (including influenza and other respiratory pathogens), relative population immunity and environmental conditions.
In the northern hemisphere, some countries have reported early starts to the influenza season.
In other countries, influenza activity is starting to increase, but has not yet reached the epidemic threshold.
In the southern hemisphere, some countries have had unusually long seasons compared to previous years, with virus activity remaining higher than usual in recent months.
Global influenza surveillance and monitoring is conducted through the Global Influenza Surveillance and Response System (GISRS), a WHO-coordinated network of over 160 institutions in 131 Member States.
GISRS is tasked with conducting year-round surveillance and monitoring of influenza viruses and serving as the global alert mechanism for the emergence of novel influenza viruses and other respiratory pathogens with pandemic potential.
In the northern hemisphere temperate and sub-tropical countries, areas and territories, influenza activity was generally low from June to August 2025.
Activity gradually increased in September and continued to increase through November 2025. Influenza A viruses, especially A(H3N2) viruses, predominated during this period (...).
In the southern hemisphere temperate and sub-tropical countries, areas and territories, influenza activity generally decreased from June 2025 and remained low through August.
However, a slight increase has been observed since September.
Influenza A(H1N1)pdm09 viruses predominated in June and July; however, A(H3N2) viruses have predominated since September (...).
In tropical areas, there has been sustained influenza activity from June through November.
Influenza A(H1N1)pdm09 viruses predominated through July. Since then, the proportion of influenza A(H3N2) viruses among reported detections has increased and has become predominant since the end of September (...).
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Genetic characteristics of recent seasonal influenza viruses
Influenza A(H1N1)pdm09 and influenza B/Victoria lineage viruses continue to circulate in all regions albeit at low levels.
Influenza A(H3N2) viruses
Based on genetic sequence data available in GISAID, a mixture of A(H3N2) haemagglutinin (HA) clades and subclades are currently circulating globally; however, there has been a recent and rapid rise in a particular subclade of A(H3N2), J.2.4.1 (alias subclade K Nextclade/Nextstrain nomenclature).
A(H3N2) subclade K viruses have genetically drifted from related J.2.4 viruses and have several amino acid changes in their HA in comparison.
Detections of subclade K viruses are increasing in many parts of the world, with the exception, to date, of South America.
Subclade K viruses were particularly evident from August 2025 in Australia and New Zealand and have now been detected in more than 34 countries over the last 6 months.
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Overview of seasonal influenza by WHO Region
African region
Influenza detections in the WHO African Region overall increased in October with influenza A(H3N2) predominant.
The timing and predominant virus varied by zone.
In the western part of the region, influenza detections increased in September and October with A(H3N2) predominant since October.
All seasonal subtypes have been detected continuously in the middle and eastern parts of the region.
Influenza activity peaked in May 2025 in South Africa with almost exclusively A(H3N2) detections; in recent weeks influenza activity has increased slightly but remained low.
Eastern Mediterranean Region
While influenza activity in the WHO Eastern Mediterranean Region overall increased in October with A(H3N2) viruses predominant, there were variations by zone.
In countries in the northern part of the region, influenza detections increased in October with influenza A(H1N1)pdm09 predominant and lesser proportions of influenza A(H3N2) and B virus detections reported.
In the Arabian Peninsula, influenza detections also increased in October but with influenza A(H3N2) viruses predominant.
European Region
As of 21 November 2025, reported rates of influenza-like illness (ILI) and/or acute respiratory infection (ARI) in primary care were at baseline levels for most countries and areas of the WHO European Region.
However, detections were increasing and regionally pooled test percent positivity in primary care sentinel surveillance rose above 10% in weeks 45 and 46 (ending on 15 November), marking the start of the 2025/26 influenza season for the European Region.
This was approximately four weeks earlier than the median, but not out of the ordinary, with epidemiological trends similar to those observed in the 2022/23 influenza season.
Influenza activity was variable between countries, with those in the west of the Region generally seeing earlier increases of influenza indicators compared to others.
Influenza admissions, detections, and percent positivity in hospital surveillance were also increasing from inter-seasonal levels, with a higher proportion aged 65 years or older.
A majority of influenza detections from sentinel and non-sentinel primary care and hospital surveillance systems were A(H3N2) viruses.
Region of the Americas
During the 2025 southern hemisphere season in the Americas, influenza transmission exceeded the seasonal threshold in mid-March, remaining mostly at low to moderate levels.
Circulation was driven by influenza A(H1N1)pdm09, reaching a peak positivity of 19%.
Activity then declined to low levels until the end of August, when an increase in circulation was observed, associated with influenza A(H3N2) in Brazil and Chile.
As of beginning of November, Chile remains at moderate levels of influenza A(H3N2) transmission, without evidence of increased severity or rises in outpatient consultations.
As of 4 November 2025, subclade K had not been detected in South America.
In the northern hemisphere countries of the Americas, during week 45 of 2025, seasonal influenza circulation remained low, with influenza A(H1N1)pdm09 predominating in the Caribbean and Central America.
In North America, influenza activity—although still low—was increasing, mainly driven by influenza A virus detections.
While most detections in Mexico were influenza A(H1N1)pdm09, a predominance of influenza A(H3N2) has been observed in the United States and Canada, with growing detections of the A(H3N2) subclade K.
South-East Asia Region
Influenza detections in the South-East Asia Region started increasing from June, peaked in August and since then have generally remained low with some exceptions.
During the 2025 till November, the proportion of Influenza A among all influenza viruses tested positive was 66% Influenza A(H3N2) was the predominant sub-type (43%) in transmission followed by A(H1N1)pdm09 (~20%).
In Thailand, influenza detections of predominantly A(H3N2) increased in October and November.
Influenza A(H3N2) detections also increased since July in Bangladesh and October in Sri Lanka.
While the region has seen an increase in Influenza A(H3N2), 22 sequences of subclade K have been reported in GISAID from Nepal (1), India (4) and Thailand (17) as of 30 November.
Western Pacific Region
Since the beginning of October 2025, influenza seasonal activity has increased in the Western Pacific Region.
In some countries, including Japan and the Republic of Korea, the onset of the typical seasonal influenza activity period started earlier than in previous years.
As of 9 November 2025, influenza positivity ranged from 8% to 56% in the northern hemisphere countries.
In southern hemisphere countries, influenza activity shows mixed trends; positivity has declined in Australia, remains high in New Zealand and is rapidly increasing in Fiji.
The elevated influenza activity in New Zealand and Fiji is unusual for this time of the year.
The predominant circulating influenza subtype is influenza A(H3N2), marking a shift from A(H1N1)pdm09, which predominated during the 2024-2025 northern hemisphere winter season.
The increases in influenza have predominantly been driven by the expansion of A(H3N2) subclade K, which represents 89% of sequences submitted to GISAID from the Western Pacific Region (as of 21 November 2025).
Epidemiology
Seasonal influenza (the flu) is an acute respiratory infection caused by influenza viruses that circulate globally and year-round. In temperate regions, seasonal influenza typically peaks during the winter months, whereas in tropical areas, influenza viruses can circulate year-round with seasonality and intensity that varies across countries.
There are four types of influenza viruses, types A, B, C and D. Influenza A and B viruses circulate and cause seasonal epidemics of disease:
Influenza A viruses are further classified into subtypes according to the combinations of the proteins on the surface of the virus. Currently circulating in humans are subtype A(H1N1) and A(H3N2) influenza viruses. Influenza B viruses are not classified into subtypes but can be broken down into lineages. Influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.
Influenza spreads easily between people when they cough or sneeze. Influenza disease can cause illness ranging from mild to severe, sometimes resulting in hospitalization or death. While most individuals recover within a week without need for medical care, influenza can lead to serious complication including death, especially among high-risk groups such as young children, the elderly, pregnant women and those with underlying conditions. Health and care workers are at high risk of acquiring influenza virus infection due to increased exposure to the patients, and of further spreading particularly to vulnerable individuals.
Public health response
WHO is enhancing national, regional, and global capacities for influenza preparedness and response, including:
-- continuous global monitoring of influenza viruses and disease activity;
-- issuing seasonal influenza vaccine composition recommendations for both hemispheres;
-- providing technical guidance to Member States on vaccine selection and campaign timing;
-- supporting countries in developing prevention and control strategies;
-- enhancing diagnostic capabilities and laboratory networks;
-- monitoring vaccine effectiveness and susceptibility to approved antivirals;
-- supporting disease surveillance and outbreak response activities;
-- promoting increased vaccine coverage among high-risk groups;
-- facilitating research and development of new therapeutics and countermeasures; and
-- enhancing risk communication for the onset of the influenza season.
WHO risk assessment
Seasonal influenza activity has increased globally in recent months, and influenza A(H3N2) viruses are predominant.
This rise coincides with the onset of winter in the northern hemisphere.
Epidemics and outbreaks of seasonal influenza and other circulating respiratory viruses can place significant pressure on healthcare systems.
Although global activity remains within expected seasonal ranges, early increases and higher activity than typical at this time of year have been observed in some regions.
Seasonal influenza could place significant pressure on healthcare systems even in non-temperate countries.
Genetically drifted influenza A(H3N2) viruses, known as subclade K viruses, have been detected in many countries.
While data on how well the vaccine works against clinical disease this season are still limited, vaccination is still expected to protect against severe illness and remains one of the most effective public health measures.
WHO advice
Surveillance
Due to the constantly evolving nature of influenza viruses, WHO continues to stress the importance of year-round global surveillance to detect and monitor virological, epidemiological and clinical changes associated with emerging or circulating influenza viruses that may affect human health and timely virus sharing for risk assessment. Countries are encouraged to remain vigilant to the threat of influenza viruses and review any unusual epidemiological patterns.
WHO advises Member States to maintain surveillance for respiratory pathogens through an integrated approach, considering country context, priorities, resources and capacities. WHO has published guidance on integrated respiratory virus surveillance. WHO has also updated guidance on assessing influenza epidemic and pandemic severity, including the impact on healthcare facilities.
Clinical management and prophylaxis
Clinical care for seasonal influenza focuses on identifying illness severity, assessing risk of progression, and linking to definitive care. Most cases are mild and self-limiting, but severe disease, marked by respiratory distress, sepsis, acute respiratory distress syndrome or multi-organ failure, requires urgent supportive care and often hospitalization. Clinical management of influenza involves high-quality supportive care—oxygen therapy, monitoring, hydration and respiratory support—and is foundational to improving outcomes, especially in severe cases.
Diagnostic testing should support rapid decision-making: nucleic acid amplification test (NAAT) is conditionally recommended for confirmation of suspected disease in severely unwell patients, while either NAAT or digital immunoassay may be used for non-severe cases, depending on context and resource availability. Testing should be performed early with the aim of identifying people in need of treatment and linking them to care, including antivirals where indicated.
Patients at high risk of progressing to severe disease are likely to benefit from antiviral to reduce their chance of admission to hospital. High-risk groups include adults ≥65 years, those with immunocompromising conditions, chronic cardiovascular, neurological or respiratory disease; malignancy, pregnancy and diabetes further elevate risk. Individuals ≥85 years or those with multiple risk factors are considered extremely high risk and might be considered for antiviral prophylaxis if exposed to influenza.
Infection prevention and control measures in health-care settings
Seasonal influenza is known to cause health care-associated infection outbreaks, in particular in long-term care facilities. WHO advises the use of syndromic screening at all entry points to health-care settings and as part of daily inpatient assessment to ensure that patients with suspected or confirmed infections that are transmissible in health-care settings, including influenza, are identified as soon as possible and that appropriate transmission-based precautions are implemented. WHO advises the use of droplet precautions when caring for patients with suspected or confirmed influenza. This includes appropriate patient placement (isolation) of suspected or confirmed cases, and the use of a medical mask by all health and care workers and visitors when caring for patients with suspected or confirmed influenza.
Appropriate risk assessment for additional personal protective equipment (e.g. eye protection, filtering facepiece respirators, gown, gloves) should be performed by health and care workers when caring for patients with influenza.
Increased risk of influenza transmission may occur instances where care activities or patient symptoms are likely to generate splashes or sprays of blood, body fluids, secretions and excretions onto mucosa of eyes, nose or mouth; or if in close contact with a patient with respiratory symptoms (e.g. coughing/sneezing) and sprays of secretions may reach the mucosa of eyes, nose or mouth directly, or indirectly via contaminated hands. When performing an aerosol-generating procedure on patients with suspected or confirmed influenza, patient placement in an airborne infection isolation room as well as airborne and contact precautions with eye protection are advised.
Vaccination
Vaccination is the best way to prevent influenza disease. Safe and effective vaccines have been used for more than 60 years. Influenza viruses are constantly changing, so the composition of the seasonal influenza vaccine is regularly updated to contain viruses that are more related to those circulating. WHO, through the Global Influenza Programme and GISRS, in collaboration with partners, continuously monitors influenza viruses and activity globally and recommends seasonal influenza vaccine compositions in February and September for the following northern and southern hemisphere influenza seasons, respectively.
WHO recommends annual vaccination for high-risk groups, including health and care workers. People should ideally get vaccinated just before the influenza season begins for the most effective coverage, although getting vaccinated at any time during the influenza season can still help prevent flu infections. While the effectiveness of the vaccine may vary across seasons and risk groups, it reduces disease severity and lowers the chance of complications and death. Vaccination is especially important for people at high risk of influenza complications and their caregivers.
Genetic changes or drift can occur in the circulating influenza viruses before or during the influenza season, including during the time between vaccine strain selection and the influenza season. Even if there are some genetic differences between the circulating influenza viruses and the strains that are included in the vaccines, the seasonal influenza vaccine may still provide protection against drifted viruses. Current vaccines include three influenza viruses: influenza A(H1N1)pdm09, influenza A(H3N2) and influenza B/Victoria lineage viruses. Therefore, circulation of a drifted virus does not always result in seasonal influenza vaccines being less effective in offering protection against influenza associated illness.
As of now, it remains unclear how the vaccine will protect against clinical disease during this current season. However, early vaccine effectiveness estimates show the current vaccine is 70 to 75% effective at preventing hospital attendance in children aged 2 to 17 years and 30 to 40% effective in adults.[1],[2]
Public health and social measures in the community
The implementation of appropriate and proportionate public health and social measures (PHSM) is an essential component in the overall response to seasonal influenza epidemics.
Measures such as performing hand hygiene, respiratory hygiene and cough etiquette as well as voluntary self-isolation and mask wearing of individuals who are symptomatic or have tested positive for influenza viruses can reduce transmission of influenza viruses.
Countries should consider developing a plan to scale up additional PHSM in the event of high or extraordinarily high epidemics.
Risk communication and community engagement
Member States should consider to update and strengthen their risk communication and community engagement (RCCE) strategy integrating respiratory viruses. Enhanced risk communication and community engagement approach support empowerment of individuals to make informed decisions, countering misinformation, and community-led protection strategies.
Clear, regular, evidence-based, culturally acceptable and context adapted RCCE approaches are essential for building and maintaining trust with the concerned and affected populations to ensure adoption of interventions, practices and behaviours. For RCCE efforts to be successful, it is vital that national policies for RCCE incorporate community engagement and feedback mechanisms that acknowledge and address contextual challenges faced by different population groups, particularly those made most vulnerable. The integration of RCCE approaches to promote vaccination against influenza is also recommended.
WHO does not recommend any restriction on travel to or trade with the countries named in this report, based on the information available on the current event.
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Source:
Link: https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON586
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