Two natural experiments in Canada suggest that herpes zoster (shingles or varicella zoster) vaccination averts or delays dementia diagnoses. [University of Minnesota-CIDRAP]

The analysis, published in The Lancet Neurology, was led by Stanford University researchers. The team estimated the effect of live attenuated shingles vaccination on new-onset dementia in 232,124 Canadians aged 70 years and older based on a natural experiment in Ontario. The researchers then triangulated the findings with a second natural experiment in Ontario and a quasi-experimental approach that used data from multiple provinces.

A quasi-randomized rollout of shingles vaccine took place in Canada in 2016. In Ontario, residents who turned 71 in or after January 2017 were eligible for free vaccination, while those who turned 71 before that month were ineligible.

“The date-of-birth eligibility thresholds of Ontario's herpes zoster vaccination programme created three comparison groups: ineligible because they were born before Jan 1, 1945; eligible for only 3.5 months because they were born in 1945; and eligible for at least 1 year and 3.5 months because they were born between Jan 1, 1946, and Sept 15, 1951 (ie, aged 65–70 years on Sept 15, 2016),” the study authors wrote. 

Role of microbes in dementia 

Participants were born in Canada from January 1930 to December 1960 and registered with one of 1,434 primary care providers in the Canadian Primary Care Sentinel Surveillance Network as of September 2016. 

Dementia diagnoses were counted using electronic health records from January 1990 to June 2022, and Ontario residents aged 65 years and older were surveyed to estimate shingles vaccine coverage.

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