If you’re surprised someone came down with the flu before Halloween, you’re not alone. The 2026-2027 flu season appears to be starting early and strong, experts say.
Flu activity remains low nationwide, but cases are rising rapidly in many parts of the country, especially in the West, where two states, Louisiana and Hawaii, have reported moderate levels.
As of October 21, flu infections are increasing or likely to increase in 44 states, according to the latest estimates from the Centers for Disease Control and Prevention.
This is not surprising, as flu activity typically intensifies during this time of year. However, this year’s flu season started early, with infections starting to rise as early as August.
“Early-season influenza activity is currently higher than normal for this time of year,” the CDC said in its FluView surveillance report for the week ending October 17.
Influenza A (H1N1) has caused the vast majority of cases in the United States so far this season, according to the Centers for Disease Control and Prevention. This is different from last winter, when H3N2 subclade K (also known as the “super flu”) was the dominant strain.
What’s more, the circulating H1N1 strain appears to have developed a new mutation that may affect its ability to evade pre-existing immunity.
If H1N1 sounds familiar, that’s because it’s the infamous “swine flu” virus that caused the 2009 pandemic.
As mutated H1N1 strains spread, doctors urge people to get the 2026-2027 flu vaccine.
Why does flu season come so early?
No two flu seasons look the same because the time and severity vary from year to year.
However, there are several indicators that flu activity is higher than usual for this time of year.
Cases began increasing in late August and early September and have been rising steadily since, according to the CDC.
The percentage of flu tests that came back positive for the week ending October 17 was 5.7%. During this time last year, the weekly test positivity rate was less than 1%, according to the CDC’s National Respiratory and Enterovirus Surveillance System Dashboard.
CDC data shows that the number of patients visiting the doctor or urgent care for flu-like illness increased 2.2% in the week ending October 17, which is higher than the previous two seasons. Hospitalizations and deaths from the flu are also rising.
“The number of cases is still relatively low, so we’re still in the early stages of a flu surge,” Dr. Andrew Pecos, a virologist at the Johns Hopkins Bloomberg School of Public Health, told TODAY.com.
It’s unclear why cases are rising earlier than normal.
One potential reason is that people may have lower original immunity to H1N1 strains with new mutations.
“Maybe that’s why people are getting sick faster,” Dr. Bernard Cummings, an infectious disease physician and professor at the Icahn School of Medicine at Mount Sinai, told TODAY.com.
The H1N1 strain is spreading differently than the H3N2 strain that was dominant last winter. “Even people who had the flu last year are vulnerable,” Cummings said.
When the H1N1 flu began spreading in late August, newer flu vaccines were just beginning to be rolled out. “The pattern we often see with early influenza is that the first strains to get here are the ones that have the hardest time escaping immunity,” Pecos added.
Does earlier flu season mean a more severe flu season? Experts say not necessarily.
It’s too early to tell how the 2026-2027 flu season will pan out, or whether it will be milder than last year. Last winter, the United States experienced a severe flu season, the culprit being the highly mutated H3N2 subclade K variant, known as the “super flu.”
“We still have to see how bad it is this year, but I think it’s just starting earlier,” Cummings said.
“The only thing that’s predictable about the flu is that it’s unpredictable,” NBC News medical contributor Dr. Natalie Azar told “TODAY.”
The vaccine, branded mFLUSIVA, is approved for use in people 50 and older.
What are mutated H1N1 strains?
Flu viruses are constantly mutating, which is why we get newer flu shots every year.
Last summer, a series of mutations occurred in H3N2, leading to the emergence of subclade K variants.
The CDC says H1N1 viruses generally mutate less frequently than H3N2 viruses. This year, however, H1N1 (pdm09), clade D.3.1.1, acquired a new mutation, G155E, Pekosz said.
The mutation was first discovered in Seattle, Washington, in September, according to a preprint paper posted on bioRxiv.
“It’s different from last year’s H1N1 virus in that we know the location of the mutation (G155E) is important in generating a protective antibody response,” Pekosz explained.
That means the H1N1 strain may be better able to evade immunity because antibodies people have from previous infections or vaccinations may not be able to recognize it.
“How this (mutation) will affect the spread and severity of the disease is unclear,” Pecos said.
Even if the circulating H1N1 strain does not match the strain included in the 2026-2027 flu vaccine, the shot is still expected to provide protection.
“The mismatch is not a black and white situation. We may have lower immunity, not no immunity,” Pecos added.
Historically, H1N1 influenza has hit young people especially hard. Azar said the new mutations may make children more susceptible to infection, especially if they haven’t been exposed to H1N1 influenza.
“It is too early to tell which viruses may be dominant or to tell whether the effectiveness of this season’s flu vaccine will be affected,” the CDC said in its latest Flu Watch report.
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H1N1 flu symptoms
Cummings said the flu often strikes suddenly and hard.
Common symptoms of the flu include:
- fever
- Chills
- fatigue
- body aches
- runny nose
- cough
Cummings said it can be difficult to distinguish between symptoms of a flu infection and those caused by the coronavirus, which is why testing is important.
If you test positive for the flu, you can seek treatment with antiviral medications, which can reduce the severity and duration of symptoms.
Antiviral drugs are particularly important tools for people in high-risk groups, including people over 65, children under 5, and people who are immunocompromised.
These medications are not available over-the-counter, so you need to see your doctor first. “You have to get the prescription quickly … within the first 48 hours after symptoms appear, (otherwise) they don’t work,” Pekosz said.
If you develop severe flu symptoms, please contact your physician.
When to get the flu shot
Experts stress there is no reason to panic about the mutated H1N1 strain, but it is a good reminder to protect yourself.
“Think of this as a sign to go out and get your flu shot,” Pecos said.
Getting the flu shot is the most effective way to prevent serious illness, complications, and death from the flu.
The 2026-2027 flu vaccine is available at most pharmacies, physician offices and clinics. The updated vaccine protects against three viruses: influenza A H1N1 (pdm09), influenza A H3N2 and influenza B.
There are a variety of options, including the standard flu vaccine, high-dose vaccine, nasal spray and the new mRNA flu vaccine.
“Even if there are mismatched strains, the vaccine will provide better protection against the other two strains,” Pecos said. Different flu strains may spread later in the season, so getting a shot can protect you throughout the winter and into early spring.
Cummings added that the flu vaccine is never 100 percent effective, and if you get vaccinated, you can still get infected, but you won’t get as seriously ill and won’t end up in the hospital.
It’s especially important to get vaccinated if you’re considered high-risk, but everyone can benefit.
Experts say the best time to get a flu shot is before the end of October, but you can still benefit if you get it later.
“If you haven’t been vaccinated for whatever reason, now is the time to get vaccinated because flu season starts a little earlier and it will take at least two weeks for you to get your vaccine immunity to a protective level,” Pekosz added.
If you’re not sure which flu vaccine you should get, talk to your doctor or pharmacist.
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