Houston—The annual influenza vaccine is designed to reduce the risk of contracting seasonal flu and having severe symptoms. A new study suggests that the benefits go beyond that, and a flu shot might reduce the risk of developing Alzheimer's disease (AD).

The article, accepted for publication in the August issue of the Journal of Alzheimer's Disease, compared the risk of AD incidence between patients with and without prior flu vaccination. A large nationwide sample of U.S. adults aged 65 years and older from September 1, 2009, through August 31, 2019, was used. Eligible patients, who were aged 65 years and older at the start of the study, were free of dementia during the 6-year look-back period.

"We found that flu vaccination in older adults reduces the risk of developing Alzheimer's disease for several years. The strength of this protective effect increased with the number of years that a person received an annual flu vaccine—in other words, the rate of developing Alzheimer's was lowest among those who consistently received the flu vaccine every year," explained Avram S. Bukhbinder, MD, a recent graduate of McGovern Medical School at UTHealth Houston. "Future research should assess whether flu vaccination is also associated with the rate of symptom progression in patients who already have Alzheimer's dementia."

Dr. Bukhbinder, with the Division of Child Neurology at Massachusetts General Hospital in Boston, and senior author Paul E. Schulz, MD, the Rick McCord Professor in Neurology at McGovern Medical School, previously found a possible link between the flu vaccine and reduced risk of AD; however, this study analyzed a much larger sample than the prior research.

From the unmatched sample of more than 2.3 million eligible patients, the study team produced a sample of 935,887 flu—vaccinated-unvaccinated matched pairs. The matched sample had an average age of 73.7 (SD, 8.7) years of age, and 56.9% were female. Median follow-up was 46 (IQR, 29-48) months.

Results indicate that 5.1% (n = 47,889) of the flu-vaccinated patients compared to 8.5% (n = 79,630) of those unvaccinated. Researchers calculate the risk ratio as 0.60 (95% CI, 0.59-0.61) and adjusted risk ratio of 0.034 (95% CI, 0.033-0.035), corresponding to a number needed to treat of 29.4.

"This study demonstrates that influenza vaccination is associated with reduced AD risk in a nationwide sample of US adults aged 65 and older," the authors concluded.

"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," Dr. Schulz explained. "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. Clearly, we have more to learn about how the immune system worsens or improves outcomes in this disease."

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