The data, presented at the Clinical Trials on Alzheimer's Disease conference, offered almost no new results from what the company previously released in October. However, analysts who parsed through it said the lack of any new negatives in the report means they expect the company will take the data to the Food and Drug Administration.
"The data is so confusing," said Jared Holz, a health-care strategist at Jefferies, adding the report did show a slightly better response to the drug at higher doses.
Dr. Eric Siemers, an Alzheimer's expert who previously led Alzheimer's research at Eli Lilly, also noted the company offered a more detailed look but "almost the same as they showed" previously.
Shares of Biogen closed up 3% on Thursday after falling 3% immediately on the news. Prior to the announcement, Biogen shares were halted at $286.83. The stock, which had a market value of nearly $52 billion at the time, is down nearly 5% since the start of the year.
The drug targets a compound in the brain known as beta-amyloid, which is thought to play a role in the devastating disease.
In March, Biogen pulled the plug on its drug after an analysis from an independent audit revealed the experimental medicine was unlikely to work. The news sent its stock tumbling. However, shares of the Cambridge, Massachusetts-based company soared on Oct. 22 after the drugmaker shocked investors by announcing it was seeking regulatory approval for the drug after all.
Biogen said at the time that a new analysis of a larger data set showed that aducanumab "reduced clinical decline in patients with early Alzheimer's disease" and patients who received the drug "experienced significant benefits on measures of cognition and function such as memory, orientation, and language."
The findings in October were met with skepticism from investors and analysts who said the results may have just been by chance. Some analysts remained skeptical after Thursday's results from the two late-stage studies, saying it raised more questions.
Salim Syed, a senior biotech analyst at Mizuho Securities, said in a note to clients that the new data is "not impressive," adding it "represents a highly selected patients population that may not be representative of a real world."
Experts discussing the results on a panel Thursday were largely positive.
Sharon Cohen, a primary investigator on Biogen's trial, said the data is "is exhilarating not just to the scientific community but to our patients as well."
There are currently no drugs approved that can reverse the mental decline from Alzheimer's, which is the sixth leading cause of death in the U.S. The FDA has approved Alzheimer's drugs aimed at helping symptoms, not actually reversing or slowing the disease itself.
— CNBC's Meg Tirrell contributed to this report.
This is a developing story. Please check back for updates.