Factors influencing the relationship between influenza vaccination and the risk of developing dementia: A systematic review

Background Dementia is a leading cause of mortality worldwide, with its prevalence projected to rise sharply in the coming decades. Established risk factors include advanced age, hypertension, diabetes, obesity, smoking, excessive alcohol consumption, physical inactivity, social isolation, and depression. Increasing evidence suggests that vaccination—by preventing infections that may trigger or accelerate neurodegeneration—could play a protective role against dementia, though findings remain inconsistent. Objective To systematically review observational evidence on the association between influenza vaccination and the risk of developing dementia, with attention to demographic, social, and health-related moderating factors. Methods This systematic review, conducted in accordance with PRISMA 2020 guidelines, synthesized evidence from Scopus and Web of Science. Eligible studies were observational in design, employed validated dementia diagnoses, and reported demographic, social, or health covariates. Study quality was assessed using the Newcastle–Ottawa Scale. Results Seven studies, encompassing 9,866,019 participants, met the inclusion criteria. Six of these studies reported a significant association between influenza vaccination and a reduced risk of dementia. Protective effects were particularly evident with repeated vaccinations and among individuals with chronic comorbidities. Key moderating factors included age, sex, vaccination frequency, and specific health conditions. Conclusions Current evidence suggest that influenza vaccination may confer a protective effect against dementia, especially in populations with chronic diseases who receive regular vaccinations. These results highlight the potential of influenza vaccination as a scalable, cost-effective public health measure to mitigate dementia risk and underscore the need for further high-quality longitudinal research.