Public health officials have issued urgent warnings as a highly contagious and “aggressive” new variant of the flu, known as Subclade K, tears across the United States of America. This mutation of the Influenza A (H3N2) virus is being blamed for a dramatic spike in hospitalizations and severe symptoms just as the holiday season peaks.
As of today, Saturday, 27 December 2025, the CDC reports that the variant now accounts for nearly 90% of all sequenced flu cases in the country.
Medical experts, including those from Johns Hopkins and the American Lung Association, describe this variant as particularly concerning due to its genetic evolution.
Subclade K features seven specific mutations that allow it to bypass existing immunity better than previous strains. Because this variant surged late in the Southern Hemisphere’s season, this year’s Northern Hemisphere flu shot is considered a “genetic mismatch.” While it still offers protection against severe death and hospitalization, it is less effective at preventing infection entirely.
According to growing reports, the country’s doctors are seeing “intense” symptoms that come on rapidly, including fevers reaching 104°F–105°F, persistent “phlegmy” coughs, severe joint/muscle aches, and, unusually for adults, vomiting and diarrhea.
The United States CDC’s FluView map currently shows “High” or “Very High” respiratory illness activity in 17 jurisdictions. The hardest-hit states include, New York, New Jersey, Rhode Island, Louisiana, and Colorado with other high cases in Massachusetts, Connecticut, Michigan, Maryland, New Mexico, and Georgia.
“We are at the beginning of what will probably be a pretty severe influenza season… We’re seeing Subclade K everywhere we see influenza.” – Dr. Andrew Pekosz, Johns Hopkins Bloomberg School of Public Health.
Despite the mismatch, the CDC and WHO continue to urge getting vaccinated, as they say it remains the best tool to prevent the most severe outcomes (ICU admission and death); antivirals drugs like Tamiflu are said to still be effective if taken within 48 hours of symptom onset; and monitoring children is touted as top priorities for parents as they are advised to watch for signs of dehydration and shortness of breath, as pediatric hospitalizations are on the rise.
Notably, while the “Subclade K” seasonal flu is the primary threat to the US’s general public right now, scientists are also closely monitoring a separate development: the first human case of H5N5 (Bird Flu) recently confirmed in Washington State. However, officials stress that this is currently not spreading person-to-person.
