The United States may face two flu peaks this year after an early rise in infections linked to a mutated influenza strain, a doctor has cautioned.
Dr. Alexander Greninger, who leads the Division of Infectious Disease Diagnostics at the University of Washington’s Department of Laboratory Medicine, issued the warning as flu activity on the West Coast climbed more than two months ahead of its usual schedule.
“I wouldn’t be surprised to see a second flu season, like a normal flu season,” Greninger said this week.
“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 attributed the early West Coast surge to a mutated form of H1N1, the influenza strain responsible for the 2009 pandemic. That outbreak sickened 60 million people and led to more than 240,000 hospitalizations.
Greninger said the current wave could reach an early high point before a different, unidentified variant emerges and produces another surge ahead of the traditional end of flu season in May.
Flu activity generally begins circulating in October, reaches its highest levels between December and February, and gradually eases through May. In a typical year, infections first increase in the South before spreading across the rest of the country.
This year’s pattern has been markedly different. Data from the Centers for Disease Control and Prevention (CDC) showed flu activity beginning to rise during the week ending August 8, with levels continuing to climb. The West Coast also saw the initial jump, and state health department figures indicate that roughly one in 10 people there are now testing positive for influenza.
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.”
Nationally, flu cases increased 13 percent from the previous week, while hospitalizations rose 17 percent. The CDC said H1N1 accounts for most reported infections, the same strain that drove the 2009 pandemic and sickened 60 million Americans.
Greninger said up to 98 percent of flu samples arriving at his laboratory are now linked to H1N1. The 2009 swine flu pandemic hospitalized 240,000 people and was associated with 12,400 deaths.
H1N1 is included in this year’s flu vaccine, which should offer some protection. However, the latest version carries a mutation known as G155E, which experts believe may make it harder for antibodies generated by vaccination or prior infection to recognize the virus.
The latest CDC figures show that 5.7 percent of flu tests collected during the week of October 17 were positive, up from 5 percent the week before.
Two weeks earlier, the positivity rate stood at 3.9 percent.
Respiratory illnesses accounted for about 2.2 percent of healthcare visits during the latest week. That was unchanged from the prior week but higher than the 1.8 percent recorded two weeks earlier.
Hospital admissions also continued to rise, reaching 4,811 patients this week, compared with 4,024 the previous week and 2,572 two weeks earlier.
The CDC identified H1N1 as the most frequently reported flu strain during the latest reporting period.
Flu activity increased in 16 states from the previous week: Alaska, North Dakota, Wisconsin, Vermont, Oregon, Wyoming, South Dakota, Ohio, Pennsylvania, New York, Utah, Arizona, New Mexico, Oklahoma, Hawaii and Florida.
Louisiana and Hawaii are reporting moderate levels of flu activity, while conditions remain low or minimal elsewhere. A week earlier, only Louisiana had reached the moderate category; two weeks before that, every state was reporting minimal or low activity.
‘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.
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‘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.
