New Delhi: For years, doctors and researchers worldwide noticed something curious. Older adults who regularly got certain vaccines, like the flu shot or shingles vaccine, seemed to stay sharper as they aged. Their memories held up better, and they were less likely to be diagnosed with dementia, including Alzheimer’s disease. At first, the idea raised eyebrows. How could a simple vaccine meant to prevent an infection have anything to do with the brain?
But as study after study uncovered similar patterns, the medical community began to take notice. Today, more evidence than ever points to a link between routine vaccinations and a lower risk of cognitive decline.
This realisation has sparked two major lines of inquiry. One focuses on the numbers—large-scale studies and health records that track millions of people across time. The other dives deep into biology, searching for clues in how the immune system and brain interact.
The evidence
Out of all the vaccines being studied, four have shown the strongest potential when it comes to protecting brain health.
The first is the influenza vaccine, a staple for older adults. One major study found that people over 65 who got their annual flu shot had up to a 40 per cent lower risk of developing Alzheimer’s disease over the next four years. Other data showed a 17 per cent reduction in overall dementia risk among those who received the vaccination, a remarkable return from something as routine as a yearly shot.
The shingles vaccine has drawn similar attention, especially in studies from Wales and Australia, where researchers used birthdate-based vaccine eligibility to eliminate bias. The results were clear: people aged 50 and older who got the shingles vaccine were 20 per cent less likely to develop dementia over the next seven years. The protective effect appeared even stronger in women, suggesting possible sex-based immune differences.
More recently, the spotlight has turned to the RSV vaccine, which was approved in the US in 2023 for older adults. A large study by researchers at the University of Oxford found that those who received the RSV shot—on top of the flu vaccine—had even lower rates of dementia. This suggests that each vaccine might add a different layer of protection.
The Tdap vaccine, which defends against tetanus, diphtheria, and whooping cough (hence the name), is also part of the picture. On its own and especially when given with the shingles vaccine, it’s been linked to a reduced risk of dementia.
Those “natural experiments” in Wales and Australia, based on something as arbitrary as a birthday cutoff, helped eliminate one of the trickiest challenges in vaccine research: the fact that healthier people are often more likely to get vaccinated in the first place. By controlling for that, scientists could see more clearly whether the vaccines themselves were making a difference. And they were.
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Link between brain and body
A key explanation is inflammation. Infections like the flu or shingles trigger inflammation throughout the body, including in the brain. This stress is thought to accelerate brain ageing. A 2023 study even linked serious infections to increased risks of both dementia and Parkinson’s. By preventing those infections, vaccines may indirectly protect the brain.
Another theory focuses on viral reactivation. Some viruses, like varicella-zoster (which causes chickenpox and shingles), can lie dormant in nerve cells for decades. Later in life, as immunity wanes, they can reactivate and inflame or damage the brain. The shingles vaccine helps stop this reawakening, reducing long-term risk.
The most intriguing idea is trained immunity—the notion that some vaccines can retrain the immune system to respond better across the board, reducing low-level chronic inflammation that’s been linked to cognitive decline. This effect may be stronger in women and in those who haven’t recently received other vaccines like the flu shot.
Families have long noticed that an illness like the flu or shingles can sometimes leave an older loved one changed, more forgetful, less sharp. Now, research backs up those observations: infections and the inflammation they cause can trigger or accelerate cognitive decline.
While most of the studies linking vaccines to lower dementia risk are observational—meaning they show a connection, not a definite cause—experts still find the evidence compelling.
Doctors like Dr Avram Bukhbinder (Massachusetts General Hospital, Boston) and Dr Pascal Geldsetzer (Stanford University School of Medicine) believe it’s time to think of vaccines as part of dementia prevention—especially for people over 50. For Dr Bukhbinder, “it just adds a more compelling reason” to get vaccinated routinely.
No single vaccine is a silver bullet. But together, they may offer a way to protect both the body and the brain.
(Edited by Prasanna Bachchhav)