When talking about placebo effect, it's the idea that the same thing could have happened taking a fake pill, just because the mind knows you are taking something that should have an effect.

Sure, the vitamin D in his blood may increase, but the anecdote of "it works" is about him getting sick less. And that could be a placebo effect and any sake pill would have had the same effect.

And the placebo effect can happen even when the person knows it's a fake pill.