As Americans head into the heart of sickness season, health officials are keeping a close eye on the flu strains driving an unusually early rise 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 starts in earnest around November, data used to track respiratory illnesses – like laboratory tests, hospitalizations and wastewater data – show that the season was already well underway before the clock struck Nov. 1.
A new strain, a mutated version of H1N1, appears to be driving the spread, with some experts warning that it could play a hand 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 flu cases were reported nationwide?
Flu cases began to take off in early fall this year, reaching an activity level in October not usually seen until December or January, according to wastewater data.
"Seasonal flu activity continues to increase in many parts of the country and is higher than typically observed at this time of year," the CDC said on its influenza tracking site, FluView. The agency estimates 1.7 million illnesses, 25,000 hospitalizations and 1,400 deaths from flu so far this season as of Oct. 30. Two new pediatric deaths were also reported to the agency in the last week.
As of Nov. 5, CDC projections found that flu infections were on the rise in 40 states and declining in none. The highest activity seems to be in Southern states and on the West Coast.
Flu activity detected from wastewater was still categorized as "very low" nationwide as of Oct. 30, also according to the CDC. It noted that wastewater data tracking is experiencing a "brief gap" in updates due to a new contract with a testing company. It did not clarify how long this gap would last.
Additional CDC data through Oct. 31 also showed that about 9.14% (4,694 ) of the 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 the flu.
Wastewater data maintained by WasteWaterSCAN, a Stanford University-led tool for tracking the spread of disease, categorized flu activity as "high" nationally, meaning higher concentrations of the virus have been detected in the wastewater of more communities nationwide.
The same data shows the earliest onset of flu in at least the last four years; samples positive for influenza A were at 71% in October, compared with 40% in September and just 20% in October 2025.
The CDC likewise noted that seasonal flu activity began increasing around late August to early September
Map of flu activity by state
What are the new flu strains?
The CDC said "it was too early to know what viruses may predominate" in late October, though scientists and health officials have signaled that they are paying extra attention to a few strains.
This doesn't mean this year's viruses have transformed into something dramatically more dangerous or deadly. Rather, mutations have likely made them a little better at slipping past immunity people build from previous infections and vaccines.
H1N1, a type of influenza A, is the primary diagnosis behind this year's flu season thus far. You may recognize the name from the "swine flu" pandemic of 2009, caused by a novel strain called H1N1/09. Health officials noted that this year's virus remains part of the long-circulating seasonal H1N1 lineage rather than a new pandemic strain.
The most common H1N1 strain currently is the genetic subgroup known as subclade D.3.1.1. Scientists are watching the strain because it has accumulated additional mutations in parts of the virus targeted by human antibodies, raising concerns that some newer versions may be building upon mutations and drifting away from existing immunity almost "in 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, nobody really knows, but likely it's due to mutations," Dr. Corey K. Smith, emergency physician and Regional Medical Director at Teladoc Health, told USA TODAY. "And what happens is we were all exposed to H1N1 back in 2009, and after some time, as it mutates - and even though we developed this shield, this armor - we have slightly decreased immunity to it."
Notably, early evidence shows D.3.1.1 may impact groups of people differently. Some people's antibodies appear to still "recognize" the strain pretty well, while others seem to lose protection.
In the September study, researchers noted that teenagers and younger adults seemed to be more affected than adults, specifically people between 15 and 25.
This may be thanks 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 partially on this first encounter. So, the first flu strain you were exposed to as a child is the basis for how your immune system responds now. This is why we may see some strains of common illness impacting groups differently along generational lines.
In this specific case, the 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.
"This is pretty much how viruses work. These viruses, they sit back and say, 'hey, let's attack people, let's attack animals, let's attack birds,' whatever," said Smith. "But as they encounter us and our antibodies, they kind of go back to their homes and say, 'Hey, let's mutate and see if we can come back bigger and stronger next year or next season'."
The doctors and scientists who formulate next year's flu shot review these patterns and try to make predictions on how to prevent the next wave, he continued.
What are the latest flu symptoms?
Symptoms associated with D.3.1.1 are the same as other influenza viruses and include:
- Fever
- Body aches and chills
- Cough
- Runny or stuffy nose and sneezing
- Sore throat
- Headache
- Fatigue
Smith noted that patients often wonder which signs or symptoms can help them figure out whether they have the flu or another respiratory illness, like COVID. One major distinction he noted is the presence or absence of fever and the sniffles.
"The symptoms initially start as being relatively similar as you cross different viruses: cough, congestion, malaise (feeling generally unwell), weakness, body aches and a fever," meaning a temperature of 100.4 or higher, he said. Fever, especially in adults, is a strong indicator of flu, along with the sudden onset of body aches and chills. If you felt fine earlier and suddenly "feel like you got hit by a Mack truck" an hour or two later, it is more likely the flu, he explained.
"With a common cold, you may have a cough and a runny nose, but you don't have those extreme body aches and head pain," Smith also said. "With COVID, you can have a fever or a little cough, but congestion tends to be the main thing, plus or minus headache 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 keep contagious illnesses at bay, according to the CDC. This applies to all the ailments above and others that often circulate during the wintertime, like respiratory syncytial virus, or RSV.
Health agencies recommend washing your hands with hot, soapy water for at least 20 seconds. It's also important to wash them before and after eating, before and after handling food, after using the bathroom and after being around sick people or touching surfaces that could be contaminated. Mask-wearing while sick or out and about during illness season can also help prevent the spread.
This is especially important for people at risk of more serious disease, including children and older adults, pregnant people, immunocompromised people and those with underlying conditions like 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 prominent medical institutions. Earlier this year, leading medical groups came together to release vaccine recommendations as federal agencies continue to share inconsistent information amid a guideline shakeup and legal challenges under the leadership of known vaccine skeptic and health secretary Robert F. Kennedy Jr.
The guide recommends the yearly influenza shot to everyone aged six months or older (including pregnant people), with a few exceptions for people with certain medical conditions. People 65+ should recieve a higher dose, if possible.
Even if it doesn't stop you from catching the flu entirely, vaccination has still been shown to reduce the severity and length of illness.
"That's what vaccines do. It creates a stronger immune response and bigger, stronger antibodies. It gives us a little preview, like a film, like a trailer of what's about to come," said Smith. "Please, get vaccinated. It's not too late and it does contain various mutations and strains of the H1N1, including those that are circulating this year."
This article originally appeared on USA TODAY: The flu strain driving this season's unusually early spread
