With dementia affecting about 10 million new people each year worldwide, the scientific community is intensifying efforts to identify ways to prevent or at least delay its onset. Beyond well-known lifestyle strategies, new evidence highlights an unexpected ally in brain protection: the shingles (herpes zoster) vaccine.

Researchers at Stanford University School of Medicine analyzed data from older adults and found that those vaccinated against shingles were 20% less likely to develop dementia within the next seven years compared to the unvaccinated. The study, published a few months ago in Nature, was based on the theory that viruses affecting the nervous system may increase dementia risk.

Even more intriguing were the findings of a follow-up study, published a few days ago in Cell: the same vaccine appears not only to offer preventive protection, but may also benefit individuals already diagnosed with dementia by slowing the progression of the disease.

The Relationship Between the Two Conditions

Shingles is a painful viral infection triggered by the same virus that causes chickenpox (varicella-zoster). After childhood infection, the virus remains “hidden” in nerve cells and can reactivate later in life, especially in people with weakened immune systems.

Dementia, on the other hand, affects more than 55 million people globally, with around 10 million new cases each year. Alzheimer’s disease is the most common form and has been the subject of decades of intense research, including investigations into the role of viral infections in its progression.

Previous studies had observed that the shingles vaccine was associated with a lower likelihood of developing dementia. However, they could not rule out the possibility that vaccinated individuals simply tended to lead healthier lifestyles, which are not always captured in medical records and could influence the outcomes.

A “Natural Experiment” for a Clear Answer

This bias was overcome thanks to a peculiarity of Wales’ vaccination program, launched on September 1, 2013. The program offered a live attenuated vaccine only to individuals who were exactly 79 years old on that date. Those aged 78 became eligible the following year, and so on, while those 80 or older were never eligible.

This extremely narrow age cutoff—distinguishing people who turned 80 just a week earlier or later—created a rare opportunity: researchers could compare two nearly identical groups, as if in a randomized clinical trial.

Using medical records from more than 280,000 older adults without dementia at the beginning of the program, scientists examined the vaccine’s effects by comparing individuals closest to the eligibility threshold.

What the Results Showed

Seven years of follow-up revealed striking differences:

• The vaccine reduced shingles cases by about 37%.
• One in eight older adults developed dementia by 2020, but vaccinated individuals had a 20% lower risk.
• No other differences were found between the groups—only in dementia rates.
• The analysis also revealed that the vaccine’s benefits span the entire spectrum of cognitive decline:
  • Vaccinated individuals were less likely to develop mild cognitive impairment within nine years.
  • Those who received the vaccine after being diagnosed with dementia were significantly less likely to die from the disease in the following nine years, indicating a slowing of disease progression.

Overall, nearly half of the 7,049 older adults with dementia at the program’s start died from the disease during follow-up, while this proportion dropped to about 30% among vaccinated individuals.

Promising, But Questions Remain

The protective effect appeared stronger in women, possibly due to differences in immune function or disease progression. However, the exact mechanism by which the vaccine reduces risk remains unknown. It is also unclear whether the newer generation of vaccines—modern protein-based, non-live formulations—offer equal or even greater protection.

Dr. Geldsetzer and his team have already replicated their analysis using data from England, Australia, New Zealand, and Canada, and their next goal is a large randomized clinical trial to provide a definitive answer.