Does the Flu Shot Lower Dementia Risk? What a Study of 165,000 Older Adults Found

Every year the same reminder arrives: it is time for your flu shot. We usually think of it as protection for the weeks ahead, a way to avoid a week in bed, or something worse. A study published earlier this year in the journal Neurology asked a different question: could the type of flu vaccine an older adult receives be linked to their chances of being diagnosed with Alzheimer's disease years later? It has been back in the news this month, which makes it worth a calm look.

The short answer is modest, hopeful and still uncertain.

Does the flu shot lower your risk of dementia?

Possibly a little, but nobody knows for sure yet. Flu vaccination is recommended for older adults mainly because flu can make them seriously ill, and serious illness is hard on the whole body, including the brain. Several observational studies have linked flu vaccination to a lower chance of a later dementia diagnosis. But people who get vaccinated also tend to look after their health in other ways, which can explain part of the pattern. To our knowledge, no randomised trial has shown that a flu shot prevents dementia. The study below goes one step further and compares two kinds of flu vaccine with each other, which helps with some of that problem, but not all of it.

What the researchers did

A team at UTHealth Houston, led by Paul Schulz, used a large US database of health insurance claims from 2014 to 2019. They included adults aged 65 and over with no earlier diagnosis of dementia who had received a flu vaccine: 120,775 with a high-dose vaccine and 44,022 with a standard-dose vaccine. A high-dose vaccine contains more of the active ingredient and is designed to give an older immune system a stronger response. Instead of comparing vaccinated with unvaccinated people, the researchers compared the two vaccine types with each other and followed everyone for up to three years. An Alzheimer's diagnosis was identified from medical codes and from prescriptions for approved Alzheimer's medicines.

What they found

In the first couple of years after vaccination, people who had received the high-dose vaccine were somewhat less likely to be diagnosed with Alzheimer's dementia. At its widest, around two years after the shot, the gap was about half a percentage point. That works out to roughly one fewer diagnosis for every 185 people who received the high-dose vaccine. A second, more cautious way of analysing the same data gave a smaller gap, about one in 270. The difference showed up earlier and more clearly in women, and less consistently in men. Some news reports quoted a much larger relative figure. We prefer the absolute numbers because they show what the difference could mean for one person, and it is small. The researchers also checked outcomes that a vaccine should not affect and found no link there, which suggests the method was not simply picking up noise.

What this does and does not mean

This is an association. Nothing in this study shows that a high-dose flu vaccine prevents Alzheimer's disease. Even with careful statistics, people who receive the high-dose vaccine may differ from those who receive the standard dose in ways the data cannot see, such as health, income or how often they see a doctor. The lead author himself named this kind of healthy-user effect as a concern. The authors suggest two ideas for how a vaccine might matter: fewer severe flu infections and less inflammation in the body, or a longer-lasting change in immune responses. Both are hypotheses, not findings. One outside expert also described the evidence as nowhere near as strong as for the shingles vaccine.

The honest limits

Claims data record diagnoses for billing, not after careful memory testing, and the codes used here also included unspecified dementia, so some cases may be mislabelled as Alzheimer's. Three years is short for a disease that develops over decades. The researchers had little information about lifestyle, education or biological markers. The data came from US commercial insurance, so the results may not apply to everyone, and results for mild cognitive impairment were mixed. The authors call for longer studies in more varied groups, ideally with biomarkers to confirm diagnoses.

How to use this

This study is not a reason to change anything on its own, and nobody should choose a vaccine because of it. Flu shots are recommended for older adults because of what flu itself can do, and that reason stands even without any benefit for the brain. In the United States, the CDC already recommends a higher-dose vaccine for people aged 65 and over, and availability and advice differ from country to country. If you are unsure which flu vaccine is right for you, a doctor or pharmacist can tell you.

A small, steady habit

Brain health rarely comes down to one big decision. It tends to be built from many small, steady ones: a vaccine appointment kept, a walk, a good night's sleep, a few minutes of mental play. Treeny is built around that same idea. The daily exercises are short and meant to be enjoyed, and Flo, the coach inside the app, adjusts them to how you are doing each day. Treeny was not part of this study and does not treat or prevent any disease.

A note on how to read this

This article describes research and is not medical advice. The study shows an association and cannot show that any flu vaccine lowers the risk of Alzheimer's disease. Please talk to your doctor or pharmacist about which vaccine suits you, and about any worries you have about your memory.