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When we think of the side effects of treatments or medications, our minds may naturally turn to negative side effects, like nausea or headaches. But sometimes a side effect proves beneficial. Recently, evidence has been building that adult vaccines have the unexpected side effect of lowering dementia risk.

Shingles Vaccine

The most compelling evidence to date is for the shingles vaccine. Anyone who’s had chickenpox is at risk of the virus reactivating and causing a painful condition known as shingles. Both the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health recommend the shingles vaccine to adults aged 50 or older.

The link between the shingles vaccine and dementia risk first came to light as a result of a natural experiment. In 2013, public health officials in Wales, United Kingdom, received a stock of shingles vaccine, but it wasn’t enough to vaccinate all eligible people. Because the vaccine is most effective for those younger than 80, officials set a cutoff date for eligibility. Anyone born on or after September 2, 1933, could get the shot. Those born before that date were ineligible. This decision created a real-world experiment in which the population was split into treated and untreated groups by chance, mimicking a randomized trial. Scientists followed a group of people all born within a week of the cutoff date and discovered that those who received the vaccine were 20% less likely to develop dementia over the next seven years than those who didn’t. The effect was only statistically significant in women.

The shingles vaccine used in the Wales experiment was Zostavax, a live, weakened form of the virus, which has since been replaced in most countries (including Wales) by Shingrix, a non-live vaccine that is better at preventing shingles. The rapid transition from Zostavax to Shingrix created a scenario that allowed researchers to compare the two vaccines. People who received the Shingrix vaccine had an even lower chance of being diagnosed with dementia over the next six years than those who received the older vaccine, and for those who did go on to develop dementia, the Shingrix vaccine delayed diagnosis by about 5 months. Both men and women benefited from the vaccine, but the effects were still strongest in women.

Other Adult Vaccines

There is also evidence that vaccines against the flu, pneumococcus, and Tdap (tetanus, diphtheria, and pertussis) lower the risk for dementia. However, all these studies were limited in that they analyzed past health records and made observations on the available data. So, while they show a strong link between these vaccines and a lower dementia risk, randomized controlled trials are needed to prove a direct cause-and-effect connection.

Why do Some Adult Vaccines Appear to Lower Dementia Risk?

Scientists are not exactly sure, but there are two theories:

1. Vaccines prevent infections and brain inflammation: Most infections cause inflammation, which is a sign your immune system is doing its job to fight the invaders. However, if left unchecked, uncontrolled inflammation, especially in the brain, causes harm and plays a significant role in dementia. Vaccines reduce the risk of infections and inflammation.
2. Vaccines Prime the Immune System: Vaccines directly train the adaptive immune cells to recognize a specific threat (like the virus that causes shingles) and mount a targeted attack against it. Adaptive immune cells are the immune system’s special forces: highly trained and deployed only against the enemy they were trained for. But our immune system has another half called the innate immune system, which is our first line of defense against any invader. Rather than being focused on a single target, innate immune cells attack all invaders. Vaccines also seem to prime this system to be better prepared for all threats. This is especially true of newer vaccines that may contain an ingredient called an adjuvant, which helps boost the immune response. Improved immune fitness could help maintain brain health over time.

More work is needed before scientists can say with certainty how vaccines lower dementia risk or whether they should become part of a regular treatment plan to protect the brain. However, it is reassuring that, for once, a side effect might prove to be a good one.