HERPES zoster vaccination was associated with reduced dementia diagnoses in elderly Canadian adults throughout natural experiments.
Herpes Zoster Vaccination and Incident Dementia
Researchers assessed whether live attenuated herpes zoster vaccination was linked to incident dementia using policy driven eligibility cut offs in Ontario, Canada. The analysis focused on adults aged 70 years and older and leveraged the fact that people born just before versus just after specific dates were similar in health and behaviour, aside from vaccination eligibility.
Using electronic health records from primary care practices spanning 1990 to mid 2022, the investigators examined new dementia diagnoses after the herpes zoster vaccination programme began. They also used survey data in Ontario to quantify differences in shingles vaccine uptake around the eligibility thresholds.
How the Natural Experiments Worked
The primary natural experiment compared individuals born immediately before versus immediately after January 1, 1946, because this date determined eligibility for the programme in Ontario. A second analysis used January 1, 1945, as an additional eligibility threshold. In both comparisons, baseline characteristics were reported as similar on either side of the cut off, supporting the premise that vaccination uptake, rather than underlying differences, drove any outcome divergence.
Across a 5.5 year follow up, being born on the eligibility side of the threshold was associated with an absolute 2.0 percentage point reduction in new dementia diagnoses, with confidence intervals that did not cross zero in either threshold analysis.
Clinical Takeaways and Next Questions
To triangulate findings beyond the cut off design, the team compared dementia incidence trends before versus after programme initiation in Ontario birth cohorts eligible for vaccination against the same birth cohorts in other Canadian provinces where a comparable programme was not in place. In these quasi experimental comparisons, eligible Ontario cohorts had fewer new dementia diagnoses after the programme began.
The authors interpreted the convergence of results as evidence more consistent with a causal relationship than standard observational analyses, raising the possibility that herpes zoster vaccination may prevent or delay incident dementia. They called for mechanistic research to clarify pathways and inform understanding of neuroimmune health in older age.
Reference: Pomirchy M et al. Herpes zoster vaccination and incident dementia in Canada: an analysis of natural experiments. Lancet Neurol. 2026;25(2):170-180.
