Adults who have received a tetanus, diphtheria, pertussis (Tdap) vaccination have a 42% lower risk for dementia compared with those who are not vaccinated, according to a recent study.

"The magnitude of the effect of Tdap vaccination on delaying dementia is very exciting from the geriatrician's viewpoint," said coauthor John Morley, MD, professor of geriatrics at Saint Louis University. "These effects on dementia are more pronounced than any treatments we have available for Alzheimer's disease."

The study in Journals of Gerontology points out that adult vaccinations appear to reduce risk of dementia. It has not been established, however, whether Tdap vaccination has that effect.

Researchers tested the hypothesis in a Veterans Health Administration (VHA) cohort and then repeated it in a MarketScan medical claims cohort. Included were patientsaged 65 years and older who were free of dementia for 2 years prior to the index date.

Participants had either received not received a Tdap vaccination by the start of either of the two index periods (2011 or 2012). Follow-up continued through 2018, with controls having no Tdap vaccination for the duration of follow-up.

VHA patients were, on average, 75.6 (SD ± 7.5) years of age, 4% female, and 91.2% Caucasian. MarketScan patients, meanwhile, were 69.8 (SD ± 5.6) years of age, on average, and 65.4% were female. The authors reported that, after controlling for confounding, patients with Tdap vaccination versus those who were unvaccinated had a significantly lower risk for dementia in both cohorts (VHA: hazard ratio [HR] = 0.58; 95% confidence interval [CI]: 0.54-0.63 and MarketScan: HR = 0.58; 95% CI: 0.48-0.70).

"Tdap vaccination was associated with a 42% lower dementia risk in 2 cohorts with different clinical and sociodemographic characteristics," the researchers stated. "Several vaccine types are linked to decreased dementia risk, suggesting that these associations are due to nonspecific effects on inflammation rather than vaccine-induced pathogen-specific protective effects."

The authors added that infections have been found to contribute to worsening cognitive impairment and incident dementia.

Both influenza and herpes zoster vaccination have been linked to a lower risk for dementia, according to background information in the article, but existing evidence is limited by self-reported vaccination history and inadequate control for healthy adherer bias.

One factor, the authors pointed out, is that patients who get vaccinations are more likely to get other recommended preventive care and try to prevent chronic disease, including dementia.

"These results are very impressive in suggesting that routine vaccinations can significantly reduce the risks of dementia," explained coauthor Daniel Hoft, MD, PhD, professor of internal medicine and director of the division of infectious diseases, allergy and immunology at SLU. "At this point, the mechanism for this protection is unclear, but could be related to either nonspecific effects that limit brain inflammation or specific immune effects."

In the study, the authors controlled for the number of well visits and other confounders in order to establish whether Tdap vaccination is associated with lower risk for dementia.

The study team also controlled for sustained use of anticholinergics, nonsteroidal anti-inflammatory drugs, statins, steroids, antivirals, metformin, and sulfonylurea. The group also took into account any comorbid physical and psychiatric conditions, including type 2 diabetes, obesity, hypertension, ischemic heart disease, congestive heart failure, atrial fibrillation, asthma, chronic obstructive pulmonary disease, traumatic brain injury, vitamin B12 deficiency, depression, anxiety disorders, nicotine dependence, and alcohol/drug dependence.

"Appropriate vaccination may be a cost-effective way to either prevent dementia or slow progression of cognitive decline," lead author  Jeffrey Scherrer, PhD, said, adding that prospective studies and clinical trials are needed to confirm the conclusion.

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