The flu is hitting harder than usual because multiple factors are converging at once: two aggressive influenza A strains are circulating simultaneously, the virus has mutated enough to partially dodge vaccine protection, and years of reduced exposure during the pandemic left many people with weakened immune defenses. The current season’s cumulative hospitalization rate is the third highest since the 2010-11 season, and the CDC has classified recent flu activity as high severity across all age groups.
Two Strains Circulating at Once
Most flu seasons are dominated by a single strain. This time, two influenza A subtypes are spreading at nearly equal levels: A(H1N1) accounts for about 53% of subtyped viruses and A(H3N2) makes up the remaining 47%. When two strains co-circulate like this, the overall burden climbs because your immune system’s prior exposure to one strain doesn’t protect you from the other. You can effectively get the flu twice in the same season.
Influenza B viruses have remained relatively quiet, though a small uptick in the B/Victoria lineage appeared later in the season. The real damage has come from the A strains working in tandem to push hospitalization numbers well above historical averages.
The Virus Has Outpaced the Vaccine
Flu viruses constantly accumulate small genetic changes, a process called antigenic drift. This season, both circulating strains have drifted enough that the current vaccine is a less-than-ideal match. Researchers at Johns Hopkins found that blood samples from vaccinated individuals showed reduced ability to neutralize dominant H1N1 and H3N2 isolates compared to the vaccine strains. In plain terms, the virus changed just enough that antibodies trained by the vaccine don’t latch on as tightly.
CDC interim estimates reflect this mismatch. For adults, the vaccine prevented somewhere between 36% and 54% of outpatient flu infections, depending on the study network, and between 41% and 55% of hospitalizations. Children fared somewhat better, with effectiveness against hospitalization ranging from 63% to 78%. Those numbers are lower than what public health officials aim for, but they still mean vaccinated people who do catch the flu tend to have milder illness and shorter recovery times.
The “Immunity Debt” Effect
During the pandemic years of masking, social distancing, and lockdowns, flu transmission dropped dramatically, roughly 46% globally. That was good in the short term but created a problem: millions of people, especially young children, went one or two full seasons with little to no flu exposure. Without that regular immune “refresher,” the population’s collective defenses weakened.
The payback has been steep. A large international study found that in the first winter after COVID restrictions were lifted, flu cases surged 132% above historically predicted levels. Summer flu activity, normally minimal, jumped even higher at 161% above expected rates. The researchers found this wasn’t limited to children who missed early exposures. Adults across all age groups showed the effects of reduced immune priming. Countries that enforced stricter pandemic measures saw larger rebounds, consistent with the idea that longer isolation from the virus created a bigger gap in immunity.
We’re still working through this deficit. Each season since the pandemic has been more severe than what was typical in the 2010s, and the immunity gap is a major reason why.
Children Are Being Hit Especially Hard
Pediatric flu outcomes this season are alarming. The cumulative hospitalization rate for children is the second highest in the past 15 seasons. So far, 79 children and teens have died from the flu, and roughly 90% of those who died were unvaccinated or incompletely vaccinated.
This follows the 2024-25 season, which recorded 293 pediatric flu deaths, the highest in any season since the CDC began tracking in 2004. Children born during or just after the pandemic missed early flu exposures that normally help build foundational immunity. For kids under five, whose immune systems are still learning to recognize respiratory viruses, those missed years of exposure matter enormously.
Air Pollution Amplifies the Impact
An underappreciated factor in severe flu seasons is air quality. Research across six U.S. cities found that fine particulate matter (the tiny particles from vehicle exhaust, wildfires, and industrial emissions) has a significantly stronger association with respiratory emergency visits during peak flu periods than during low flu periods. In Atlanta, for example, the risk of a respiratory emergency visit from air pollution exposure was meaningfully higher when flu was circulating widely compared to when it wasn’t.
The mechanism is straightforward: air pollution irritates and inflames the airways, making the respiratory tract more vulnerable to viral infection. When flu is already widespread, polluted air essentially opens the door wider. Cities experiencing poor air quality during winter months can expect flu to hit their populations harder than the national averages suggest.
Multiple Viruses Circulating Together
The flu isn’t spreading in isolation. COVID-19 and RSV continue to circulate, and getting infected with more than one virus at a time worsens outcomes considerably. Studies show that patients co-infected with influenza and COVID-19 face roughly double the risk of ICU admission and more than double the risk of needing mechanical ventilation compared to those with COVID alone. Among hospitalized children under one year old, about one in three COVID patients also had a viral co-infection, most commonly RSV.
This “triple threat” dynamic strains hospital capacity even when individual virus severity might be manageable. Emergency departments dealing with simultaneous surges of flu, COVID, and RSV face resource constraints that can delay care for all respiratory patients.
Standard Antivirals Still Work
One piece of genuinely good news: the flu viruses circulating right now remain fully susceptible to available antiviral treatments. CDC surveillance testing of 266 virus samples found zero cases of reduced effectiveness for the three main antiviral medications used to treat flu. This means that if you’re prescribed an antiviral within the first 48 hours of symptoms, it should work as expected to shorten illness and reduce the risk of complications.
The one exception involves an older class of antivirals called adamantanes, which have been ineffective against circulating flu strains for years and are no longer recommended.
Where the Season Stands Now
As of the most recent surveillance data, about 17.9% of respiratory specimens tested at clinical laboratories are coming back positive for influenza. The weekly hospitalization rate has started to decrease, suggesting the peak may be passing. Still, the cumulative toll is severe: more than 25,500 laboratory-confirmed flu hospitalizations have been reported since the season began in October, with a cumulative rate of 73.3 per 100,000 people.
To put that in context, that cumulative rate is the third highest in 15 years of tracking. The combination of dual circulating strains, pandemic-era immunity gaps, partial vaccine mismatch, and co-circulating respiratory viruses has produced a season that stands out even among historically bad ones. The decline in weekly numbers is encouraging, but flu seasons can produce secondary waves, particularly if a strain that was less dominant early on gains ground later.