Flu surge driven by new H3N2 subclade K prompts school closures and masking across U.S.

A new wave of influenza has swept across the United States earlier than usual, driven by a dangerous H3N2 subclade K. Health officials say cases are skyrocketing in several large cities, prompting schools to close and hospitals to reinstate masking and other precautions.

CDC data dated December 6 show very high influenza transmission in New York City, with high activity in surrounding New Jersey and New York state. The data also show Colorado and Louisiana reporting high levels, with moderate activity in Idaho and Connecticut. The surges, which come weeks ahead of typical holiday spikes, are thought to be largely driven by H3N2 subclade K, which is rising in more than three dozen states. New York City's acting health commissioner Dr. Michelle Morse said the city's cases are skyrocketing and urged residents to get their annual flu shot and to wear a mask if symptoms develop.

Poly Prep Country Day School in Brooklyn closed its Dyker Heights campus for two days last week after at least 200 students were absent or sent home with flu or flu-like symptoms. Earlier this month, Hackensack Meridian Health and RWJ Barnabas Health in New Jersey reinstated masking protocols for patients, visitors and healthcare workers. Detroit Medical Center also restricted visitor access to protect patients and staff from the spreading virus.

In Missouri and Iowa, other schools were forced to shut down or cancel classes for several days due to high flu rates. Villa Duchesne Catholic School in St. Louis, Missouri, and the Moulton-Udell school district in Iowa were among those affected.

In Texas, health officials reported that the week ending December 6 was the third in a row to surpass 5,000 hospital and clinic visits for influenza, a figure that doubles the amount seen last year. During the same period, 7.4 per 100,000 people over age 65 were hospitalized, up from 2.2 per 100,000 last month. The Hawaii Department of Health also reported an increase in 911 calls for respiratory-related and flu-like symptoms; Garrett Hall, the department’s acting state EMS Chief, said responders recorded 332 respiratory-related encounters for the week ending December 4, roughly a 45 percent increase compared with June.

Nationwide, 8.1 percent of flu tests were positive during the week of December 6, up from 7.1 percent the week before. The rate of outpatient respiratory illness rose from 2.9 percent to 3.2 percent. About 3 million people have contracted the illness so far, and there have been 1,200 deaths from the flu this season. The influenza hospitalization rate was 6.9 per 100,000 people nationwide, up from 4.3 per 100,000 the week before. At this time last year, 5.1 percent of tests were positive, and the hospitalization rate was 3.1 per 100,000. The disease typically peaks between December and February after winter holidays.

The season’s dominant flu strain is a brand-new H3N2 subclade K, which has left many people with little immunity and a higher risk of severe illness. Public health officials are urging vigilance for classic flu symptoms, which may appear abruptly and include fever, fatigue, chills, headache, body aches, congestion, runny nose and coughing. While influenza A accounts for about three of four flu cases, vaccination remains the primary tool for prevention, with current estimates of vaccine effectiveness typically in the 40 to 60 percent range depending on the strain and population.

Health experts emphasize that the situation is unfolding rapidly and vary by region, underscoring the importance of vaccination, masking in high-risk settings, and staying home when sick. Officials also note that healthcare systems are adapting to an earlier-than-expected surge, which could strain capacity during the peak months of December through February. As communities continue to monitor CDC data and local guidance, residents are urged to follow public health recommendations and seek care if flu symptoms worsen or fail to improve.