Editorial: Increased Respiratory Infections in the 2025-2026 Influenza Season and the (A)H3N2 Virus, a Past Cause of Pandemic Influenza

During the last century, there were four influenza pandemics, and in 1968, the A(H3N2) influenza virus caused the third-largest pandemic of the 20th century (Hong Kong 'flu). On December 10, 2025, the World Health Organization (WHO) reported a global increase in seasonal influenza, predominantly driven by the H3N2 subclade K (J.2.4.1) viral variant, which is now detected in most countries. On November 20, 2025, the European Centre for Disease Prevention and Control (ECDC) published an assessment of the threat posed by circulating H3N2 subclade K in the European Union and the European Economic Area (EU/EEA), which was identified in up to 50% of cases. On January 5, 2026, the US Centers for Disease Control and Prevention (CDC) reported that the 2025-2026 influenza season, which ended on December 27, 2025, reached a 30-year high in respiratory illness incidence. The 21st century has yet to experience a pandemic-level influenza outbreak. This editorial aims to highlight the status of infections caused by the H3N2 subclade K influenza virus and highlights the importance of influenza surveillance, monitoring, and the development of effective influenza vaccines.