Influenza vaccination reduces dementia risk: A systematic review and meta-analysis.

Animal models have indicated that influenza vaccination may prevent or delay the onset of dementia. However, the epidemiological evidence in human beings is still limited. Given this background, this systematic review and meta-analysis aimed to summarize the current state of the art of observational studies investigating the association between influenza vaccination and the risk of dementia. We searched Scopus and Pubmed/Medline until 24 September 2021 for studies investigating the risk of dementia by influenza vaccination status. After adjustment for potentially important confounding variables, data were reported as risk ratios (RRs) with 95% confidence intervals (CIs). Among 273 articles initially evaluated, five were included for a total of 292,157 older people free from dementia at baseline (mean age=75.5 ± 7.4 years; 46.8% females). All studies were of high quality. Over a mean follow-up of 9 years, influenza vaccination mitigated the risk of dementia (RR=0.97; 95%CI: 0.94-1.00; I2 =99%). This association held after adjustment for a mean of nine potential confounders (RR=0.71; 95%CI: 0.60-0.94; I2 =95.9%). In sensitivity analysis, removing one study from the adjusted analyses, the adjusted RR remained similar (RR= 0.67; 95%CI: 0.63-0.70), but the heterogeneity disappears (I2 =0%). In conclusion, influenza vaccination was associated with a significantly lower risk of dementia suggesting that the vaccination of older people against influenza may also aid in the prevention of dementia.