Washington, D.C., 12 December 2025 (PAHO/WHO) – The Pan American Health Organization (PAHO) has issued a briefing note, updating countries on the increasing circulation of the influenza A(H3N2) subclade K (J.2.4.1) virus in several regions of the world, and to reiterate the call to strengthen surveillance and promote vaccination—particularly among older adults and people with risk factors. PAHO also urges countries to ensure timely clinical management of cases and prepare health services for the possibility of early, or more intense, respiratory disease activity.
According to the latest data, circulation of subclade K has risen rapidly in Europe and several Asian countries, where it now represents a substantial proportion of detected influenza A(H3N2) viruses. Health authorities in those countries have not reported significant changes in clinical severity; however, seasons dominated by the A(H3N2) subtype have historically been associated with greater impact among older adults.
In North America, the United States and Canada are also reporting a progressive increase in detections of subclade K. To date, similar circulation has not been observed in South America, based on data reported to the Global Initiative on Sharing All Influenza Data (GISAID).
PAHO noted that the genetic evolution observed in subclade K is part of the natural variation process of seasonal influenza viruses. Although evidence on vaccine effectiveness for the current season remains limited, preliminary data from Europe indicate that vaccination continues to provide protection comparable to previous years against severe disease, including hospitalization.
Recommendations for countries
PAHO urged Member States to maintain and strengthen epidemiological, virological, and genomic surveillance; ensure high vaccination coverage; provide timely treatment of cases; and reinforce the preparedness of health services for the possibility of early or more intense activity during the 2025–2026 season in the Northern Hemisphere.
The Organization reiterated the importance of seasonal influenza vaccination for older adults, people with chronic conditions, pregnant women, and other groups at higher risk of complications, emphasizing that vaccinating these populations also helps reduce pressure on hospitalization services.
PAHO also recalled that the recommendations outlined in the December 4 Epidemiological Alert remain in effect. These include strengthening surveillance for influenza, respiratory syncytial virus (RSV), and SARS-CoV-2; ensuring continuous reporting of data; and regularly submitting samples for sequencing to detect variants that may alter virus transmissibility or severity.
The Organization further underscored the importance of early diagnosis, reinforced prevention and control measures, availability of antivirals for at-risk groups, and the immediate investigation and reporting of unusual respiratory events, in accordance with the International Health Regulations.
Personal preventive measures—such as handwashing, covering coughs and sneezes, and staying home when experiencing fever or respiratory symptoms—remain essential to limit transmission.
