Varicella-zoster virus causes chickenpox and herpes. The virus can remain dormant in nerve cells for decades.

As people get older and their immune systems get weaker, the virus can reactivate. When that happens, it can trigger an outbreak of shingles, a disease characterized by often extremely painful blisters, burning, tingling and numbness. If you get a bad case, it may become chronic and disabling.

According to the U.S. Centers for Disease Control and Prevention, an estimated one in three people in the U.S. develop at least one case of shingles in their lifetime. Fortunately, vaccines for shingles have been available since 2006.

As you get older and your immune system gets weaker, the virus that causes chicken pox can reactivate.

The arrival of vaccines has provided an insight into a link between the shingles vaccine and declines in memory, thinking and other cognitive abilities known as dementia.

A recent British study found that people who received the shingles vaccine were 20 percent less likely to develop dementia in the seven years after they were vaccinated, compared with those people who didn't get the shots.

This study came about as the result of unusual circumstances. When the first shingles vaccine rolled out in Wales on September 1, 2013, officials had to establish a strict age cut-off because there was a limited supply of the vaccine. Only people who were 79 on that date were eligible for one year, but those 80 and older were ineligible because the vaccine was then considered less effective for folks over eighty years. As younger people turned 79, they became eligible for the vaccine for the year.

This protocol allowed the researchers to compare relatively equal groups: those people eligible for the vaccine alongside people just slightly older who were prohibited from getting it.

This set up a “natural experiment,” according to Paul Geldsetzer, the study's senior author and an assistant professor of medicine at Stanford. “If I take 1,000 people born one week and 1,000 people born one week later, there shouldn't be any difference between them except for the large difference in the vaccination uptake,” he explained in a press release.

In total, the researchers tracked health records of about 280,000 people who were aged 71 to 88 and didn't have dementia when the vaccine rolled out. During the next seven years, nearly half of those eligible for the vaccine received it, while only a tiny number from the ineligible group were vaccinated. This provided a clear picture of before and after.

A newer version of the vaccine, Shingrix, contains an inactivated portion of virus. Research shows the new vaccine is even more effective and lasting.

The vaccine's association with lower rates of dementia appeared to mark it as beneficial against developing dementia — but why?

There are different theories, but one possibility has to do with inflammation. By preventing shingles, vaccines reduce the neuroinflammation caused by reactivation of the virus; and as Geldsetzer put it, “Inflammation is a bad thing for many chronic diseases, including dementia.”

It's important to note that the study involved an older form of the shingles vaccine, Zostavax, that contains a modified version of the live virus. Zostavax has since been discontinued in the U.S. and other countries. A newer version of the vaccine, Shingrix, contains an inactivated portion of virus. Research shows the new vaccine is even more effective and lasting.

There are other ways to reduce the risk of dementia according to Alzheimers.gov. They include:

  • Controlling high blood pressure
  • Managing blood sugar
  • Maintaining a healthy weight
  • Being physically and mentally active
  • Staying connected with family and friends
    • Treating hearing problems
  • Getting at least 7 to 8 hours of sleep each night
  • Protecting your head from injury
  • Reducing alcohol
  • Refraining from tobacco use
The study is published in Nature.