Updated on January 8, 2024.
For more than 20 years, U.S. health officials have recommended with rare exceptions that everyone ages 6 months and older get an annual flu shot to help protect themselves and others against this seasonal virus and related complications. But recent research adds to a growing pile of evidence that the flu shot—as well as some other widely available vaccines—could help reduce the risk for Alzheimer’s disease or other forms of dementia.
What does the flu shot have to do with the brain?
Scientists are still working to answer this question, but there is evidence that certain viral and bacterial infections may be linked to a higher risk for dementia.
An August 2022 study from UTHealth Houston published in the Journal of Alzheimer’s Disease, which included more than 2.3 million adults aged 65 and older, found those who got an annual flu shot were 40 percent less likely than their unvaccinated peers to develop Alzheimer’s disease within four years.
This protective effect may increase over time with consistent vaccination, according to the study’s first author, Avram S. Bukhbinder, MD. “In other words, the rate of developing Alzheimer’s was lowest among those who consistently received the flu vaccine every year,” Dr. Bukhbinder said in a news release.
Other vaccines tied to lower dementia risk
The protective effect against dementia isn’t specific to the flu shot, the UTHealth Houston researchers explained, noting that other vaccines may also help protect against Alzheimer’s or other dementias. Results from other studies have found a link between lower risk for dementia and vaccination against shingles, pneumococcal pneumonia, tetanus, diphtheria, and pertussis (whooping cough).
“Clearly, vaccines are demonstrating in large population studies that overall, there is a decreased risk for subsequent dementias. What is not very clear is how exactly these vaccines may minimize risk in general,” says Brian Balin, PhD, professor of neuroscience and neuropathology at the Philadelphia College of Osteopathic Medicine.
Over the past 30 years, several studies have found an association between various infectious diseases and dementia, Dr. Balin points out. One large January 2023 study published in JAMA Network Open, which included more than 15,000 people with an average age of 55, found that those who were hospitalized with an infection were almost six times more likely to be diagnosed with dementia three to 20 years later, compared to those who were not hospitalized with an infection. Dementia rates were higher with infections involving the lungs, urinary tract, skin, blood, and circulatory system, as well as infections that were acquired during a hospital stay.
Another large study published in Neuron in January 2023 found 22 connections between viral infections and neurodegenerative disorders—chronic conditions that destroy parts of the brain or nervous system over time.
For the study, researchers analyzed a biobank of more than 300,000 people in Finland, searching for those who had been diagnosed with one of six different conditions: Alzheimer’s disease, amyotrophic lateral sclerosis (ALS), generalized dementia, vascular dementia, Parkinson’s disease, and multiple sclerosis (MS). Of these people, the researchers identified those who had been hospitalized for a viral infection. Next, they compared their findings to a biobank of more than 500,000 people in the UK.
The strongest connection was found between viral encephalitis— a rare viral infection of the brain—and Alzheimer’s. People treated for viral encephalitis were approximately 31 times more likely to develop this form of dementia. Hospitalization for severe flu was also associated with a higher risk for Alzheimer’s Disease, dementia, Parkinson’s Disease, and amyotrophic lateral sclerosis (ALS). The study also showed that meningitis and the varicella-zoster virus (which causes chickenpox and shingles) were linked to neurodegenerative diseases.
In some cases, the impact on the brain persisted for up to 15 years after infection. The researchers noted, however, there are vaccines for several of the viruses implicated.
And, a large 2021 study published in Lancet Healthy Longevity that included data from nearly one million older adults in the UK found that being diagnosed with a common infection was tied to a 53 percent higher risk for dementia within five years, compared to not being diagnosed with an infection. Risk for dementia was highest for those with sepsis, pneumonia, and hospitalization for infection.
How some infections may harm the brain
Several pathogens have been linked to dementia, including the oral Herpes virus (herpes simplex 1, or HSV1), certain bacteria that can cause pneumonia (usually chlamydophila pneumoniae), as well as the bacteria that cause syphilis, Lyme Disease, and gum disease.
Some organisms that can cause systemic infection throughout the body, such as certain fungal infections and COVID, may increase the risk for dementia, according to Balin. “I believe that these associations with dementia are plausible and quite possible,” he says.
Scientists have different theories that could help explain how certain infections might increase the risk for dementia over time. Some place the blame on the virus or bacteria itself, while others think that certain infections may affect the immune system in a way that worsens this form of cognitive decline. Others think various risk factors may also contribute, such as autoimmune mechanisms, cardiovascular factors, diabetes, expression of certain genes, and trauma to the blood-brain barrier, Balin adds.
Some types of viruses such as HIV and persistent measles are neurotropic, he explains. This means they can directly infect brain (nerve) cells, causing infection, neuroinflammation, and harming brain tissue either right away—or even years later. In the 2023 study published in Neuron, researchers found that the majority (81 percent) of associations between viral infection and neurodegenerative conditions involved neurotrophic viruses.
One example is a type of herpes simplex virus that causes chicken pox upon initial infection and can hide out in nerve cells for years, then reactivate later in life to cause painful shingles. Some scientists suggest this reactivation of dormant herpes virus may trigger a process that ultimately damages brain cells and contributes to the development of dementia.
Some infections may cause indirect damage by promoting inflammation which, in turn, harms the brain. Another more controversial theory is that the accumulation of sticky plaques (beta-amyloid proteins) in the brain, which has long been linked to Alzheimer’s disease, may be triggered by certain infections. These plaques may form to protect the brain —effectively trapping and containing harmful pathogens. Based on this hypothesis, these plaques would be similar to cholesterol in that some may be essential but high levels could promote disease.
Another reason to get vaccinated
Research into the link between infection and dementia is ongoing, but some experts suggest that routine vaccines may decrease risk for dementia by preventing infection in the first place, or by training the immune system to keep neuroinflammation in check and provide protection against amyloid plaque.
While scientists continue this investigation, one thing is already clear: Staying up to date with routine vaccinations can help prevent potentially deadly infections and related complications. In addition to getting a seasonal flu shot each year, ask your healthcare provider about other vaccines that may be available for you, depending on your age, vaccination history, and current health status.
For overall brain health, Balin says that “maintaining a healthy immune system through diet, exercise, vaccines, minimizing exposure to air pollution and known sick individuals (especially for respiratory illnesses) are all beneficial and recommended.”
