US Could Face Two Flu Seasons This Winter as Cases Rise Early on West Coast, Doctor Warns

The United States could experience two separate flu waves this year after a mutated influenza strain triggered an unusually early rise in infections, a doctor has warned. 

Dr Alexander Greninger, who leads the Division of Infectious Disease Diagnostics at the University of Washington’s Department of Laboratory Medicine, raised the possibility this week as flu activity climbed on the West Coast more than two months ahead of schedule.

‘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.’ 

Public health officials have attributed the early West Coast surge to a mutated form of H1N1, the influenza strain responsible for the 2009 swine flu pandemic. That outbreak sickened 60 million people and led to more than 240,000 hospitalizations. 

Greninger said the current strain could drive an early peak, followed by a second surge caused by another, as-yet unidentified variant before the flu season concludes in May. 

In a typical year, influenza begins circulating in October, reaches its highest levels between December and February, and gradually fades by May. Activity usually begins in the South before moving across the rest of the country.

This year, however, Centers for Disease Control and Prevention (CDC) data showed flu activity starting to climb during the week ending August 8 and continuing upward. The West Coast also saw the first unexpected increase, with state health department figures indicating that roughly one in 10 people is now testing positive. 

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.’ 

Flu cases rose 13 percent overall from the previous week, while hospitalizations increased 17 percent. The CDC says H1N1 accounts for most infections, the same strain that drove the 2009 pandemic and sickened 60 million Americans.

At Greninger’s laboratory, as many as 98 percent of incoming flu samples are now identified as H1N1. The strain was linked to 60 million illnesses, 240,000 hospitalizations and 12,400 deaths during the 2009 swine flu pandemic.

H1N1 is included in this year’s flu vaccine, offering some protection. However, the latest version carries a mutation known as G155E, which experts believe may prevent antibodies generated by earlier vaccines or infections from recognizing it effectively. 

Nationally, 5.7 percent of flu tests collected during the week of October 17 were positive, up from 5 percent the week before, according to the latest CDC figures. 

Two weeks earlier, the positivity rate stood at 3.9 percent.  

Respiratory illness 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. 

Hospitals admitted 4,811 flu patients during the week, compared with 4,024 the previous week and 2,572 two weeks earlier.

The CDC also reported H1N1 as the most frequently identified flu strain during the latest reporting period.  

Sixteen states recorded higher flu activity than the week before: 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 flu levels, while activity remains low or minimal elsewhere. A week earlier, Louisiana was the only state in the moderate category, and two weeks ago every state reported 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. 

Across Arizona, California, Hawaii and Nevada, at least 10 percent of flu tests were positive last week, a sign of substantial influenza activity in the region.  

With flu circulating early, experts are urging Americans to receive this season’s vaccine as soon as possible, preferably before Halloween, to maximize protection through the 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 United States last saw two clearly separated flu waves during the 2021-2022 season. The first peaked in late December 2021, followed by another high point in April 2022. The 2018-2019 season also produced two waves, with peaks in December 2018 and March 2019.

Influenza, better known as the flu, is among the most common and contagious respiratory infections, affecting about 8 percent of the US population each year.

Typical symptoms include fever, coughing, sore throat, congestion, headache and fatigue. In some cases, the infection can progress to serious complications such as pneumonia or influenza-associated encephalopathy, a severe form of brain dysfunction.

Doctors advise people to get vaccinated against flu every year. The vaccine generally provides protection for six to eight months and helps reduce the risk of infection.

Three influenza virus types can cause illness in people: influenza A, B and C.

Influenza A infects both humans and animals. Type B is primarily found in humans and is typically associated with milder illness.

Type C is far less common than types A and B and generally causes only mild illness.

The seasonal flu vaccine is reformulated each year as circulating viruses change. It targets influenza A and B, the types most commonly associated with severe disease.

Although most people recover within about a week, others can develop potentially dangerous complications, including pneumonia, which may require medical care.

According to the CDC, vaccination is one of the strongest tools for preventing flu. It reduces the risk of a doctor visit by 34 percent and hospitalization by 30 percent. Because the body may need up to two weeks to produce antibodies after vaccination, protection is not immediate.

For that reason, doctors recommend getting vaccinated several weeks before attending an indoor event or gathering where large numbers of people are expected.

Last year’s flu season was particularly severe, with a ‘super flu’ surge driven by subclade K, a heavily mutated variant of influenza A (H3N2) that produced a rapid nationwide rise in infections.

Because the strain was unfamiliar to people’s immune systems, many were more vulnerable to serious illness requiring hospitalization.

The symptoms resembled those caused by other flu strains, but experts said the ‘super flu’ variant appeared to cause more severe disease, particularly in children. Reported signs included fevers lasting a week and coughing that made breathing difficult.

The CDC estimates that the 2025-2026 respiratory season produced at least 32 million flu infections, approximately 390,000 hospitalizations and 24,000 deaths. 

H1N1, the leading strain so far this year, was last the dominant virus during the 2023-2024 flu season.

During the 2009 swine flu pandemic, H1N1 was associated with 60 million illnesses, 240,000 hospitalizations and 12,400 deaths.

To reduce the risk of infection, health officials recommend washing hands regularly and avoiding close contact with people showing flu-like symptoms.

Treatment typically involves rest, plenty of fluids and over-the-counter medicines. In more serious cases, doctors may prescribe antiviral drugs such as Tamiflu.