Subclade K: What to Know About This Year’s Flu

If it feels like everyone you know is sick right now, you aren’t imagining it. A new, highly mutated version of the flu called subclade K is driving one of the most intense flu seasons in years, with doctors’ offices and hospitals seeing record numbers of patients.

The good news? It is not too late to protect yourself. While this variant is different from what experts expected, the flu shot remains your best defense against a severe case that could land you in the hospital.

“By some metrics, this is the worst we have had in years and possibly, decades in terms of inpatient flu numbers and test positivity rates at the hospital,” says Scott Roberts, MD, a Yale Medicine infectious diseases specialist.

Subclade K is a newly emerged variant of the H3N2 subtype of influenza A. Think of it like the Omicron variant of COVID-19, but for the flu: it belongs to a familiar family but has developed seven distinct mutations that make it different from what experts predicted when this year’s vaccine was designed.

“Subclade K is within the H3N2 family. It’s not a completely new flu strain,” Dr. Roberts says.

Because it has mutated significantly, it is spreading more easily, leading to higher test positivity rates at hospitals and increased inpatient hospitalizations.

While the flu affects all ages, this season’s surge is primarily driven by children. According to CDC data, the highest incidence of infection is currently among children ages 0 to 4, and school-aged children and teenagers.

While anyone can become severely ill with flu, certain groups are at much higher risk, including infants, young children and older adults, immunocompromised individuals, and people with preexisting conditions such as asthma or chronic obstructive pulmonary disease (COPD).

Adults often have “immunological memory” from decades of previous flu infections and vaccinations, Dr. Roberts says. Because children have had less exposure to various flu strains, their immune systems are more vulnerable to new variants like subclade K.

Although some reports are using the term “super flu,” Dr. Roberts says he has not seen any data that shows that the subclade K variant is more severe than other types of flu.

“The reason people are saying ‘super flu’ is because it’s a bad flu wave and many people are getting infected and hospital admissions are high,” he says. “That’s all reflective of the fact that it's more mutated and there's increased transmissibility, not that it’s more severe.”

No. Flu season typically lasts through the spring, and we may see a second wave of flu B—the other main type of influenza—in the coming months. Getting vaccinated now is the best way to reduce the severity of your symptoms if you do get sick and to keep you out of the hospital.

Subclade K presents with traditional flu symptoms, Dr. Roberts says, which often appear more suddenly than a common cold:

• High fever or feeling feverish/chills
• Cough and sore throat
• Muscle or body aches
• Headache and fatigue
• Vomiting and diarrhea, which are more common in children than in adults

“One of the main things I tell people to watch out for is a very abrupt onset of fever, so abrupt that you can remember the exact moment you started to feel it,” he says. “This differs from COVID-19 and RSV [respiratory syncytial virus], where you usually have a slow, gradual buildup. The three main symptoms of flu are fever, cough, and runny nose.”

Flu symptoms typically start in the upper respiratory tract. If it progresses to the lower respiratory tract, it can lead to pneumonia and other complications. “If you have trouble breathing, that’s always a sign to head to the emergency department or urgent care,” Dr. Roberts says.

If you feel sick, you should test as soon as possible. Knowing whether you have the flu or COVID-19 is essential because effective medical treatments exist for both, but they work best when started early. Over-the-counter combination tests that check for both viruses are now available at most pharmacies.

How to get the most accurate result:

• Don’t wait: You generally have a 48-hour window from the start of symptoms for flu medications to be most effective.
• Test again if negative: Home tests are sometimes less sensitive than hospital tests. If you feel sick but get a negative result, test again the following day.
• Consult a professional: If you still feel very ill despite a negative home test, contact your doctor for a more sensitive clinical test.

Most people recover from the flu at home with rest and fluids. However, you should contact a health care provider or seek urgent care if you experience:

• Difficulty breathing or shortness of breath
• Persistent chest or abdominal pain
• Signs of dehydration (no urine for many hours, dry mouth)
• Seizures or severe muscle pain
• Worsening of chronic medical conditions, such as asthma or heart disease

For parents, seek immediate care if your child has bluish lips or face, ribs pulling in with each breath, or is not alert or interacting when awake.

If you feel symptoms starting, contact your doctor within the first 48 hours. Antiviral medications, such as Tamiflu, are most effective when started early.

In addition to Tamiflu, there are three other antivirals for flu, including Xofluza, Relenza, and Rapivab. Dr. Roberts recommends talking to your doctor about these medications, as each has different side effects and potential out-of-pocket costs.

He also notes that many of these antiviral medications can be taken preventively for high-risk individuals who have been exposed to a household member with the flu.

Key points to protect yourself and your family this flu season:

• Get vaccinated: Even if the current flu vaccine is not a perfect match for subclade K, it helps your body recognize the virus and prevents the most life-threatening outcomes
• Practice hand hygiene: Frequent hand washing is still the best way to avoid getting infected
• Stay home if you’re sick: This prevents further spread to high-risk groups