By Nidhi Subbaraman

Mounting evidence suggests that vaccination against the varicella zoster virus -- which causes chickenpox in children and triggers shingles in adults -- also protects the brain.

Several recent studies suggest that the vaccines reduce the risk of dementia in older adults, but key questions remain, including: How the vaccines might work to stop or delay the condition, and whether the benefit is limited to people of a certain age.

The vaccines studied, Zostavax and Shingrix, both appeared to offer protection.

The latest study found that among 70- and 80-year-olds in Australia, people who were eligible to get the Zostavax shot were 1.8 percentage points less likely to get a dementia diagnosis in the next 7.4 years than those who were ineligible. The study was published in the journal JAMA Wednesday.

The Stanford University team that did the work zoomed in on a government initiative -- launched in Australia starting Nov. 1, 2016 -- that offered the shingles vaccine Zostavax free to people between 70 and 79 years old. People who were born after Nov. 1, 1936, were able to get the free shot, but people born on or before that date weren't. The study included 18,400 people in their 70s and 80s when the program launched.

The effectiveness of Zostavax for preventing shingles drops after the age of 80.

"All that's different between them is whether they were born a few days earlier or a few days later," said Pascal Geldsetzer, a Stanford University epidemiologist and an author of the study.

Geldsetzer and his colleagues published a similar study in Nature earlier this month that included nearly 300,000 adults in Wales, around a similar vaccination program. People born before Sept. 2, 1933, aged 80 or older, were ineligible for vaccination with Zostavax, whereas those born on or after that date, could get the shot. Vaccinated people were 20% less likely to get a dementia diagnosis in the next seven years than people who didn't get the shot.

"We keep seeing this in data set after data set," Geldsetzer said.

In the Wales study, the protective effect was stronger in women than men, though it isn't clear why. In Australia, the researchers didn't see this difference.

"They've really done all you can do to try and kick the tires and make sure that the effects you're seeing are real and robust," said Dr. Paul Harrison, a psychiatrist and researcher at the University of Oxford who wasn't involved in the JAMA or Nature studies.

Many countries, including the U.S., have replaced Zostavax, which contains a live, weakened version of the varicella zoster virus, with a newer vaccine called Shingrix. Shingrix, approved in the U.S. in 2017, contains a portion of viral protein mixed with a cocktail of chemicals -- an adjuvant -- that amplifies the body's immune response. Merck makes Zostavax; Shingrix is made by GSK.

A spokesperson for GSK said the company has studies planned and under way to study the link between vaccination and dementia risk.

The U.S. Centers for Disease Control and Prevention recommends the Shingrix vaccine for shingles to people 50 and older.

Shingrix might have an even stronger protective effect than Zostavax, according to a recent study that Harrison co-authored. That team studied 103,000 U.S. people around the time, in October 2017, when the U.S. replaced Zostavax with Shingrix. In the six years following vaccination, people who got the Shingrix shot were 18% less likely to get a dementia diagnosis than people who got Zostavax. The results were published in 2024 in the journal Nature Medicine.

It isn't clear yet if younger people who are vaccinated against shingles would be protected against dementia, according to Dr. Anupam Jena, a health economics researcher and physician at Harvard Medical School, who wasn't involved with the studies. Also, it is possible the vaccine only delays the onset of the disabling condition. Studies so far haven't followed people for long enough to make clear how long any protective effect lasts, Harrison said.

Theories about the role of the vaccines in dementia fall in two buckets. It might be that they hold off the virus, whose activation and infection has been linked to dementia. Another possibility is that the vaccines train the broader immune system in some way that allows it to stave off the disabling neurological condition.

Despite the promise of these observational studies, some scientists are still awaiting a trial that tests the vaccines against placebos in two groups of people who are then watched for dementia to cement the link.

For now, "The biggest treason to get the shingles vaccine is to reduce the risk of shingles," Jena said.

Write to Nidhi Subbaraman at nidhi.subbaraman@wsj.com

(END) Dow Jones Newswires

April 23, 2025 16:40 ET (20:40 GMT)

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