Biogen shareholders are rejoicing over a clinical trial’s positive results.

Shares of the biotech company were up 19.6 percent Friday, a day after Biogen and Tokyo-based Eisai announced positive results from a Phase II study with BAN2401, an anti-amyloid beta protofibril antibody, in 856 patients with early Alzheimer's disease. The stock move added more than $12 billion in value to Biogen shares.

"The prospect of being able to offer meaningful disease-modifying therapies to individuals suffering from this terrible disease is both exciting and humbling," Biogen’s chief medical officer, Alfred Sandrock, said in a release. "These BAN2401 18-month data offer important insights in the investigation of potential treatment options for patients with Alzheimer's disease and underscores that neurodegenerative diseases may not be as intractable as they once seemed."

J.P. Morgan told its clients the announcement is a good sign for the Biogen’s pipeline.

“Bigger picture, the data today adds to our confidence that there may indeed be a correlation between amyloid beta levels and cognitive functioning which, in our view, positions BIIB nicely for a potentially substantial wave of anticipation trade well ahead of the future binary aducanumab readouts (as we believe prevailing levels reflect minimal contribution from Alzheimer’s),” analyst said in a note to clients Thursday.

The analyst reiterated his overweight rating and $400 price target for Biogen shares, representing 34 percent upside to Thursday’s close. The company had a market value of $63 billion as of Thursday’s close.

Biogen shares were down 6 percent this year through Thursday versus the S&P 500’s 2 percent gain.