The US could end up suffering two flu seasons this year due to a mutated strain of the influenza virus, a doctor has warned.
Dr Alexander Greninger, head of the Division of Infectious Disease Diagnostics at the University of Washington’s Department of Laboratory Medicine, raised the prospect this week as flu levels rose on the West coast more than two months early.
‘I wouldn’t be surprised to see a second flu season, like a normal flu season,’ Greninger said.
‘You’ll have [the current uptick in] cases that will peak in 2026, and then who knows what comes in January or February or March.’
Health officials have linked the early surge in cases on the West coast to a mutated version of the H1N1 flu virus. H1N1 was the strain behind the 2009 flu pandemic that sickened 60 million and hospitalized more than 240,000.
He warned that this could cause an early peak in cases, before another – as yet unknown – variant could then emerge and cause a second peak before flu season ends in May.
Typically, flu starts to circulate in October, peaks between December and February and then declines up to May. Cases also normally rise in the South first before spreading to the rest of the country.
But this year, Centers for Disease Control and Prevention (CDC) data showed that flu levels had already begun to rise in the week ending August 8 and have continued to increase since. There was also an unexpected increase on the West coast first, where figures from the state’s health department show about one in 10 people are now testing positive for the flu.
Experts are warning the US could face two flu seasons due to earlier-than-expected increases in virus activity
Greninger told CNN: ‘In Seattle, we’re pretty far north. We see a pretty strong peak in January. We’re basically seeing January levels of flu in the last week.’
Overall, cases are up by 13 percent, and there have been 17 percent more hospitalizations, compared to the week prior. The CDC reports that most cases are H1N1, which caused a pandemic in 2009 that sickened 60 million Americans.
Greninger noted that as much as 98 percent of the flu samples now arriving at his lab derive from the strain H1N1, which caused the 2009 swine flu pandemic that sickened 60 million Americans, hospitalized 240,000 and was linked to 12,400 deaths.
While H1N1 is included in this year’s flu shot, which would provide some protection, the newest version of the virus has a new mutation called G155E that experts believe prevents preexisting antibodies from vaccines from recognizing it.
Nationally, 5.7 percent of flu tests collected during the week of October 17 came back positive, up from five percent the week before, the latest CDC data shows.
Two weeks prior, this level was 3.9 percent.
About 2.2 percent of visits to a healthcare provider this past week were for respiratory illness, in line with the previous week but up from 1.8 percent two weeks prior.
There were 4,811 patients admitted to the hospital this week compared to 4,024 the previous week and 2,572 two weeks prior.
The CDC also notes that H1N1 is the most frequently reported strain this week.
Cases have increased in 16 states compared to the week prior: Alaska, North Dakota, Wisconsin, Vermont, Oregon, Wyoming, South Dakota, Ohio, Pennsylvania, New York, Utah, Arizona, New Mexico, Oklahoma, Hawaii and Florida.
Two states, Louisiana and Hawaii, are reporting moderate flu virus levels, while the rest of the country has either low or minimal levels. Last week, only Louisiana had moderate levels, and two weeks prior, all states were either minimal or low.
‘All of the Pacific states, I would say, started their season very early, but we are now seeing as the season progresses a spread eastward, and so all regions of the country are seeing increases at this point,’ Dr Caitlin Rivers, an epidemiologist who directs the Center for Outbreak Response Innovation at Johns Hopkins Bloomberg School of Public Health, told CNN.
In the region including Arizona, California, Hawaii and Nevada, the proportion of flu tests that came back positive reached at least 10 percent last week, indicating significant flu activity.
Experts are now urging Americans to get this season’s flu shot as soon as possible, ideally before Halloween, to provide the best protection through spring.
‘A lot of people, like even myself, would wait until late October, which we’re sort of in already,’ Greninger said.
‘But really, the time to get vaccinated was actually two to three weeks ago.’
The last time a flu season featured two distinct waves was during the 2021-2022 season, where the first peaked in late December 2021 and there was a later peak in April 2022. Prior to that, there were two waves during the 2018-2019 season: one in December 2018 and another in March 2019.
Influenza, commonly known as the flu, is one of the most common and highly contagious respiratory infections, impacting around eight percent of the US population each year.
The infection can cause symptoms such as fever, cough, sore throat, stuffy nose, headaches and fatigue, and can lead to more serious complications including pneumonia and influenza-associated encephalopathy, a severe brain dysfunction condition.
Doctors recommend an annual flu vaccination, which provides protection for six to eight months, to help lower the chances of being infected.
There are three different flu virus strains that cause illness: influenza types A, B and C.
Influenza type A can infect people and animals, while type B is usually found only in humans and is generally associated with less severe infections.
Type C occurs much less frequently than A and B and causes minor illness in humans.
The seasonal flu shot, which is updated every year as the virus mutates, protects against influenza types A and B, as these are the most common and severe strains.
While most people recover within a week, some may develop complications such as pneumonia, which can be life-threatening and require medical treatment.
The flu shot is one of the most effective preventative measures against infection, reducing the risk of doctor visits by 34 percent and the risk of hospitalization by 30 percent, per the CDC, but it can take up to two weeks for the body to develop antibodies after receiving the vaccination.
Because of this, doctors recommend getting the shot a few weeks before an event or gathering where large numbers of people will be indoors.
The strain behind the 2009 swine flu pandemic is being linked to an early uptick of flu cases in the US this year
Last year saw a particularly severe flu season as a ‘super flu’ surge was driven by subclade K, a highly mutated variant of the Influenza A (H3N2) virus that caused a rapid and severe spike in cases nationwide.
The strain was brand new to people’s immune systems, making them highly vulnerable and at an increased risk of severe, hospitalizing illness.
While symptoms were the same as other flu strains, experts warned the ‘super flu’ variant was causing more severe illness, especially in children, with signs including week-long fevers and a cough that led to trouble breathing.
The 2025-2026 respiratory season saw at least 32 million flu cases, according to the CDC. There were about 390,000 hospitalizations and 24,000 deaths.
H1N1, the main strain this year, was last dominant over the 2023 to 2024 flu season.
In 2009, during the swine flu pandemic, it was linked to 60 million illnesses, 240,000 hospitalizations and 12,400 deaths.
To avoid the flu, health officials recommend regular handwashing and avoiding contact with people who have flu-like symptoms.
Treatment includes rest, staying hydrated, taking over-the-counter medication or in severe cases, prescription antivirals such as Tamiflu.