Can Flu Shots Protect Your Brain Against Dementia? How Strong Is the Evidence?

I did an updated and targeted meta-analysis to find out.

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Highlights:

• Most studies investigating the neuroprotective effects of flu vaccines focused on dementia (only one examined Parkinson’s disease).

• The most recent meta-analysis (2023) synthesized six studies and found that flu vaccination was associated with a 31% reduced risk of dementia.

• Incorporating five new studies into an updated meta-analysis of 11 studies lowered the estimated benefit to a 21% risk reduction.

• A more targeted meta-analysis of studies that adjusted for healthcare access or utilization—to account for the healthy vaccinee bias—found the protective effect was no longer statistically significant.

• When further restricting to studies that also examined the number of vaccine doses, a significant dose-dependent effect emerged: receiving at least 4 flu shots was associated with an 18% lower risk of dementia.

• Caveats and the real-life context of these findings are then discussed.

Background

Every year, millions of people get their annual flu shot, primarily to avoid a miserable week in bed or, worse, a trip to the hospital. But research suggests that flu shots might also protect against neurodegenerative diseases. Could a flu shot provide long-term benefits for brain health, or is this just another case of correlation being mistaken for causation?

In this article, I’ll assess how strong the evidence is for the potential neuroprotective effects of the flu vaccine. This piece also builds on my previous two articles, in which I conducted an updated meta-analysis to quantify the impact of influenza infection on the future risk of Parkinson’s disease (PD) and dementia, the two most common neurodegenerative diseases worldwide. (Read them here and here.)

Methods

I first scanned PubMed, the leading biomedical literature database, using the following keywords: (influenza[tiab] OR flu[tiab]) AND (vaccination[tiab] OR vaccine[tiab]) AND (dementia[tiab] OR Alzheimer[tiab] OR Parkinson[tiab] OR neurodegenerative[tiab]). The code [tiab] means searching for the term in the title or abstract. As of March 26, 2025, the search returned 150 results.

Going through them informed me that most studies examined the association between flu vaccines and dementia (with only one study investigating PD). The latest meta-analysis on this topic was published in 2023, which scoured the literature as of September 14, 2022. But since then, five additional relevant studies have been published.

I then performed an updated meta-analysis with Review Manager, as I did previously in “Influenza and Parkinson’s: How Big Is the Risk? An Updated Meta-Analysis” and “Does Influenza Raise the Risk of Dementia? An Updated Meta-Analysis.” As dementia is not a prevalent disease (with a <10% incidence), various risk measurement—risk ratio (RR), odds ratio (OR), and hazard ratio (HR)—are treated equally in the meta-analysis.

I also conducted a more targeted meta-analysis to synthesize studies that controlled for the healthy vaccinee bias to some extent. This bias occurs when people who choose to get vaccinated are generally more health-conscious and have accessible healthcare, which could already lower the risk of dementia by itself. Next, I performed an even more targeted meta-analysis on studies that not only adjusted for healthy vaccinee bias but also considered the number of flu vaccinations involved.