They thought it was the plaque in the brain that caused Alzheimer's. They invented a vaccine to get rid of the plaque. They used it on people who already had dementia. It got rid of the plaque. But the patients still had dementia.
But... maybe the vaccine could work preventively? So they aren't giving up on the vaccine yet.
What's clear is that they're making progress but that they don't really understand this disease very well yet.
At least they have a way to stop plaque.
Two steps forward, one step back.