Older adults who received the shingles vaccine after entering a skilled nursing facility were less likely to be diagnosed with dementia during the next four years, according to a large new study.
The researchers found that vaccinated adults had about a 24% lower risk than those who did not receive the vaccine.
The study was led by Kaley Hayes of Brown University’s School of Public Health and was published in Annals of Internal Medicine. The research team also included scientists from the University of Delaware, the Providence Veterans Affairs Medical Center, and other institutions.
Shingles is caused by the same virus that causes chickenpox. After a person recovers from chickenpox, the virus can remain inactive in nerve tissue for many years and later become active again, usually in older age.
When this happens, shingles can cause a painful skin rash and sometimes long-lasting nerve pain. Because the risk increases with age, vaccination is recommended for many older adults.
The current shingles vaccine, called Shingrix, was introduced in 2017. It is a recombinant vaccine, meaning it does not contain a live form of the virus, and it is now the only shingles vaccine available in the United States.
Earlier studies had suggested that older shingles vaccines might be linked to a lower risk of dementia. The new study focused only on Shingrix and on a particularly vulnerable group: older adults entering skilled nursing facilities.
The researchers used Medicare claims and electronic health records from more than 5,500 skilled nursing facilities across the country. They included 509,926 adults aged 66 and older who had no previous dementia diagnosis and were eligible for shingles vaccination.
Among these patients, only 8,843 received at least one dose of Shingrix after entering a facility. The rest remained unvaccinated during the period studied.
After four years, dementia had been diagnosed in 18.8% of vaccinated participants. By comparison, 24.6% of unvaccinated participants received a dementia diagnosis.
The researchers estimated that this difference was equal to a 24% lower risk among vaccinated adults. Hayes said the result could translate to about one dementia case avoided for every 17 cases that might otherwise occur.
To strengthen the analysis, the researchers used a method known as target trial emulation. In simple terms, they tried to make the observational data resemble a clinical trial as closely as possible by carefully comparing similar groups and adjusting for known differences.
This matters because a true randomized trial of hundreds of thousands of nursing facility residents would be difficult and expensive. However, an observational study still cannot fully rule out differences between people who chose vaccination and those who did not.
For example, vaccinated adults in this study tended to be slightly younger and healthier. These differences could partly explain why they were less likely to develop dementia.
The researchers adjusted for many of these factors and found that the association remained. Even so, the study cannot prove that the vaccine itself prevented dementia.
Scientists are still trying to understand why shingles vaccination might be linked to brain health. One possibility is that preventing shingles reduces inflammation or other stress on the nervous system, but this remains uncertain.
Another possibility is that vaccination may stimulate parts of the immune system in ways that indirectly help protect the brain. More research is needed before scientists can say which explanation, if any, is correct.
The findings are interesting because dementia is a major health problem in older adults and currently has no simple method of prevention. If an existing vaccine also turns out to provide some protection for the brain, that could have important public health value.
However, the results should be interpreted carefully. The study was not designed to prove cause and effect, and it relied on health records, which can miss some diagnoses or differences in care.
The study also received funding from GlaxoSmithKline, the manufacturer of Shingrix. The authors reported that the company had no control over the study design, analysis, or decision to publish, but this funding relationship is still important to disclose.
Overall, the research adds to a growing group of studies linking shingles vaccination with a lower rate of dementia. The association is large enough to deserve further testing, but clinical trials or other strong studies are still needed before the vaccine can be considered a proven way to reduce dementia risk.
Source: KSR.
