Background: It has been suggested that changes to the immune system could be a factor in age-related conditions such as Alzheimer's disease. Our objective was to examine the association between past exposure to conventional vaccines and risk of Alzheimer's disease.
Methods: We analyzed data from a representative community sample of subjects 65 years of age or older participating in the Canadian Study of Health and Aging, a prospective cohort study of dementia. Screening and clinical evaluations were done at both baseline and follow-up. Past exposure to vaccines was assessed at baseline by means of a self-administered questionnaire.
Results: Of the 4392 eligible subjects who were cognitively unimpaired and for whom vaccine information was available at baseline (in 1991-1992) and who completed follow-up 5 years later (in 1996-1997), 527 were diagnosed as having cognitive impairment or dementia other than Alzheimer's disease and were excluded from these analyses. Of the remaining subjects, 3682 were cognitively unimpaired at follow-up and 183 were newly diagnosed as having Alzheimer's disease. After adjustment for age, sex and education, past exposure to vaccines against diphtheria or tetanus, poliomyelitis and influenza was associated with lower risk for Alzheimer's disease (odds ratio [OR] 0.41, 95% confidence interval [CI] 0.27-0.62; OR 0.60, 95% CI 0.37-0.99; and OR 0.75, 95% CI 0.54-1.04 respectively) than no exposure to these vaccines.
Interpretation: Past exposure to vaccines against diphtheria or tetanus, poliomyelitis and influenza may protect against subsequent development of Alzheimer's disease.