Summary: Wales used a cutoff date in 1933 to set eligibility for the vaccine: born before, ineligible; born after, eligible. The authors analyzed dementia rates in the populations born one week before and one week after and found a significant decrease in the vaccination population born one week after.

Given the unlikelihood of any other salient differences in the two populations (they were all born within 14 days of each other), they conclude that the vaccine had prophylactic effects against dementia and further conclude that Alzheimer’s may be caused by a virus.