So residents of Wales born on either side of the dividing date were matched according to their use of preventative health services, past diagnoses, and educational level. The incidence of dementia was then compared between people on either side of September 2, 1933. As a first step, the researchers confirmed that the vaccine was effective at reducing the incidence of shingles, with numbers similar to those in the vaccine’s clinical trials.
Overall, being eligible for the vaccine was associated with a 1.3 percent reduction in the absolute risk of a dementia diagnosis. That translates to a 8.5 percent reduction of relative risk; when scaled to account for the fact that fewer than half of those eligible received the vaccine, that works out to be a 20 percent reduction in relative risk, which is pretty substantial.
To make sure that it was real, the researchers repeated the analysis using a difference-in-difference approach and came up with roughly the same numbers. That also eliminates the possibility that people who came in for health care (for shingles or some other condition) were more likely to incidentally receive a dementia diagnosis. They also compared the before-and-after populations in terms of a collection of common health outcomes and found that none of those showed any change in the two populations. And nothing else related to NHS policy was changed based on the September 2 date.
Separately, in a draft manuscript the researchers posted on the Med arXiv, the researchers find a similar effect when using UK HNS data to search for a protective effect of the shingles vaccines when it comes to deaths diagnosed to result from dementia. So by all indications, the effect was real.
What’s going on?
The researchers suggest three potential explanations. One of them is the obvious: Suppressing the reactivation of the varicella zoster virus reduces dementia onset. But it’s also possible that the effect is indirect—that dementia is associated with immune activity, and the vaccine alters that in some way. Finally, there’s the possibility that being treated for shingles could promote the onset of dementia or increase the frequency of diagnoses.
