A large new study has revealed a possible explanation for why the shingles vaccine may reduce the risk of developing dementia. Researchers found that the vaccine helps prevent reactivation of the varicella-zoster virus — the same virus that causes chickenpox and shingles — which may, in turn, protect the brain.

The varicella-zoster virus can remain dormant in the body for decades before reactivating as shingles, a painful condition also known as herpes zoster. The new study found that individuals who received the shingles vaccine had a significantly lower risk of developing dementia than those who were unvaccinated. Scientists believe the vaccine’s ability to reduce viral reactivation and inflammation may play a key role in preserving cognitive health.

The analysis, published in Nature Medicine, examined health records from more than 100 million Americans between 2007 and 2023. Researchers found that people who received the shingles vaccine had a 27% to 33% lower risk of dementia over three years. Those who received multiple doses had even better protection than individuals who received just one.

This was one of the largest studies ever conducted on the link between shingles vaccination and brain health. Researchers also found that people who experienced multiple bouts of shingles had a higher risk of dementia — between 7% and 9% — for up to nine years after the second outbreak, compared to those who had shingles only once.

Study author Pascal Geldsetzer, an assistant professor of medicine at Stanford University, explained that as people age, the virus can reactivate, though the immune system typically suppresses it. However, when it fully reactivates, it can cause the hallmark symptoms of shingles, including burning, tingling, blistering, and rash.

While the exact connection between the virus and dementia remains unclear, experts have theories. Some believe repeated reactivation of the virus damages brain regions involved in memory and cognition. Others suspect it causes ongoing inflammation in the brain.

This isn’t the first study to link shingles vaccination to a lower dementia risk. Earlier this year, a Nature analysis of 280,000 people found that those who received the shingles vaccine were 20% less likely to develop dementia over seven years compared to those who did not. Similarly, a 2024 Nature Medicine study found that people who received Shingrix— the enhanced shingles vaccine that boosts immune response — were at least 17% less likely to be diagnosed with dementia within six years.

“Shingles is a painful illness that can lead to debilitating pain,” says Dr. Amesh A. Adalja, a senior scholar at the Johns Hopkins Center for Health Security says. “This is another study adding more data to the benefits of the shingles vaccine, moving beyond just prevention of shingles, which in itself is valuable.”

The shingles vaccine, administered in two doses, is more than 90% effective at preventing shingles, according to the Centers for Disease Control and Prevention (CDC). The CDC recommends getting the vaccine at age 50, with doses given two to six months apart. Adults 19 and older with weakened immune systems are also advised to receive it.