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.