Does shingles increase a person's risk of dementia?

Researchers then determined which participants developed dementia up to 21 years after their shingles diagnosis. Of people who had shingles, 9.7% developed dementia. Of people who did not have shingles, 10.3%, developed dementia.

After adjusting for other health conditions such as diabetes, cancer, and traumatic head injury, researchers found that people with shingles had a 7% lower risk of dementia than people who did not have shingles.

"We were surprised by these results," said Schmidt. "The reasons for this decreased risk are unclear, but it could be explained by missed diagnoses of shingles in people with undiagnosed dementia. Shingles vaccination is encouraged for older people because it can prevent complications from the disease, but our study suggests it is unlikely to reduce dementia risk."

Researchers did find that people who had shingles that had spread to the central nervous system had nearly twice the risk of developing dementia. However, Schmidt said such complications are rare, affecting below 0.1% of those with shingles.

A limitation of the study was that participants were identified based on antiviral prescriptions or hospital visits for the disease so results may not be the same for people with milder cases and those who are not treated for the disease.