IMO that’s part of what’s interesting about this study design — they tested vitamin D blood levels and adjusted the supplement dose based on that. This seems like a much better approach than taking a high dose blindly.
I think the headline is accurate. The 52% number is from the experiment arm (participants who received a vitamin D supplement, with the quantity guided by blood testing). While it’s technically possible for the supplement dosage to be calculated as zero, 85% of participants were deficient at baseline, so this isn’t the main effect.