Building on recent surges in Europe and the US, the World Health Organization has issued a formal alert for the 2025-2026 flu season due to the rapid global spread of influenza A(H3N2) subclade K (J.2.4.1) since August 2025. While not more severe, its advance prompts preparations in the Northern Hemisphere and anticipation in the Southern Hemisphere, including Brazil.
The WHO's alert highlights subclade K's genetic changes enhancing transmissibility, now prominent in global sequencing and driving early flu activity in Europe. Global flu remains within norms, but some regions face intensified early surges straining health systems.
In the Southern Hemisphere, the variant is absent so far, but experts warn of imminent arrival via holiday travel. "We can only imagine that this subclade will reach the country... the chance of this clade entering Brazil and spreading rapidly is very high," says Rosana Richtmann, head of infectology at Grupo Santa Joana.
Through the Global Influenza Surveillance and Response System (GISRS), involving over 130 countries, WHO data confirms vaccine effectiveness at 70-75% against child hospitalizations (ages 2-17) and 30-40% in adults, despite drifts.
Annual vaccination, hygiene, and antivirals remain key, especially with Brazil's low 2025 coverage signaling urgency for 2026 campaigns. No travel bans advised.
