If you’re surprised to learn that people came down with the flu before Halloween, you’re not alone. According to experts, the 2026-2027 flu season appears to have started early and strong.
Flu activity is still low nationally, but cases are rising rapidly in many parts of the country, particularly in the West, and two states, Louisiana and Hawaii, are already reporting moderate levels.
As of Oct. 21, flu infections are increasing or likely to increase in 44 states, according to the latest estimates from the U.S. Centers for Disease Control and Prevention.
This is not surprising, given that flu activity typically increases this time of year. However, this year’s flu season started much earlier than expected, with infections surging as early as August.
“Early season influenza activity is currently higher than typically seen at this time of year,” the CDC said in its FluView surveillance report for the week ending October 17.
H1N1 influenza has caused the vast majority of cases in the United States so far this season, according to CDC data. That’s a change from last winter, when H3N2 subclade K – aka the “superflu” – was the dominant strain.
Additionally, it appears that the circulating H1N1 strain has acquired a new mutation that could affect its ability to evade pre-existing immunity.
If the H1N1 virus looks familiar to you, it’s because it’s the famous strain of “swine flu” that caused a pandemic in 2009.
As the mutated H1N1 strain spreads, doctors are urging people to get the 2026-2027 flu vaccine.
Why does flu season start so early?
No two flu seasons are the same, as the timing and severity can vary from year to year.
However, several indicators indicate that flu activity is higher than usual for this time of year.
Cases began increasing in late August and early September and have increased steadily since, according to the CDC.
The percentage of positive flu tests was 5.7% in the week ending October 17. During the same period last year, the weekly test positivity rate was less than 1%, according to the CDC’s National Respiratory and Enteric Virus Surveillance System dashboard.
Outpatient visits to doctor’s offices or urgent cares for flu-like illnesses reached 2.2% in the week ending Oct. 17, higher than the previous two seasons, according to CDC data. Hospitalizations and deaths from the flu are also on the rise.
“Cases are still relatively low, so we’re still in the early stages of a flu wave,” Andrew Pekosz, Ph.D., a virologist at the Johns Hopkins Bloomberg School of Public Health, told TODAY.com.
It’s unclear why cases are increasing earlier than normal.
One potential reason is that people may have less pre-existing immunity to the H1N1 strain if there is a new mutation.
“Maybe that’s why people are getting sick earlier,” Dr. Bernard Camins, an infectious disease physician and professor at the Icahn School of Medicine at Mount Sinai, told TODAY.com.
The spread of the H1N1 strain is different from the H3N2 strain that dominated last winter. “Even people who got the flu last year are vulnerable,” Camins says.
When the H1N1 virus began spreading in late August, updated flu vaccines had just begun to be rolled out. “The pattern we often see with flu early in the season is that the first strain that arrives is the one that evades immunity the most,” adds Pekosz.
Does an early flu season mean a more severe flu season? Not necessarily, experts say.
It’s too early to tell how the 2026-2027 flu season will go or if it will be milder than last year. The United States experienced a tough flu season last winter, fueled by the highly mutated subclade K variant of H3N2, aka the “super flu.”
“We still have to see how bad it will be this year, but I think it’s just starting earlier,” Camins says.
“The only predictable thing about the flu is that it’s unpredictable,” NBC News medical contributor Dr. Natalie Azar said on TODAY.
What is the mutated H1N1 strain?
Flu viruses are constantly mutating, which is why we get an updated flu vaccine every year.
Last summer, H3N2 detected a host of mutations, leading to the emergence of the K subclade variant.
H1N1 viruses generally change less often than H3N2 viruses, according to the CDC. However, this year the H1N1 virus (pdm09), subclade D.3.1.1, acquired a new mutation, G155E, Pekosz explains.
The mutation was first identified in September in Seattle, Washington, according to a preliminary paper published in bioRxiv.
“It is different from last year’s H1N1 virus and the position of the mutation (G155E) that we know is important for generating protective antibody responses,” says Pekosz.
That means the H1N1 strain might be better able to evade immunity, because antibodies people have from a previous infection or vaccination might not recognize it as well.
“How this (mutation) will affect the spread and severity of the disease is unclear,” says Pekosz.
Even if the circulating H1N1 strain does not match that included in the 2026-2027 flu vaccines, the vaccine should still provide protection.
“A mismatch is not a black and white situation. We may have less immunity, but not no immunity,” adds Pekosz.
Historically, H1N1 has hit young people particularly hard. The new mutation could make children more vulnerable, especially if they haven’t yet been exposed to H1N1, Azar said.
“It is too early to know which viruses might predominate or to say whether there might be implications for flu vaccine effectiveness this season,” the CDC said in its latest FluView report.
H1N1 Flu Symptoms
The flu often strikes suddenly and intensely, Camins explains.
Common symptoms of the flu include:
- Fever
- Chills
- Fatigue
- Body aches
- Runny nose
- Cough
It can be difficult to distinguish signs of a flu infection from those caused by COVID, which is why testing is important, Camins says.
If you test positive for the flu, you may seek treatment with antiviral medications, which can reduce the severity and duration of symptoms.
Antivirals are a particularly important tool for high-risk groups, including people over 65, children under 5, and immunocompromised people.
These medications are not available over the counter, so you will need to consult a doctor first. “You need to get that prescription quickly…within 48 hours of symptoms appearing, (otherwise) they don’t work as well,” Pekosz says.
If you have serious flu symptoms, contact your doctor.
When to get the flu vaccine
There’s no reason to panic about the mutated strain of H1N1, experts emphasize, but it’s a good reminder to protect yourself.
“Take (this) as a sign to go out and get your flu shot,” says Pekosz.
Getting a flu vaccine is the most effective way to prevent serious illness, complications and death from the flu.
The 2026-2027 flu vaccines are available at most pharmacies, doctor’s offices and clinics. The updated vaccines protect against three viruses: influenza A H1N1 (pdm09), influenza A H3N2 and influenza B.
There are several options available, including standard flu vaccines, high-dose vaccines, a nasal mist, and the new mRNA flu vaccine.
“Even if there is one incompatible strain, there are two other strains against which the vaccine will provide better protection,” says Pekosz. Different strains of flu may circulate later in the season, so the vaccine may protect you throughout the winter and early spring.
The flu vaccine is never 100% effective, Camins adds, and you can still get infected if you’ve been vaccinated — but you won’t get as sick or end up in the hospital.
Vaccination is especially important if you are considered high risk, but everyone can benefit from it.
The best time to get a flu shot is before the end of October, but you can still benefit from it if you get it later, experts note.
“If you haven’t received it yet for whatever reason, now is the time to get vaccinated because the flu season starts a little earlier and you need at least two weeks for vaccine immunity (to reach) protective levels,” Pekosz adds.
If you are not sure which flu vaccine you should get, talk to your doctor or pharmacist.
Gn Health
