Shingles Vaccine Guards Against Dementia, Study Reveals

A vaccine for shingles could protect against dementia according to a new study that took advantage of a quirk in Wales' vaccine rollout scheme.

Shingles is a condition characterized by localized blistering, which largely affects adults who've previously had chickenpox. The varicella zoster virus that causes both of the conditions invades the nervous system before falling dormant, only to emerge again long after the symptoms of chickenpox are gone.

There's mounting evidence that common viral infections like shingles may contribute to the development of Alzheimer's disease by seeding the amyloid plaques thought to trigger neurodegeneration. If true, vaccines protecting people from the virus may come with the added benefit of reducing dementia risk, nabbing two diseases for the price of one.

Testing such a theory is challenging, but a team led by Stanford University medical researchers has now presented data that's about as solid as it gets in lieu of controlled randomized trials.

"To provide causal as opposed to correlational evidence, we take advantage of the fact that, in Wales, eligibility for the zoster vaccine was determined on the basis of an individual's exact date of birth," the authors write.

As of 1 September 2013, every Welsh citizen aged 79 and under was eligible for the shingles vaccine for one year, while everyone aged 80 or older was exempt due to the vaccine's limited supply.

The team then compared the detailed health records of 282,541 vaccinated and non-vaccinated adults of very similar ages, just a few weeks on either side of the cutoff date.

By the end of the study period in 2020, one in eight participants had been newly diagnosed with dementia; people who had been diagnosed with dementia prior to 2013 were excluded from the results.

Among those who had received the shingles vaccine, dementia was 20 percent less likely.

Though similar findings have surfaced in the past in associational studies on vaccines, lifestyle factors associated with individuals choosing to be vaccinated or not have made it difficult to confidently identify causal links.

The Welsh shingles vaccine policy, on the other hand, created a natural experiment that isolated the vaccine's effects within two groups that are more or less statistically identical.

Aside from being far more likely to have ever received the vaccine due to eligibility, there was little else – for instance, education level, health treatments like opioid use, occurrence of other health conditions – that differed statistically between the two large groups.

The team also investigated whether the drop in dementia risk might be the result of the vaccine reducing reactivation of the dormant virus.

"This might occur if the virus is directly toxic to neurons, or promotes a generalized inflammatory response in the nervous system that is neurotoxic, or both," Harvard health care policy expert Anupam Jena, who was not involved in the study, wrote in a commentary for Nature News and Views.

"Reactivation of the virus is thought to cause amyloid-β deposition, tau aggregation and damage to blood vessels – key features of Alzheimer's disease."

The results support this theory, with higher rates of dementia among people with multiple shingles episodes, an effect that was mitigated among people who took antiviral medicine during episodes.

"What makes the study so powerful is that it's essentially like a randomized trial with a control group – those a little bit too old to be eligible for the vaccine – and an intervention group – those just young enough to be eligible," says Stanford medical professor Pascal Geldsetzer.

His team has replicated the method with data from countries where the vaccine rollout followed a similar approach, including England, Canada, Australia, and New Zealand. Geldsetzer claims the "strong protective signal for dementia" has shown up in multiple datasets, though these findings are yet to be published.

While this research ultimately doesn't confirm shingles as a factor in dementia, nor that a shingles vaccine inherently offers protection, it certainly provides a strong basis for further investigation.

This research was published in Nature.