Flu activity has surged in the U.S. and Canada following a notable increase in cases around mid-December 2025.
Even though flu cases in Canada are showing a downward trend as of January 9, 2026, the United States has not yet reached its peak for the season, as indicated by data from the Centers for Disease Control and Prevention.
As an emergency room pediatrician in central Massachusetts, I’ve witnessed a dramatic rise in flu cases lately. I’ve also been in touch with colleagues across the nation, and they report experiencing a similar spike in patients with flu symptoms.
In early January, New York state saw the highest recorded number of flu cases in a single week. Other states, including Colorado, are also facing record levels of flu, with 44 out of 55 states reporting high or very high flu activity, according to the CDC.
This rise in flu cases, often referred to as a “superflu,” is attributed mainly to a new subtype of the virus called subclade K.
What’s different about the current flu season?
Flu seasons differ each year. For example, during the 2024-2025 season, cases spiked quickly starting in October. In contrast, this year’s season began more slowly but accelerated suddenly in mid-December.
By the last week of December, over 8% of doctor visits in the U.S. were for flu-related symptoms—the highest rate recorded since 2005. Emergency department visits for flu are rising sharply, especially in the South and Northeast, where figures surpass the national average.
In Canada, more than 26% of flu tests conducted during this period came back positive.
As of January 15, the CDC estimates that this flu season has resulted in 15 million illnesses, 180,000 hospitalizations, and about 7,400 deaths. A small bit of good news is that this season hasn’t coincided with significant outbreaks of RSV or COVID-19, which often complicates flu seasons.
Typically, the flu poses a greater risk to people over 65 or those with existing health conditions, with younger children usually experiencing milder symptoms. However, this year’s strain seems to be hitting children harder. They’ve been coming in with high fevers of over 104 degrees lasting several days, or with complications such as febrile seizures, croup, and severe dehydration.
The CDC reports that there is currently an unusually high number of children hospitalized due to flu. So far this season, there have been 17 pediatric deaths reported.
Last year, which was the deadliest on record for children, the flu caused 289 pediatric deaths.
What is subclade K?
Influenza subvariants evolve each year, and this year, the most common variant is subclade K, a subtype of H3N2. First detected in Australia in July 2025, it accounts for 91.5% of infections in the U.S. to date and is also driving the current peak in Canada.
This variant has proven to be particularly contagious, evidenced by the rapid increase in flu cases. Some researchers believe that mutations have made it distinct from older strains, and that lower levels of circulation in previous years may have left the population with less immunity.
At my emergency department, we’ve noted particularly high fever spikes—104 to 105 degrees—which aren’t inherently dangerous but can lead to more instances of febrile seizures and dehydration in young kids.
How late is too late to get vaccinated?
I generally encourage people to get vaccinated against the flu, as recommended by the American Academy of Pediatrics. Getting the vaccine can significantly reduce the risk of severe illness, hospitalization, and death. Those who receive it are often more likely to have milder cases.
If you’re still considering the flu shot this season, I highly recommend getting it soon. Delaying vaccination diminishes the time when it can be most effective since it takes a couple of weeks to reach maximum efficacy. If vaccinated in mid-January, you might expect peak protection by late January or early February.
Flu season typically peaks between December and February in North America.
However, it’s worth noting that the strains selected for the 2025-2026 flu vaccine don’t match up perfectly with the predominant subclade K. Still, there’s evidence that this year’s vaccine does offer some level of protection.
Should you take an at-home flu test?
In my view, at-home flu tests are generally not very beneficial.
People often seek flu tests to determine whether they should start antiviral treatments like Tamiflu. While these medications might lessen symptoms slightly, they are not particularly effective and can come with side effects like upset stomach and diarrhea. Most of us in the emergency room rarely recommend them.
Whether you have the flu or another similar illness, such as RSV or COVID-19, the recommended home treatment and when to seek medical attention remain the same.
Given the current high levels of flu, if you’re feeling unwell with fever and upper respiratory symptoms, it’s reasonable to assume it’s the flu.
However, for those with pre-existing conditions like heart disease or severe asthma, antiviral treatment may be advised, making testing more beneficial in those cases.
Basic precautions can protect you and your community
The standard precautions for all flu-like illnesses apply here, but it’s especially crucial to recognize how contagious subclade K is.
To safeguard yourself and others, frequent handwashing can help reduce the risk of exposure to flu droplets. Most importantly, individuals feeling unwell, especially those with a fever, should stay home from work or school until they’ve been fever-free for 24 hours.
