Review of: Wiemken TL et al, Vaccine 2021;39(39):5524–5531

Study Type: Retrospective cohort study

A growing body of literature links various vaccinations (eg, diphtheria, tetanus, polio, and influenza) to a lower risk of dementia through unclear mechanisms—one theory is that vaccines activate microglia, which clear amyloid-beta. However, many of these studies are small or geographically limited.

In this study, researchers analyzed Veterans Health Administration (VHA) records of 123,747 veterans age 65 years and older without dementia from 2011 to 2019 (covering seven influenza seasons) to examine the association between influenza vaccination and incident dementia. Records of Medicare claims and Part D pharmacy claims were also included to catch any diagnoses of dementia that might have occurred outside VHA care. This sample was overwhelmingly White (91%) and male (96%).

The number of annual influenza vaccinations received was compared to incidence of dementia over the study period. 54% of the study sample received at least one influenza vaccination; the study did not differentiate between standard and high-dose vaccinations. The researchers controlled for confounding factors including physical and psychiatric comorbidities, as well as the use of medications associated with cognitive function. The overall incidence of any type of dementia was 13% over the eight-year study period.

The study found that patients who received six or more influenza vaccinations had a 12% lower risk of developing dementia compared to those who did not receive any influenza vaccinations. The mechanism behind this association is speculated to involve the vaccines training the immune system to generate nonspecific protection.

Carlat Take

Similar to other vaccinations, there appears to be a link between influenza vaccination and a lower dementia risk. However, association is not causation, and many confounding factors exist in this type of retrospective research. For example, missing vaccinations could be a marker of early memory issues that later become full-blown dementia. In addition, people who are engaged in preventive care are more likely to take better care of their physical health, thus impacting modifiable risk factors for dementia. As annual influenza vaccinations are already highly recommended for elderly patients, we can continue to encourage our patients to get their annual flu shots.