How do vaccines affect the brain?

For many years, vaccines were viewed primarily as tools to prevent infections. However, a growing number of observational studies show that people who get certain vaccines in middle age and old age have a lower risk of dementia than those who do not get vaccinated.

This relationship received more attention after studies on the shingles vaccine. Shingles is caused by the Varicella-Zoster virus, which is also the virus that causes chickenpox. After a person has chickenpox, the virus does not disappear completely but can lie dormant in the nervous system for many years. When immunity declines, the virus reactivates and causes shingles.

According to experts, vaccines can protect the brain in two main ways: preventing infections that increase inflammation in the nervous system and activating the immune system to clean up toxic substances in the brain.

Mechanisms that support brain protection

Pascal Geldsetzer, an epidemiologist at Stanford University who has researched the shingles vaccine, said there is growing evidence that the virus can “hide” in the nervous system for years and is linked to neurodegenerative disease.

For example, the Varicella-Zoster virus continuously interacts with the immune system, causing neuroinflammation; Getting the shingles vaccine can help reduce the risk of dementia. Not only shingles, a number of other infections have also been linked to the risk of dementia, such as pneumonia, syphilis, Lyme disease and periodontal disease. Some studies also suggest that some strains of influenza virus can affect brain cells and be linked to neurodegenerative diseases such as Parkinson’s or dementia.

This leads scientists to ask: if chronic infection and inflammation can contribute to brain weakness, could vaccines against these diseases help reduce the risk of long-term brain damage?

The easiest explanation is that vaccines help the body avoid infections that can harm the brain. When a person has an infection, the body must activate an inflammatory response to fight the pathogen. If this condition occurs repeatedly, is prolonged or is related to a virus that can exist in the nervous system, the brain may be affected.

Professor Rudolph Tanzi, a neurologist at Harvard Medical School, believes that Alzheimer’s disease often develops silently over decades. From the time the first plaques and protein tangles appear in the brain until symptoms of dementia become apparent, it can take about 20 years.

So if some vaccines have only been used for a few years but are already associated with a lower risk of dementia, the protection may not come solely from preventing a particular infection.

Another theory is that vaccines may trigger a broader immune response in the body. When vaccinated, the immune system not only learns to recognize a specific pathogen, but is also awakened to a certain degree.

According to Professor Tanzi, this peripheral immune response can help white blood cells enter the brain and clear amyloid plaques. This type of protein plaque is thought to play an important role in Alzheimer’s disease.

In a study on mice genetically engineered to have an Alzheimer’s-like condition, scientists found that when the peripheral immune response is activated, white blood cells can move into the brain and begin processing plaques.

From there, Professor Tanzi believes that vaccination can partly reduce the amyloid burden in the brain. Simply put, the vaccine does not directly cure Alzheimer’s, but it can make the immune system more active in clearing substances related to neurodegeneration.

Recommended vaccines

Although the brain protection mechanism still needs further research, experts say that getting vaccinated as recommended still has clear benefits. Vaccines help reduce the risk of infections, reduce complications, reduce hospitalizations and may provide additional benefits for brain health.

For middle-aged and elderly people, commonly mentioned vaccines include influenza vaccine, shingles vaccine, pneumococcal pneumonia vaccine and RSV vaccine depending on age, personal risk and recommendations in each country.

Professor Paresh Malhotra, a neurologist at Imperial College London, said that as evidence about the impact of vaccines on the brain increases, it may be necessary to consider getting some vaccines earlier, from age 40-50, instead of just waiting until old age.