New research suggests even more benefits for flu vaccination in older people. The study found that adults over 65 who received at least one flu shot were noticeably less likely to be diagnosed with Alzheimer’s disease up to four years later. More research will be needed to confirm a cause-and-effect link and to figure out where this protective effect is coming from, however.
Alzheimer’s disease is the most common form of dementia, and is thought to currently affect more than 5 million Americans. There are lifestyle habits that may reduce a person’s risk of Alzheimer’s, such as regular exercise, and there are medications that can help manage its symptoms. But there are no treatments known to substantially prevent or reverse its progression. However, some research has pointed to a relationship between certain infections and Alzheimer’s, which has led to hopes that preventing or treating these infections may lower its incidence or delay its onset.
In 2020, researchers at the University of Texas Health Science Center at Houston analyzed medical records and found a link between flu vaccination and a lower associated risk of diagnosed Alzheimer’s. This time, they turned to an even larger database of medical claims and were able to compare the outcomes of nearly a million pairs of adults over 65 throughout the U.S. who were either vaccinated or unvaccinated for the flu. The pairs were matched in factors like age and were tracked for an average of 46 months.
During the study period, 8.5% of unvaccinated adults were diagnosed with Alzheimer’s or received medication often used to manage it, compared to 5.1% of vaccinated individuals—about a 40% lower relative risk. There also appeared to be a cumulative effect, such that people who consistently got vaccinated annually during the study period were the least likely to develop Alzheimer’s. The findings were published online this month in the Journal of Alzheimer’s Disease.
Studies in recent years have suggested that certain germs can hide in the brain following infection earlier on in life and directly trigger the development of Alzheimer’s, particularly herpesviruses. But the authors speculate that the connection seen in this study isn’t necessarily unique to influenza. Rather, it’s more about the relationship between our immune system and the aging brain.
“Since there is evidence that several vaccines may protect from Alzheimer’s disease, we are thinking that it isn’t a specific effect of the flu vaccine,” said study author Paul Schulz, director of the Neurocognitive Disorders Center at UT’s McGovern Medical School, in a statement. “Instead, we believe that the immune system is complex, and some alterations, such as pneumonia, may activate it in a way that makes Alzheimer’s disease worse. But other things that activate the immune system may do so in a different way—one that protects from Alzheimer’s disease.”
These kinds of observational studies can only demonstrate a correlation between two things, not clearly prove a cause-and-effect link between vaccination and Alzheimer’s risk. But other research teams have found a similar connection. And of course, vaccines for the flu and other diseases remain effective at preventing severe illness from the germs they target—benefits that tend to be even more pronounced for older people.
It will take time to figure out exactly why vaccines may help keep our brain in good shape, but they’re doing plenty of good for us already. The researchers, for their part, may next plan to study whether covid-19 vaccines can offer a similar buffering effect against dementia.