Scientists: A common vaccine appears to have a surprising impact on brain health

A new scientific commentary suggests that annual influenza vaccination could serve as a practical and accessible strategy to help delay or prevent the onset of dementia in older adults. By mitigating the risk of severe cardiovascular events and reducing systemic inflammation, the seasonal flu shot may offer neurological protection that extends well beyond respiratory health. This perspective article was published in the journal Aging Clinical and Experimental Research.

Dementia poses a significant and growing challenge to aging societies worldwide, creating an urgent need for scalable prevention strategies. While controlling midlife risk factors like high blood pressure remains a primary focus, medical experts are looking for additional tools that can be easily integrated into existing healthcare routines.

Lorenzo Blandi from the Vita-Salute San Raffaele University and Marco Del Riccio from the University of Florence authored this analysis to highlight the potential of influenza vaccination as a cognitive preservation tool. They argue that the current medical understanding of the flu shot is often too limited. The researchers propose that by preventing the cascade of physical damage caused by influenza, vaccination can help maintain the brain’s vascular and cellular health.

The rationale for this perspective stems from the observation that influenza is not merely a respiratory illness. It is a systemic infection that can cause severe complications throughout the body. The authors note that influenza infection is associated with a marked increase in the risk of heart attacks and strokes in the days following illness.

These vascular events are known to contribute to cumulative brain injury. Consequently, Blandi and Del Riccio sought to synthesize existing evidence linking vaccination to improved cognitive outcomes. They posit that preventing these viral insults could modify the trajectory of dementia risk in the elderly population.

To support their argument, the authors detail evidence from four major epidemiological studies that demonstrate a link between receiving the flu shot and a lower incidence of dementia. The first piece of evidence cited is a 2023 meta-analysis. This massive review aggregated data from observational cohort studies involving approximately 2.09 million adults.

The participants in these studies were followed for periods ranging from four to thirteen years. The analysis found that individuals who received influenza vaccinations had a 31 percent lower risk of developing incident dementia compared to those who did not.

The second key study referenced was a claims-based cohort study. This research utilized propensity-score matching, a statistical technique designed to create comparable groups by accounting for various baseline characteristics. The researchers analyzed data from 935,887 matched pairs of older adults who were at least 65 years old.

The results showed that those who had received an influenza vaccination had a 40 percent lower relative risk of developing Alzheimer’s disease over a follow-up period of roughly four years. The study calculated an absolute risk reduction of 3.4 percent, suggesting that for every 29 people vaccinated, one case of Alzheimer’s might be prevented during that timeframe.

The third study highlighted in the perspective used data from the Veterans Health Administration. This study was significant because it used time-to-event models to address potential biases related to when vaccinations occurred.

The researchers found that vaccinated older adults had a hazard ratio for dementia of 0.86. This statistic indicates a risk reduction of roughly 14 percent. The data also revealed a dose-response relationship. This means that the protective signal was strongest among participants who received multiple vaccine doses across different years and seasons, rather than just a single shot.

The fourth and final study cited was a prospective analysis of the UK Biobank. This study modeled vaccination as an exposure that varies over time, allowing for a nuanced view of cumulative effects.

The researchers observed a reduced risk for all-cause dementia, with a hazard ratio of 0.83. The reduction in risk was even more pronounced for vascular dementia, showing a hazard ratio of 0.58. Similar to the veterans’ study, this analysis supported the idea of a dose-response relationship. The accumulation of vaccinations over time appeared to correlate with better cognitive outcomes.

Blandi and Del Riccio explain several biological mechanisms that could account for these protective effects. The primary pathway involves the prevention of vascular damage. Influenza infection is a potent trigger for inflammation and blood clotting.

Research shows that the risk of acute myocardial infarction can be six times greater in the first week after a flu infection. By preventing the flu, the vaccine likely prevents these specific vascular assaults. Since vascular health is closely tied to brain health, avoiding these events helps preserve cognitive reserve. The cumulative burden of small strokes or reduced blood flow to the brain is a major predictor of cognitive decline.

In addition to vascular protection, the authors discuss the role of neuroinflammation. Studies in animal models have shown that influenza viruses can trigger activation of microglia, which are the immune cells of the brain. This activation can lead to the loss of synapses and memory decline, even if the virus itself does not enter the brain.

Systemic inflammation caused by the flu can cross into the nervous system. The authors suggest that vaccination may dampen these inflammatory surges. There is also a hypothesis known as “trained immunity,” where vaccines might program the immune system to respond more efficiently to threats, reducing off-target damage to the brain.

Based on this evidence, the authors propose several policy changes and organizational strategies. They argue that public health messaging needs to be reconceptualized. Instead of framing the flu shot solely as a way to avoid a winter cold, health officials should present it as a measure to reduce heart attacks, strokes, and potential cognitive decline. This approach addresses the priorities of older adults, who often fear dementia and loss of independence more than respiratory illness.

The authors also recommend specific clinical practices. They suggest that health systems should prioritize the use of high-dose or adjuvanted vaccines for adults over the age of 65. These formulations are designed to overcome the weaker immune response often seen in aging bodies.

Additionally, the authors advocate for making vaccination a default part of hospital discharge procedures. When an older adult is leaving the hospital after a cardiac or pulmonary event, vaccination should be a standard component of their care plan. This would help close the gap between the known benefits of the vaccine and the currently low rates of uptake in many regions.

Despite the promising data, Blandi and Del Riccio acknowledge certain limitations in the current body of evidence. The majority of the data comes from observational studies. This type of research can identify associations but cannot definitively prove causality.

There is always a possibility of “healthy user bias,” where people who choose to get vaccinated are already more health-conscious and have better lifestyle habits than those who do not. While the studies cited used advanced statistical methods to control for these factors, residual confounding can still exist.

The authors also note that studies based on medical claims data can suffer from inaccuracies in how dementia is diagnosed and recorded. Furthermore, the precise biological mechanisms remain a hypothesis that requires further validation. The authors call for future research to include pragmatic randomized trials that specifically measure cognitive endpoints. They suggest that future studies should track biological markers of neuroinflammation in vaccinated versus unvaccinated groups to confirm the proposed mechanisms.

The study, “From breath to brain: influenza vaccination as a pragmatic strategy for dementia prevention,” was authored by Lorenzo Blandi and Marco Del Riccio.