The Health Information and Quality Authority (HIQA) has published a scoping review examining whether there is evidence that shingles (herpes zoster) vaccination may be associated with a lower risk of dementia. This review follows HIQA’s 2024 Health Technology Assessment of Herpes Zoster Vaccination and was commenced by HIQA in response to emerging evidence reporting a link between shingles vaccination and lower dementia risk.

This review identified three considerations to help understand how shingles vaccination might be associated with a lower risk of dementia. These included the link between shingles and dementia, between shingles vaccination and dementia, and between adult vaccination generally and dementia. The review looked for studies to address each of these three topics.

HIQA’s review found that some studies reported that shingles increases the risk of dementia, while others reported no clear link. For the second question, the available studies reported a lower incidence of dementia in people with a history of shingles vaccination or in groups who were eligible for vaccination compared with those who were not. While these studies can show an association, they cannot prove that shingles vaccination reduces dementia risk. For the third question, many reviews showed that other adult vaccines, including influenza, are also linked with a lower risk of dementia. Again, they cannot prove that vaccination alone resulted in lower risk.

In the absence of randomised clinical trials, HIQA concluded that it is not possible to determine whether shingles vaccination by itself directly reduces the risk of dementia. This is because the available evidence only looked at association and not causation. Other factors, including vaccinated individuals having healthier lifestyles and or lower levels of underlying risk, were also identified as potential explanations for the lower observed incidence of dementia.

HIQA has advised that the findings of this scoping exercise should be considered in the planned review of any new evidence of the clinical effectiveness and safety of shingles vaccination since the publication of its 2024 HTA. This planned review will inform any reconsideration of whether a national shingle vaccination programme is a good use of healthcare resources. Read the full scoping review at hiqa.ie.