Why would people who underwent shingles vaccination in later life exhibit a lower risk of later developing dementia? Firstly, one has to elect to take this vaccine, and so this could be selecting for people who tend to take better care of their health, and thus are in better shape than their peers, on average. Secondly, vaccination produces an effect called trained immunity that makes people incrementally more resistant to the chronic inflammation of aging. Lastly, there is some evidence for persistent viruses such as the varicella-zoster virus that causes shingles to contribute to the onset and progression of dementia, for reasons yet to be fully understood. Epidemiological studies such as this one don't shed much light on biological mechanisms, but do add expected value to seeking vaccination rather than passing it up.
Shingles, a viral infection that produces a painful rash, is caused by the same virus that causes chicken pox - varicella-zoster. After people contract chicken pox, usually in childhood, the virus stays dormant in the nerve cells for life. In people who are older or have weakened immune systems, the dormant virus can reactivate and cause shingles. Previous studies based on health records have linked the shingles vaccine with lower dementia rates, but they could not account for a major source of bias: People who are vaccinated also tend to be more health conscious in myriad, difficult-to-measure ways. Behaviors such as diet and exercise, for instance, are known to influence dementia rates, but are not included in health records.
But two years ago, researchers recognized a fortuitous "natural experiment" in the rollout of the shingles vaccine in Wales that seemed to sidestep the bias. The vaccination program specified that anyone who was 79 was eligible for the vaccine for one year. People who were 80 were out of luck - they would never become eligible for the vaccine. The researchers looked at the health records of more than 280,000 older adults who did not have dementia at the start of the vaccination program. They focused their analysis on those closest to either side of the eligibility threshold - comparing people who turned 80 in the week before with those who turned 80 in the week after. The same proportion of both groups likely would have wanted to get the vaccine, but only half, those almost 80, were allowed to by the eligibility rules.
Over the next seven years, the researchers compared the health outcomes of people closest in age who were eligible and ineligible to receive the vaccine. By factoring in actual vaccination rates - about half of the population who were eligible received the vaccine, compared with almost none of the people who were ineligible - they could derive the effects of receiving the vaccine. As expected, the vaccine reduced the occurrence over that seven-year period of shingles by about 37% for people who received the vaccine, similar to what had been found in clinical trials of the vaccine. Seven years later, one in eight older adults, who were by then 86 and 87, had been diagnosed with dementia. But those who received the shingles vaccine were 20% less likely to develop dementia than the unvaccinated.
Link: https://med.stanford...n-dementia.html
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