Every three seconds, someone, somewhere in the world, develops dementia. The number of people living with the condition is projected to rise dramatically, doubling from 78 million in 2020 to 139 million by 2050, making dementia an urgent public health concern of our time. Now, a Canadian study published in The Lancet Neurology found that herpes zoster vaccination, known by the brand name Zostavax, reduced or delayed dementia diagnosis by 2 percentage points over 5.5 years. The benefits were more pronounced in women than in men.
The researchers initially extracted records for nearly 230,000 elderly patients born between 1930 and 1960 who were living in Ontario and registered with primary care providers.
They carried out a natural experiment—where researchers study the effects of naturally occurring events without manipulating variables—that took advantage of a policy change in Canada in 2016, creating a clear comparison group. Ontario residents who turned 71 on or after January 1, 2017, became eligible for a free herpes or shingles vaccine, while those who reached the same age just before that date did not, allowing researchers to compare outcomes between the two groups.
They observed that, following the initiation of the vaccination program, there was a decline in dementia diagnoses among the Canadian population eligible for the free shingles vaccination compared with those who were not.
A clear reduction in new dementia cases was observed only among individuals born in 1945 and 1946—the birth cohorts eligible for Ontario’s publicly-funded live-attenuated shingles (varicella-zoster virus) vaccine. This pattern was not observed at other birthdate thresholds, nor in other Canadian provinces that did not offer the vaccine free of charge.
Targeting microbes to reduce dementia risk
There is a long-standing scientific theory that microbes play a major role in the development of dementia. Neurotropic herpesviruses have attracted research attention because they target the nervous system and are prone to reactivation with age.
Studies have found that these herpes viruses can trigger the brain to produce junk proteins called β-amyloid and tau in the brains of people—hallmark of Alzheimer’s disease.
The herpes zoster (shingles) vaccine is currently the only vaccine in clinical use that targets a neurotropic herpesvirus. While earlier studies have suggested that shingles vaccination may be associated with a lower risk of dementia, these findings have remained inconclusive.
Most studies on this topic were observational and relied on comparisons between vaccinated and unvaccinated individuals, making their findings vulnerable to bias. Some experts also suggest that since individuals who seek vaccination often have healthier lifestyles, it could also contribute to dementia risk reduction.
This new study, however, leveraged a natural experiment based on arbitrary birthdate eligibility rules, comparing individuals born just weeks apart who were otherwise similar, thereby reducing potential bias arising from vaccine access or choice. Furthermore, the team directly collected data from primary care providers rather than relying on insurance claims, thereby providing a holistic view of patient health over a longer period.
The results showed that seniors eligible for a free shingles vaccine were less likely to be diagnosed with dementia over the next 5.5 years. Their risk was 2 percentage points lower, and they spent more years dementia-free on average. One of the most striking revelations was that the protective effect was statistically significant in women (p-value: 0.029) but not in men (p-value: 0.52).
The researchers note that this natural experiment provides stronger evidence of a causal relationship than standard observational analyses. Further studies exploring this relation might help inform public health vaccination policies for aging populations.