New Flu Variant Drives Surge in Cases Across US as Vaccination Rates Drop

Unprecedented Early Surge

A mutated strain of influenza is spreading rapidly across the United States, driving a dramatic spike in flu cases months before peak season typically arrives. New York City alone reported nearly 14,000 flu cases in the first week of December, marking a roughly 460 percent increase in infections from the same time last year when the city recorded about 2,500 cases. The Centers for Disease Control and Prevention reported an 8.1% uptick in positive flu tests nationwide this week, with the new strain leading to a rise in cases and hospitalizations, particularly from the inland western states to the Northeast.

The culprit is a mutated, potentially severe strain of influenza A called H3N2 "subclade K" that has spread rapidly worldwide and already gained dominance in several countries in the Northern hemisphere. "Right now we're seeing clade K everywhere we are seeing influenza" in the U.S., said Andrew Pekosz, a professor and vice chair of the department of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health.

The variant has already caused devastating outbreaks internationally. In Australia, where subclade K was the predominant virus this year, flu hit a record with more than 443,000 cases. The strain has also caused a rise in flu cases in the United Kingdom, Canada and Japan.

Vaccine Challenges and Declining Protection

The variant wasn't a major player when scientists decided which strains should be in the annual flu shots, so the vaccines cover a related but slightly different group of viruses. Because subclade K was identified only in August—months after vaccine development—experts said it may not be an ideal match. Early findings from the UK Health Security Agency suggest the variant carries seven genetic changes on a major part of the virus, making it a bit harder for the body's immune system to recognize.

Despite the mismatch, vaccination still provides crucial protection. UK data found that the flu shot has reduced the risk of hospitalization or emergency care by about 75% in children and 30% to 40% in adults so far this season. When there's a vaccine mismatch, you might still get sick, but the vaccine decreases the likelihood that you will be hospitalized and decreases the likelihood that you will die from complications of the flu.

The timing couldn't be worse for declining vaccination rates. Retail pharmacies administered roughly 26.5 million flu shots between August and the end of October, which is more than 2 million fewer shots than the 28.7 million given over the same time frame in 2024. In Colorado, only about 25% of people have gotten their flu shots so far this fall, down from about 27% of people who had gotten their flu shots by this time last year.

Severe Season Ahead

Last winter's flu season was extreme, with the US experiencing its highest rates of flu hospitalizations in nearly 15 years and at least 280 children dying of influenza, the highest number since pediatric death numbers were required to be shared in 2004. The 2024-2025 flu season in the U.S. was classified as "a high severity" season according to the CDC, and while we don't generally see two severe flu seasons back-to-back, there are no rules in this game, so it's certainly possible we're going to have another severe season.

Symptoms are similar for both influenza A and its subclade K variant, both of which are typically more serious than influenza B, including fever, cough, sore throat, congestion, body aches, headaches and fatigue. "It's becoming evident that this is a pretty severe variant of the flu," said Dr. Neil Maniar, professor of public health practice at Boston's Northeastern University. "Certainly in other parts of the world where this variant has been prevalent, it's caused some severe illness, and we're seeing an aggressive flu season already."

Health experts emphasize that despite vaccine limitations, getting the flu shot remains the best defense. "It's not too late. Go and get your flu shot," said Dr. Adam Lauring, chief of the division of infectious diseases at the University of Michigan Medical School. With holiday gatherings approaching and the variant already circulating widely, the combination of reduced vaccination rates and an aggressive new strain creates what experts describe as a perfect storm for a challenging flu season ahead.