At an industry conference today, Biogen provided some updates to its Phase III Alzheimer's study, including plans to add an additional 510 patients across the two aducanumab studies, Jefferies analyst Michael Yee tells investors in a research note. While these changes do not necessarily dramatically change the probability of success, it does change Wall Street's "sense of confidence," Yee contends. He believes the market will "swing back to uncertainty" on the Alzheimer's pendulum based on trial changes. Yee thinks Biogen needs to acquire de-risked "neuro/orphan companies" to "change the narrative" from being a binary Alzheimer's company to one with products even if Alzheimer's doesn't work. Yee keeps a Hold rating on Biogen shares with a $375 price target. The stock in afternoon trading is down 7% to $295.42.