Seasonal influenza is surging earlier than expected this year, driven by a highly contagious strain of the H3N2 virus that is causing sharp increases in cases and hospitalizations across the Northern Hemisphere, according to microbiology expert Athanasios Tsakris.
Each winter, flu outbreaks are mainly caused by three variants: influenza A subtypes H1N1 and H3N2, and influenza B. While H1N1 and H3N2 often circulate together, one typically dominates. This season, an H3N2 strain called subclade K has taken hold, triggering a major epidemic wave three to four weeks earlier than usual, the professor explains in an analysis in Kathimerini.
In the UK, hospitals reported record flu-related admissions in December, averaging 2,660 patients per day with symptoms including sudden malaise, high fever, chills, muscle and joint pain, headache, sore throat, runny nose and persistent dry cough. New York City officials said flu cases reached the highest level in a decade. Greece has seen a particularly strong surge, unusually early, and even the Southern Hemisphere is reporting higher-than-expected flu activity.
Subclade K is more transmissible because of its genetic diversity. It carries 10 new mutations in the hemagglutinin gene compared with earlier H3N2 strains, reducing the immune system’s ability to recognize it. This has contributed to earlier and more intense outbreaks and increased hospitalizations, though there is no evidence it causes higher mortality.
H3N2 generally causes more severe illness in vulnerable groups, including older adults and people with chronic conditions. About 90% of cases involve people under 59, but most hospitalizations are among those over 69 or children under 14. Infants, young children and pregnant women are also at higher risk.
This year’s flu vaccine was formulated before subclade K became dominant, so the match is not exact.
Tsakris is a professor of microbiology, director of the Microbiology Laboratory at Athens University’s School of Medicine and president of the Scientific Council of the Hellenic Foundation for Research and Innovation (ELIDEK).