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Sum of boxes is a 19 point scale.

It's an 18 point scale, but more to the point: the decline in the placebo group was only 1.66 points over those 18 months, and the mean score at baseline was just over 3 points. So even 100% efficacy could only possibly have slowed decline by 1.66 out of 18 points (what you would call a 9.2% absolute reduction) in the 18 months of that experiment. And full reversal (probably unattainable) would have only slowed decline by about 3 points.

I agree that the side effects of anti-amyloid therapies are a serious concern. The reasons for this are being understood and corrected in the next generation of such therapies. For example, I expect trontinemab to achieve better efficacy with much greater safety, and there is already preliminary evidence of that. Furthermore, there are improved dosing regimens of donanemab which improve side effects significantly.

Note that my claim is not that the existing drugs are stellar, and certainly not that they're panaceas. Simply that the amyloid hypothesis is true and there has been tremendous progress based on that hypothesis as of late.