November 7, 2026, 8:02 a.m. ET
As Americans head into the heart of illness season, health officials are closely monitoring flu strains, which are leading to an unusually early surge in infections.
Flu activity began increasing in parts of the United States in late August and early September, earlier than in recent years, according to the Centers for Disease Control and Prevention. While flu season typically begins in earnest around November, data used to track respiratory illnesses — like lab tests, hospitalizations and wastewater data — show the season was well underway before the clock struck November 1.
A new strain, a mutated version of H1N1, appears to be causing the spread, with some experts warning it could play a role in an earlier or longer season.
Here’s what that means for this winter’s flu season and what experts say people should know.
How many cases of flu have been reported nationwide?
Flu cases began to take off in early fall this year, reaching a level of activity in October usually seen before December or January, according to wastewater data.
“Seasonal flu activity continues to increase in many areas of the country and is higher than typically seen at this time of year,” the CDC said on its flu tracking site, FluView. The agency estimates 1.7 million cases of illness, 25,000 hospitalizations and 1,400 deaths from the flu since the start of the season as of Oct. 30. Two new pediatric deaths were also reported to the agency last week.
As of November 5, CDC projections found that flu infections were up in 40 states and down in none. The highest activity appears to be in the Southern states and on the West Coast.
Flu activity detected in wastewater was still classified as “very low” nationally as of October 30, also according to the CDC. He noted that wastewater data tracking is seeing a “brief gap” in updates due to a new contract with a testing company. It was not specified how long this gap would last.
Additional CDC data through Oct. 31 also showed that approximately 9.14% (4,694) of 51,351 flu tests administered that week were reported positive to the National Respiratory and Enteric Virus Surveillance System. The vast majority of these confirmed illnesses (97%) were influenza A, the most common form of influenza.
Wastewater data maintained by WasteWaterSCAN, a Stanford University-led tool for tracking the spread of disease, classified flu activity as “high” nationally, meaning higher concentrations of the virus were detected in wastewater from more communities nationwide.
The same data shows the earliest onset of influenza in at least the last four years; samples positive for influenza A were 71% in October, compared to 40% in September and only 20% in October 2025.
The CDC also noted that seasonal flu activity began to increase between late August and early September.
Influenza activity map by state
What are the new strains of flu?
The CDC said in late October that it was “too early to know which viruses might predominate,” although scientists and health officials indicated they were paying particular attention to a few strains.
That doesn’t mean this year’s viruses have morphed into something considerably more dangerous or deadly. If anything, the mutations likely made them a little more capable of overcoming the immunity that people develop from previous infections and vaccines.
The H1N1 virus, a type of influenza A, is so far the main diagnosis behind this year’s flu season. You may recognize the name of the 2009 “swine flu” pandemic, caused by a new strain called H1N1/09. Health officials noted that this year’s virus is still part of the long-circulating seasonal H1N1 lineage rather than being a new pandemic strain.
The most common H1N1 strain currently is the genetic subgroup known as the D.3.1.1 subclade. Scientists are monitoring the strain because it has accumulated additional mutations in parts of the virus targeted by human antibodies, raising concerns that some newer versions could rely on mutations and shift away from existing immunity in almost “real time,” according to a recent study. The study was published on the preprint server bioRxiv, meaning the research has not yet completed the peer review process.
“Why we think we’re seeing this now, no one really knows, but it’s probably due to mutations,” Dr. Corey K. Smith, an emergency medicine physician and regional medical director of Teladoc Health, told USA TODAY. “And what happens is we were all exposed to H1N1 in 2009, and after a while, as it mutated – and even though we developed this shield, this armor – our immunity to this virus diminished slightly.”
Notably, early evidence shows that D.3.1.1 may impact groups of people differently. Some people’s antibodies still seem to “recognize” the strain quite well, while others seem to be losing their protection.
In the September study, researchers noted that adolescents and young adults appeared to be more affected than adults, particularly people between 15 and 25 years old.
This may be due to something called “immune imprinting” – a phenomenon in which the body “remembers” the first time it encountered a pathogen and shapes every reaction for the rest of your life based, at least in part, on that first encounter. So the first strain of flu you were exposed to as a child is the basis for your immune system’s current response. This is why we can see that certain strains of common diseases impact groups differently across generations.
In this case, researchers believe that people who were likely first exposed to H1N1 during or shortly after the 2009 pandemic were more affected by D.3.1.1 than people who caught the older strain of H1N1 before 2009.
“That’s pretty much how viruses work. These viruses, they sit down and say, ‘Hey, let’s attack people, let’s attack animals, let’s attack birds,’ whatever,” Smith said. “But when they meet us and our antibodies, they go home and say, ‘Hey, let’s mutate and see if we can come back bigger and stronger next year or next season.’
Doctors and scientists developing next year’s flu vaccine are looking at these patterns and trying to make predictions about how to prevent the next wave, he continued.
What are the latest symptoms of the flu?
Symptoms associated with D.3.1.1 are the same as other influenza viruses and include:
- Fever
- Aches and chills
- Cough
- Runny or stuffy nose and sneezing
- Sore throat
- Headache
- Fatigue
Smith noted that patients often wonder what signs or symptoms can help them determine if they have the flu or another respiratory illness, like COVID. A major distinction he noted is the presence or absence of fever and sniffles.
“The symptoms start out being relatively similar as you cross paths with different viruses: cough, congestion, malaise (general malaise), weakness, body aches and fever,” meaning a temperature of 100.4 or higher, he said. Fever, especially in adults, is an important indicator of the flu, as is the sudden onset of body aches and chills. If you felt fine earlier and suddenly “feel like you’ve been hit by a Mack truck” an hour or two later, it’s probably the flu, he said.
“With a cold, you may have a cough and a runny nose, but you don’t have those extreme aches and headaches,” Smith also said. “With COVID, you may have a fever or a little cough, but congestion tends to be the main problem, plus or minus headaches and anosmia (loss of taste and smell).”
How to protect yourself from the flu.
You may be tired of hearing it, but the old cliché is true: Washing your hands well — and often — is one of the best ways to ward off contagious diseases, according to the CDC. This applies to all of the above illnesses and others that often circulate in winter, such as respiratory syncytial virus, or RSV.
Health agencies recommend washing your hands with warm, soapy water for at least 20 seconds. It is also important to wash them before and after eating, before and after handling food, after using the toilet, and after being around sick people or touching surfaces that may be contaminated. Wearing a mask when sick or traveling during illness season can also help prevent the spread.
This is especially important for people at risk of more severe illness, including children and the elderly, pregnant people, immunocompromised people, and those with underlying conditions such as obesity and diabetes.
The most effective way to prevent serious illness from the flu is vaccination, according to the World Health Organization, CDC and other leading medical institutions. Earlier this year, leading medical groups came together to issue vaccine recommendations as federal agencies continue to share inconsistent information amid a guideline shakeup and legal challenges under known vaccine skeptic and Health Secretary Robert F. Kennedy Jr.
The guide recommends the annual flu vaccine for everyone six months or older (including pregnant people), with some exceptions for people with certain medical conditions. People 65 and older should receive a higher dose, if possible.
While it doesn’t completely prevent you from getting the flu, vaccination has been shown to reduce the severity and duration of illness.
“That’s what vaccines do. They create a stronger immune response and bigger, stronger antibodies. It gives us a little glimpse, like a movie, like a trailer of what’s about to happen,” Smith said. “Please get vaccinated. It’s not too late and it contains various mutations and strains of H1N1, including those circulating this year.”
Gn Health
