In a large propensity-matched cohort of older adults, those who had received at least one influenza inoculation were 40% less likely than unvaccinated peers to develop AD over the course of 4 years.
“Influenza infection can cause serious health complications, particularly in adults 65 and older. Our study’s findings – that vaccination against the flu virus may also reduce the risk of Alzheimer’s dementia for at least a few years – adds to the already compelling reasons get the flu vaccine annually,” Avram Bukhbinder, MD, of the University of Texas, Houston, said in an interview.
The new findings support earlier work by the same researchers that also suggested a protective effect of flu vaccination on dementia risk.
The latest study was published online in the Journal of Alzheimer’s Disease.
40% lower risk
Prior studies have found a lower risk of dementia of any etiology following influenza vaccination in selected populations, including veterans and patients with serious chronic health conditions.
However, the effect of influenza vaccination on AD risk in a general cohort of older U.S. adults has not been characterized.
Dr. Bukhbinder and colleagues used claims data to create a propensity-matched cohort of 935,887 influenza-vaccinated adults and a like number of unvaccinated adults aged 65 and older.
The median age of the persons in the matched sample was 73.7 years, and 57% were women. All were free of dementia during the 6-year look-back study period.
During median follow-up of 46 months, 47,889 (5.1%) flu-vaccinated adults and 79,630 (8.5%) unvaccinated adults developed AD.
The risk of AD was 40% lower in the vaccinated group (relative risk, 0.60; 95% confidence interval, 0.59-0.61). The absolute risk reduction was 0.034 (95% CI, 0.033-0.035), corresponding to a number needed to treat of 29.4.
Mechanism unclear
“Our study does not address the mechanism(s) underlying the apparent effect of influenza vaccination on Alzheimer’s risk, but we look forward to future research investigating this important question,” Dr. Bukhbinder said.
“One possible mechanism is that, by helping to prevent or mitigate infection with the flu virus and the systemic inflammation that follows such an infection, the flu vaccine helps to decrease the systemic inflammation that may have otherwise occurred,” he explained.
It’s also possible that influenza vaccination may trigger non–influenza-specific changes in the immune system that help to reduce the damage caused by AD pathology, including amyloid plaques and neurofibrillary tangles, he said.
“For example, the influenza vaccine may alter the brain’s immune cells such that they are better at clearing Alzheimer’s pathologies, an effect that has been seen in mice, or it may reprogram these immune cells to respond to Alzheimer’s pathologies in ways that are less likely to damage nearby healthy brain cells, or it may do both,” Dr. Bukhbinder noted.
Alzheimer’s expert weighs in
Heather M. Snyder, PhD, vice president of medical and scientific relations for the Alzheimer’s Association, said this study “suggests that flu vaccination may be valuable for maintaining cognition and memory as we age. This is even more relevant today in the COVID-19 environment.
“It is too early to tell if getting flu vaccine, on its own, can reduce risk of Alzheimer’s. More research is needed to understand the biological mechanisms behind the results in this study,” Dr. Snyder said in an interview.
“For example, it is possible that people who are getting vaccinated also take better care of their health in other ways, and these things add up to lower risk of Alzheimer’s and other dementias,” she noted.
“It is also possible that there are issues related to unequal access and/or vaccine hesitancy and how this may influence the study population and the research results,” Dr. Snyder said.
The study had no specific funding. Dr. Bukhbinder and Dr. Snyder disclosed no relevant financial relationships.
A version of this article first appeared on Medscape.com.