US Flu season intensifies: New dominant strain H3N2 subclade K drives record hospitalizations
TOI GLOBAL DESK | TOI GLOBAL | Jan 08, 2026, 23:52 IST
Flu cases are skyrocketing across the U.S., with an unprecedently virulent strain, subclade K, leading to a significant uptick in both doctor appointments and hospital admissions. This surge is putting immense pressure on our healthcare infrastructure. Authorities are emphasizing the importance of staying alert and practicing fundamental prevention measures such as hand hygiene and vaccinations.
Flu season is intensifying across the United States with accelerating spread and alarming increases in doctor visits and hospitalizations, reaching levels not recorded in decades in some regions due to a newly dominant strain, subclade K, which stems from influenza A H3N2.
New federal data from the Centers for Disease Control and Prevention shows flu activity surging nationwide at levels rarely seen in modern tracking. The week ending December 27 marked a milestone not recorded since flu surveillance began in 1997. For the first time, 8% of all visits to medical professionals nationwide were linked to the flu or flu-like illness. This figure represents a sharp jump as cases accelerated rapidly through December. Hospitalizations climbed even more dramatically, rising 48% from the previous week.
So far this season, the CDC reports approximately 120,000 flu-related hospitalizations and 5,000 deaths. Separate CDC data also estimates at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths nationwide, underscoring how widespread the virus has become. The CDC now categorizes flu activity as “high” nationwide, signaling sustained and intense transmission from coast to coast.
While flu activity is elevated across the country, some regions are being hit especially hard. CDC data through December 27 shows that nearly 33% of flu tests conducted nationwide came back positive, a steep rise from just 9% during the first week of December. In several states, the positivity rate climbed even higher. Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming reported positivity levels exceeding 45%, placing them among the hardest-hit areas in the country.
The CDC also noted that flu activity was “very high” in the Northeast during the same period, including in New York, Massachusetts and North Carolina.
Beyond clinical testing, wastewater surveillance is telling a similar story. WasteWaterSCAN, a Stanford University-led tracking system, categorized flu activity as “high” after detecting a 146% increase in median virus concentration between the first and last weeks of December. This rise indicates broader community spread across more locations nationwide.
As of December 30, CDC projections showed flu infections increasing in 17 states, unchanged or unreported in seven states, and declining or likely declining in 24 states.
A newly mutated strain is playing a major role in this flu season’s severity. Known as subclade K, the strain stems from influenza A, specifically the H3N2 subtype, which has long been associated with seasonal flu outbreaks. Influenza A is the virus most people refer to when they talk about the flu, and H3N2 has circulated for decades. However, its latest mutation is structurally different enough that the flu vaccine formulated for the 2025–2026 season is not as effective against it.
Because of those differences, subclade K may also be better at evading immune defenses built from prior infections or earlier vaccines. That immune escape potential has led experts and patients alike to refer to the strain as part of a so-called “super flu,” a term used to describe strains that spread quickly and cause widespread disruption. Despite concerns, health officials stress that this year’s vaccine is not useless against subclade K, even if its effectiveness is reduced compared to previous seasons.
Symptoms tied to H3N2 subclade K closely resemble those seen with other influenza viruses. Common symptoms include fever, cough, runny nose, fatigue, muscle aches and chills. Health officials warn that while symptoms may seem familiar, the rapid spread and high hospitalization rates make early detection and care especially important this season.
Health agencies continue to emphasize basic prevention strategies as flu and other illnesses circulate simultaneously. Washing hands thoroughly and often remains one of the most effective ways to reduce transmission, according to the CDC. Officials recommend washing hands with hot, soapy water for at least 20 seconds, especially before and after eating, after using the bathroom, after handling food, and after contact with sick individuals or potentially contaminated surfaces. Mask-wearing while sick or during periods of high illness activity can also help slow the spread.
Vaccination remains the most effective tool for preventing severe flu illness. The World Health Organization, CDC and other major medical institutions continue to recommend flu shots for everyone six months and older, unless a specific medical condition prevents vaccination, such as an allergy to vaccine ingredients or a prior severe reaction.
Unlike much of the country, San Diego has not yet experienced the same level of flu surge seen elsewhere. While flu cases and hospitalizations are rising locally and across California, the numbers have not reached the dramatic highs observed in other regions. That relative calm has not eased concerns. In December, county health officials warned that low vaccination rates could make this one of the worst flu seasons in recent memory. The warning comes after last year’s severe season, when 220 county residents, including seven children, died from flu-related complications. Health officials caution that peak flu season is not over and urge residents to get vaccinated if they have not already done so.
The sharp rise in flu cases is unfolding as other illnesses — including COVID, whooping cough and the so-called “winter vomiting bug," are also spreading across the U.S. The combination is placing additional strain on hospitals and healthcare systems. With flu activity still elevated and subclade K continuing to circulate, health officials say vigilance remains critical as winter continues.
Yes. The CDC now classifies flu activity as high across the U.S.
Most cases are linked to influenza A H3N2 subclade K.
"Flu activity is surging across the United States at levels rarely seen in modern tracking."
"According to new data from the Centers for Disease Control and Prevention, the week ending Dec. 27 marked a milestone not recorded since flu surveillance began in 1997, as per a report by USA Today."
"For the first time, 8% of all visits to medical professionals nationwide were linked to the flu or flu-like illness."
"That figure represents a sharp jump as cases accelerated rapidly through December."
"Hospitalizations climbed even more dramatically, rising 48% from the previous week."
"So far this season, the CDC reports approximately 120,000 flu-related hospitalizations and 5,000 deaths."
"Separate CDC data also estimates at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths nationwide, underscoring how widespread the virus has become."
"The CDC now categorizes flu activity as “high” nationwide, signaling sustained and intense transmission from coast to coast, as per a report."
"While flu activity is elevated across the country, some regions are being hit especially hard."
"CDC data through Dec. 27 shows that nearly 33% of flu tests conducted nationwide came back positive, a steep rise from just 9% during the first week of December."
"In several states, the positivity rate climbed even higher."
"Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming reported positivity levels exceeding 45%, placing them among the hardest-hit areas in the country."
"The CDC also noted that flu activity was “very high” in the Northeast during the same period, including in New York, Massachusetts and North Carolina, as quoted in a report by USA Today."
"Beyond clinical testing, wastewater surveillance is telling a similar story."
"WasteWaterSCAN, a Stanford University-led tracking system, categorized flu activity as “high” after detecting a 146% increase in median virus concentration between the first and last weeks of December."
"This rise indicates broader community spread across more locations nationwide."
"As of Dec. 30, CDC projections showed flu infections increasing in 17 states, unchanged or unreported in seven states, and declining or likely declining in 24 states, as quoted in a report by USA Today."
"A newly mutated strain is playing a major role in this flu season’s severity."
"Known as subclade K, the strain stems from influenza A, specifically the H3N2 subtype, which has long been associated with seasonal flu outbreaks."
"Influenza A is the virus most people refer to when they talk about the flu, and H3N2 has circulated for decades."
"However, its latest mutation is structurally different enough that the flu vaccine formulated for the 2025–2026 season is not as effective against it."
"Because of those differences, subclade K may also be better at evading immune defenses built from prior infections or earlier vaccines."
"That immune escape potential has led experts and patients alike to refer to the strain as part of a so-called “super flu,” a term used to describe strains that spread quickly and cause widespread disruption."
"Despite concerns, health officials stress that this year’s vaccine is not useless against subclade K, even if its effectiveness is reduced compared to previous seasons."
"Symptoms tied to H3N2 subclade K closely resemble those seen with other influenza viruses."
"Common symptoms include fever, cough, runny nose, fatigue, muscle aches and chills."
"Health officials warn that while symptoms may seem familiar, the rapid spread and high hospitalization rates make early detection and care especially important this season."
"Health agencies continue to emphasize basic prevention strategies as flu and other illnesses circulate simultaneously."
"Washing hands thoroughly and often remains one of the most effective ways to reduce transmission, according to the CDC."
"Officials recommend washing hands with hot, soapy water for at least 20 seconds, especially before and after eating, after using the bathroom, after handling food, and after contact with sick individuals or potentially contaminated surfaces."
"Mask-wearing while sick or during periods of high illness activity can also help slow the spread."
"Vaccination remains the most effective tool for preventing severe flu illness."
"The World Health Organization, CDC and other major medical institutions continue to recommend flu shots for everyone six months and older, unless a specific medical condition prevents vaccination, such as an allergy to vaccine ingredients or a prior severe reaction, as per a report by Axios."
"Unlike much of the country, San Diego has not yet experienced the same level of flu surge seen elsewhere."
"While flu cases and hospitalizations are rising locally and across California, the numbers have not reached the dramatic highs observed in other regions, as per a report by Axios."
"That relative calm has not eased concerns."
"In December, county health officials warned that low vaccination rates could make this one of the worst flu seasons in recent memory."
"The warning comes after last year’s severe season, when 220 county residents, including seven children, died from flu-related complications."
"Health officials caution that peak flu season is not over and urge residents to get vaccinated if they have not already done so."
"The sharp rise in flu cases is unfolding as other illnesses — including COVID, whooping cough and the so-called “winter vomiting bug," are also spreading across the U.S."
"The combination is placing additional strain on hospitals and healthcare systems, as per a report by Axios."
"With flu activity still elevated and subclade K continuing to circulate, health officials say vigilance remains critical as winter continues."
New federal data from the Centers for Disease Control and Prevention shows flu activity surging nationwide at levels rarely seen in modern tracking. The week ending December 27 marked a milestone not recorded since flu surveillance began in 1997. For the first time, 8% of all visits to medical professionals nationwide were linked to the flu or flu-like illness. This figure represents a sharp jump as cases accelerated rapidly through December. Hospitalizations climbed even more dramatically, rising 48% from the previous week.
So far this season, the CDC reports approximately 120,000 flu-related hospitalizations and 5,000 deaths. Separate CDC data also estimates at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths nationwide, underscoring how widespread the virus has become. The CDC now categorizes flu activity as “high” nationwide, signaling sustained and intense transmission from coast to coast.
While flu activity is elevated across the country, some regions are being hit especially hard. CDC data through December 27 shows that nearly 33% of flu tests conducted nationwide came back positive, a steep rise from just 9% during the first week of December. In several states, the positivity rate climbed even higher. Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming reported positivity levels exceeding 45%, placing them among the hardest-hit areas in the country.
The CDC also noted that flu activity was “very high” in the Northeast during the same period, including in New York, Massachusetts and North Carolina.
Beyond clinical testing, wastewater surveillance is telling a similar story. WasteWaterSCAN, a Stanford University-led tracking system, categorized flu activity as “high” after detecting a 146% increase in median virus concentration between the first and last weeks of December. This rise indicates broader community spread across more locations nationwide.
As of December 30, CDC projections showed flu infections increasing in 17 states, unchanged or unreported in seven states, and declining or likely declining in 24 states.
A newly mutated strain is playing a major role in this flu season’s severity. Known as subclade K, the strain stems from influenza A, specifically the H3N2 subtype, which has long been associated with seasonal flu outbreaks. Influenza A is the virus most people refer to when they talk about the flu, and H3N2 has circulated for decades. However, its latest mutation is structurally different enough that the flu vaccine formulated for the 2025–2026 season is not as effective against it.
Because of those differences, subclade K may also be better at evading immune defenses built from prior infections or earlier vaccines. That immune escape potential has led experts and patients alike to refer to the strain as part of a so-called “super flu,” a term used to describe strains that spread quickly and cause widespread disruption. Despite concerns, health officials stress that this year’s vaccine is not useless against subclade K, even if its effectiveness is reduced compared to previous seasons.
Symptoms tied to H3N2 subclade K closely resemble those seen with other influenza viruses. Common symptoms include fever, cough, runny nose, fatigue, muscle aches and chills. Health officials warn that while symptoms may seem familiar, the rapid spread and high hospitalization rates make early detection and care especially important this season.
Health agencies continue to emphasize basic prevention strategies as flu and other illnesses circulate simultaneously. Washing hands thoroughly and often remains one of the most effective ways to reduce transmission, according to the CDC. Officials recommend washing hands with hot, soapy water for at least 20 seconds, especially before and after eating, after using the bathroom, after handling food, and after contact with sick individuals or potentially contaminated surfaces. Mask-wearing while sick or during periods of high illness activity can also help slow the spread.
Vaccination remains the most effective tool for preventing severe flu illness. The World Health Organization, CDC and other major medical institutions continue to recommend flu shots for everyone six months and older, unless a specific medical condition prevents vaccination, such as an allergy to vaccine ingredients or a prior severe reaction.
Unlike much of the country, San Diego has not yet experienced the same level of flu surge seen elsewhere. While flu cases and hospitalizations are rising locally and across California, the numbers have not reached the dramatic highs observed in other regions. That relative calm has not eased concerns. In December, county health officials warned that low vaccination rates could make this one of the worst flu seasons in recent memory. The warning comes after last year’s severe season, when 220 county residents, including seven children, died from flu-related complications. Health officials caution that peak flu season is not over and urge residents to get vaccinated if they have not already done so.
The sharp rise in flu cases is unfolding as other illnesses — including COVID, whooping cough and the so-called “winter vomiting bug," are also spreading across the U.S. The combination is placing additional strain on hospitals and healthcare systems. With flu activity still elevated and subclade K continuing to circulate, health officials say vigilance remains critical as winter continues.
Yes. The CDC now classifies flu activity as high across the U.S.
Most cases are linked to influenza A H3N2 subclade K.
"Flu activity is surging across the United States at levels rarely seen in modern tracking."
"According to new data from the Centers for Disease Control and Prevention, the week ending Dec. 27 marked a milestone not recorded since flu surveillance began in 1997, as per a report by USA Today."
"For the first time, 8% of all visits to medical professionals nationwide were linked to the flu or flu-like illness."
"That figure represents a sharp jump as cases accelerated rapidly through December."
"Hospitalizations climbed even more dramatically, rising 48% from the previous week."
"So far this season, the CDC reports approximately 120,000 flu-related hospitalizations and 5,000 deaths."
"Separate CDC data also estimates at least 7.5 million illnesses, 81,000 hospitalizations and 3,100 deaths nationwide, underscoring how widespread the virus has become."
"The CDC now categorizes flu activity as “high” nationwide, signaling sustained and intense transmission from coast to coast, as per a report."
"While flu activity is elevated across the country, some regions are being hit especially hard."
"CDC data through Dec. 27 shows that nearly 33% of flu tests conducted nationwide came back positive, a steep rise from just 9% during the first week of December."
"In several states, the positivity rate climbed even higher."
"Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming reported positivity levels exceeding 45%, placing them among the hardest-hit areas in the country."
"The CDC also noted that flu activity was “very high” in the Northeast during the same period, including in New York, Massachusetts and North Carolina, as quoted in a report by USA Today."
"Beyond clinical testing, wastewater surveillance is telling a similar story."
"WasteWaterSCAN, a Stanford University-led tracking system, categorized flu activity as “high” after detecting a 146% increase in median virus concentration between the first and last weeks of December."
"This rise indicates broader community spread across more locations nationwide."
"As of Dec. 30, CDC projections showed flu infections increasing in 17 states, unchanged or unreported in seven states, and declining or likely declining in 24 states, as quoted in a report by USA Today."
"A newly mutated strain is playing a major role in this flu season’s severity."
"Known as subclade K, the strain stems from influenza A, specifically the H3N2 subtype, which has long been associated with seasonal flu outbreaks."
"Influenza A is the virus most people refer to when they talk about the flu, and H3N2 has circulated for decades."
"However, its latest mutation is structurally different enough that the flu vaccine formulated for the 2025–2026 season is not as effective against it."
"Because of those differences, subclade K may also be better at evading immune defenses built from prior infections or earlier vaccines."
"That immune escape potential has led experts and patients alike to refer to the strain as part of a so-called “super flu,” a term used to describe strains that spread quickly and cause widespread disruption."
"Despite concerns, health officials stress that this year’s vaccine is not useless against subclade K, even if its effectiveness is reduced compared to previous seasons."
"Symptoms tied to H3N2 subclade K closely resemble those seen with other influenza viruses."
"Common symptoms include fever, cough, runny nose, fatigue, muscle aches and chills."
"Health officials warn that while symptoms may seem familiar, the rapid spread and high hospitalization rates make early detection and care especially important this season."
"Health agencies continue to emphasize basic prevention strategies as flu and other illnesses circulate simultaneously."
"Washing hands thoroughly and often remains one of the most effective ways to reduce transmission, according to the CDC."
"Officials recommend washing hands with hot, soapy water for at least 20 seconds, especially before and after eating, after using the bathroom, after handling food, and after contact with sick individuals or potentially contaminated surfaces."
"Mask-wearing while sick or during periods of high illness activity can also help slow the spread."
"Vaccination remains the most effective tool for preventing severe flu illness."
"The World Health Organization, CDC and other major medical institutions continue to recommend flu shots for everyone six months and older, unless a specific medical condition prevents vaccination, such as an allergy to vaccine ingredients or a prior severe reaction, as per a report by Axios."
"Unlike much of the country, San Diego has not yet experienced the same level of flu surge seen elsewhere."
"While flu cases and hospitalizations are rising locally and across California, the numbers have not reached the dramatic highs observed in other regions, as per a report by Axios."
"That relative calm has not eased concerns."
"In December, county health officials warned that low vaccination rates could make this one of the worst flu seasons in recent memory."
"The warning comes after last year’s severe season, when 220 county residents, including seven children, died from flu-related complications."
"Health officials caution that peak flu season is not over and urge residents to get vaccinated if they have not already done so."
"The sharp rise in flu cases is unfolding as other illnesses — including COVID, whooping cough and the so-called “winter vomiting bug," are also spreading across the U.S."
"The combination is placing additional strain on hospitals and healthcare systems, as per a report by Axios."
"With flu activity still elevated and subclade K continuing to circulate, health officials say vigilance remains critical as winter continues."