Both work. Neither is clearly superior.
Among the most vulnerable in any flu season — adults over 65, who account for the vast majority of influenza deaths — a long-standing question has quietly persisted: does it matter which enhanced vaccine they receive? A landmark real-world study across Northern California, enrolling nearly 430,000 older adults, now offers a measured answer: both the adjuvanted and high-dose influenza vaccines protect with near-identical effectiveness, a finding that transforms clinical uncertainty into practical freedom.
- Older adults bear 70–85% of all U.S. flu deaths each year, making every percentage point of vaccine effectiveness a matter of life and death at population scale.
- For years, clinicians lacked direct evidence comparing the two recommended enhanced vaccines, leaving vaccine selection guided more by availability than by outcome data.
- Researchers designed an elegant rotating study across 65 clinics — alternating vaccines week by week — to mirror real-world conditions rather than controlled laboratory settings.
- The results landed with quiet force: just 1.5% relative difference in confirmed flu cases between 429,595 participants, with serious outcomes like hospitalizations showing no statistically significant gap.
- The finding hands health systems and clinicians something rare — genuine, evidence-backed flexibility to choose whichever enhanced vaccine they can reliably supply and deliver.
Every year, influenza claims the lives of older Americans at a disproportionate rate — between 70 and 85 percent of all flu deaths in the United States occur in adults over 65. Public health guidance has long recommended enhanced vaccines for this group, either an adjuvanted formula that stimulates the immune system with a special additive, or a high-dose version containing more antigen. Yet until now, no large real-world study had directly compared the two.
Researchers at Kaiser Permanente Northern California set out to answer that question with unusual rigor. Between August 2023 and April 2024, they enrolled 429,595 adults aged 65 and older across 65 medical facilities. Rather than assigning patients arbitrarily, clinics alternated which vaccine they administered week by week throughout the season — a design that closely mirrors how vaccination actually unfolds in practice.
The findings were striking in their sameness. Among those who received the adjuvanted vaccine, confirmed influenza occurred at a rate of 3.93 cases per 1,000 people; among those given the high-dose vaccine, the rate was 4.00 per 1,000 — a relative difference of just 1.5 percent. Hospitalizations, emergency visits, and pneumonia complications told the same story: no meaningful gap between the two.
The significance lies not in perfection but in parity. Clinicians now have clear, real-world evidence that neither vaccine is superior — a conclusion with direct implications for supply chains, patient access, and public health planning. The study's authors note it was conducted within a single health system and covers only one flu season, with a second year of data still forthcoming. But the early message is unambiguous: choose whichever enhanced vaccine you can reliably deliver. The protection will be essentially the same.
Every year, influenza kills more people over 65 than any other age group—between 70 and 85 percent of all flu deaths in the United States occur in this population. For decades, public health officials have recommended enhanced vaccines for older adults: either an adjuvanted formula, which uses an immune-boosting additive, or a high-dose version with more antigen. But no one had actually tested whether one worked better than the other in a real-world setting. A massive study conducted across Northern California now has an answer: they work about the same.
Researchers at Kaiser Permanente Northern California enrolled 429,595 adults aged 65 and older between August 2023 and April 2024. The study was elegantly simple in design. Sixty-five medical facilities participated. During the first week of flu season, half the clinics gave the adjuvanted vaccine while the other half gave the high-dose version. Then, week by week throughout the season, the facilities alternated which vaccine they administered. This meant that by the end of the season, each clinic had offered both vaccines in a balanced, rotating pattern—mimicking how vaccination actually happens in practice rather than forcing an artificial laboratory setup.
The researchers tracked what happened over the next several months. They counted confirmed influenza cases using PCR tests, the gold standard for diagnosis. They also looked at serious outcomes: hospitalizations and emergency department visits related to flu, and hospitalizations for community-acquired pneumonia, a common complication in older people who get sick. The study population was diverse and representative of the broader Northern California region, with an average age of 75 years.
The results were striking in their similarity. Among the 212,875 people who received the adjuvanted vaccine, 836 developed confirmed influenza—a rate of 3.93 cases per 1,000 people. In the 216,720 people who got the high-dose vaccine, 867 came down with flu, or 4.00 cases per 1,000. The relative difference between the two groups was just 1.5 percent. When the researchers looked at serious outcomes—hospitalizations and emergency visits—the adjuvanted group had 411 cases compared to 466 in the high-dose group, a difference that also failed to reach statistical significance. For pneumonia hospitalizations, the vaccines performed nearly identically as well.
What makes this finding important is not that the vaccines are perfect—they're not. Rather, it's that clinicians and public health systems now have genuine flexibility. For years, the question of which enhanced vaccine to use has been somewhat open. This study closes that loop. Both work. Neither is clearly superior. That matters for vaccine supply chains, for patient access, and for the practical business of keeping older Americans protected during flu season.
The study does have limits. It was conducted within a single integrated health system in Northern California, so results may not apply everywhere, particularly in regions with different flu strains or in uninsured populations. The researchers also note this was only the first season of a planned two-year study. Second-season data could refine the picture further, though early indications suggest the pattern will hold. For now, the message to doctors and health systems is clear: choose whichever enhanced vaccine you can reliably access and administer. The protection will be essentially the same.
Notable Quotes
Both enhanced vaccines are reasonable options for vaccinating older adults, and neither showed statistically better effectiveness than the other in the 2023-2024 season.— Study findings published in JAMA Network Open
The Hearth Conversation Another angle on the story
Why did it take until 2024 to actually compare these two vaccines head-to-head?
Because doing it right is hard. You need a huge population, you need real-world conditions, and you need to avoid bias—like one group seeking testing more than the other. Most previous studies just showed that enhanced vaccines beat standard-dose vaccines. Nobody had actually run a fair comparison between the two enhanced options.
So the finding is basically that they're interchangeable?
Functionally, yes. The difference in protection was 1.5 percent—well within the margin of statistical noise. For serious outcomes like hospitalization, the picture was the same. Both vaccines kept older people out of the hospital at nearly identical rates.
Does that mean the vaccines are failing? A 4 per 1,000 infection rate sounds low, but it's not zero.
It's not about failure. These are older adults, many with chronic conditions. The vaccines are preventing the majority of infections. What matters is that they're preventing the severe outcomes—the hospitalizations, the pneumonia, the deaths. That's where the real protection shows up.
Why does it matter that both vaccines work equally well? Doesn't that just create more confusion?
The opposite. It removes a bottleneck. If one vaccine had been clearly better, you'd want to prioritize it. But since they're equivalent, health systems can use whichever one they have in stock, whichever is easier to distribute. That flexibility actually gets more people vaccinated.
What happens next with this research?
They're running the study for a second season. Flu strains shift year to year, so they want to see if the pattern holds. But based on what we know about how these vaccines work biologically, there's no reason to expect a different answer.