Biogen’s aducanumab is more likely to be granted conditional versus full FDA approval in Alzheimer’s disease (AD) based on recently presented data. The results presented in the Phase III EMERGE study need confirmation in order for the drug to not face a regulatory rejection.

Still, an expected advisory committee (AdCom) meeting vote will be split, and it is unlikely that the panel will be unanimous in their recommendations for aducanumab given the current data. Despite this caveat, high unmet needs in AD and patient advocacy group support would mean a rejection inviting much criticism for the FDA, creating an incentive for the agency to consider the conditional approval option...

Even if aducanumab were conditionally approved, the CTAD data did not address the ApoE4 noncarrier subset, prompting questions on label restrictions, all three investigators said. EMERGE had about 30% noncarriers, but there was no efficacy data or dropout rate on this subgroup, they said. The dataset does not provide strong evidence for efficacy or even safety for the noncarrier group, since non-carriers are hypothesized not to respond as well to beta-amyloid clearing strategies than carriers.

https://www.clinicaltrialsarena.com/comment/aducanumab-alzheimer/

The most likely outcome is a conditional approval for ApoE4 carriers.  That is probably also the best decision given the current data.