The flu has been unusually severe. The CDC classified the 2024–25 influenza season as high severity, the worst since 2017–18, with elevated rates of outpatient visits, hospitalizations, and deaths across the board. More than 39,000 flu-related hospitalizations were recorded, and the season produced the highest number of pediatric flu deaths (280) since tracking began in 2004, second only to the 2009 H1N1 pandemic.
Several factors collided to make this season hit so hard: two aggressive virus subtypes circulating at the same time, a vaccine that offered only modest protection against them, and a population whose immune defenses had been reshaped by years of reduced flu exposure during the pandemic.
Two Strains Circulating at Once
Most flu seasons are dominated by a single subtype. This season was different. Two influenza A subtypes, H1N1 and H3N2, circulated at nearly equal levels nationwide. H1N1 accounted for about 53% of subtyped samples, while H3N2 made up roughly 47%. That dual circulation meant the virus had, in effect, two separate chances to find vulnerable hosts throughout the season.
H1N1 drove the majority of hospitalizations, accounting for nearly 59% of subtyped cases among the 39,319 people admitted to hospitals. It also carried a higher risk of severe outcomes. Large-scale studies have found that people infected with H1N1 are about 42% more likely to end up in intensive care and 79% more likely to need mechanical ventilation compared to those infected with H3N2. H1N1 tends to cause more direct damage to the lining of the airways, which can escalate to pneumonia more readily.
H3N2, meanwhile, is notorious for being harder to match in vaccines because its surface proteins mutate quickly. Globally, H3N2 has now become the predominant circulating subtype heading into the next season, with a rapidly spreading new genetic subclade (called K) detected in multiple countries. While current data don’t suggest this new subclade causes more severe illness, its rapid genetic drift is a signal that immunity from prior infection or vaccination may not hold up as well.
The Vaccine Offered Limited Protection
Flu vaccines work better in some years than others, depending on how well the vaccine strains match what’s actually circulating. This season, the match was mediocre at best. For adults, the vaccine reduced the risk of a flu-related doctor visit by only 22% to 34%, depending on the study network. Protection against hospitalization was about 30% for adults overall and 31% for those 65 and older.
Children fared slightly better but not by much. Vaccine effectiveness against flu-related hospitalization in kids was around 42% for influenza A overall and 38% specifically against H3N2. Protection against influenza B was considerably stronger, ranging from 45% to 71% in children and around 63% in adults, but influenza B caused only a small fraction of illness this season.
To put those numbers in context, a vaccine that’s 30% effective still prevents roughly 3 out of every 10 infections it encounters, which translates to thousands of avoided hospitalizations at the population level. But it also means most vaccinated people who were exposed to the virus had a real chance of getting sick anyway. That’s a noticeable gap compared to seasons where effectiveness runs closer to 50% or 60%.
Pandemic-Era Immunity Gaps
The years of masking, social distancing, and reduced mixing during the COVID-19 pandemic meant far fewer people were exposed to seasonal flu from 2020 through 2022. That created what epidemiologists call an “immunity gap,” particularly in young children who missed their early exposures to influenza entirely. The concept of immune imprinting plays a role here too: the specific flu subtype you encounter first in childhood shapes how your immune system responds to flu for the rest of your life. Children born during the pandemic years may not have had that formative first encounter, leaving them more vulnerable when flu returned in force.
The 2023–24 season had already shown warning signs, recording 210 pediatric deaths, the highest non-pandemic total at that point. This season surpassed it by a wide margin, reaching 280 pediatric deaths, a rate of 3.8 per million children nationwide.
Antivirals Still Work
One piece of reassuring news: the circulating viruses remained highly susceptible to available antiviral treatments. Among thousands of H1N1 samples tested in Europe, fewer than 1% showed any reduction in susceptibility to standard antivirals. H3N2 resistance rates were similarly low, at around 0.2%. This means that if you’re prescribed antiviral medication within the first 48 hours of symptoms, it’s very likely to work as expected, shortening the duration of illness and reducing the risk of complications.
Why It Felt Worse Than Usual
Beyond the epidemiological data, there are practical reasons this season felt particularly rough for so many people. When two subtypes co-circulate, it’s possible to catch the flu twice in one season, once from each subtype. Anecdotally, many people reported exactly that: recovering from one bout only to get hit again weeks later. The season also stretched over a long period rather than peaking sharply and fading, which meant sustained pressure on hospitals, clinics, and workplaces.
The combination of factors, dual strain circulation, a vaccine that underperformed against both subtypes, and a population still rebuilding its baseline immunity, created a perfect storm. None of these factors alone would have produced the worst season in seven years. Together, they compounded into a season that was genuinely more dangerous, especially for children, older adults, and people with chronic health conditions.