Some immunizations may be quietly protecting us from cognitive decline.
How the medicine might do that is a mystery scientists are desperate to solve. A new study on two vaccines for older adults gives us a crucial clue.
The retrospective cohort study included more than 130,000 people in the US. It reveals that the shingles vaccine (called Shingrix) and the respiratory syncytial virus (RSV) vaccine (Arexyv) are associated with a reduced risk of dementia compared to the annual flu vaccine.
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Both Shingrix and Arexyv are recommended for older adults, and they contain the AS01 adjuvant, which helps stimulate the immune system after vaccination. The flu vaccine does not.
Because the link to dementia was noticed soon after receiving the jab, it’s unlikely that the vaccines’ protection from direct viral exposure is behind the dementia link.
Instead, the findings from the University of Oxford suggest “that the AS01 adjuvant itself plays a direct role in lowering dementia risk.”
Within 18 months of receiving just the Shingrix vaccine, participants showed an 18 percent reduction in dementia risk compared to those who received only the flu vaccine.
Meanwhile, those who received the RSV vaccine showed a 29 percent reduction in dementia risk compared to the flu vaccine.
Participants who received both the Shingrix and the Arexyv vaccine showed a 37 percent reduction in risk.
This combined effect was not statistically greater than one vaccine on its own. In other words, protection from two viruses didn’t significantly increase the protection against dementia.
The findings suggest that some vaccines “protect against dementia through mechanisms unrelated to (or at least in addition to) the prevention of their [target virus]”, write the study authors, led by psychiatrist Maxime Taquet from the University of Oxford.
If that’s true, then certain vaccines may protect against dementia by triggering important pathways in the immune system.
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The conclusions align with an emerging hypothesis: that dementia is not actually a brain disease but a disorder of the immune system within the brain.
Perhaps vaccines can help get that system up and running again, even if a threatening virus never comes along.
In recent years, studies have shown that exposure to several common viruses, like those behind cold sores, shingles, mono, pneumonia, and COVID-19, can lead to a higher risk of cognitive decline down the road. Moreover, vaccines seem to reduce that risk by a significant amount.
But why that is has remained a mystery.
In 2024, for instance, a study from the United Kingdom found that Shingrix delayed dementia onset by 17 percent compared to older, less effective shingles vaccines.
At the time, this was interpreted as indicating that the more effective a shingles vaccine is at reducing viral exposure, the more the brain is protected against cognitive decline.
This older version of the shingles vaccine (called Zostavax), however, doesn’t include the AS01 immune-booster, and that may have influenced the results.
In the US, it is generally recommended that adults over the age 50 receive two doses of the shingles vaccine to protect themselves against the varicella-zoster virus. This is the same virus that causes chicken pox, and it can lie latent in the brain for years before re-emerging in adults.
It is also recommended that adults over age 75 receive the RSV vaccine.
Both of these vaccines can protect from dangerous infections, but it seems that may not be all they do.
“It is likely that both the AS01 shingles and RSV vaccines provide some protection against dementia,” conclude Taquet and his colleagues.
“The mechanisms underpinning this protection remain to be determined.”
Vaccines have saved a staggering 154 million lives around the world in the last half century from deadly viruses.
If we’re lucky, that’s just the tip of the iceberg.
The study was published in npj Vaccines.
