Shares of Biogen and other drugmakers researching Alzheimer’s disease soared Wednesday after Japan’s Eisai Co. said its potential treatment appeared to slow the fatal disease in a late-stage study.

The drugmaker said early results showed that its treatment, lecanemab, reduced patient clinical decline by 27% when compared to a placebo or fake drug after 18 months of the infused treatment.

Eisai announced results late Tuesday from a global study of nearly 1,800 people with early-stage Alzheimer’s.

The prospect of an effective new Alzheimer's treatment came roaring back this week with the announcement of preliminary clinical trial data, giving millions of seniors renewed hope after a tumultuous year.

Why it matters: Alzheimer's is a devastating disease, and the topline results boosted analysts' expectations for an entire class of drugs targeting the condition. But they also resurrect enormous questions about who'll cover the costs and how the U.S. will oversee what's likely a multi-billion dollar market.

A new drug can slow the insidious impact of Alzheimer’s disease, a major clinical trial has found.

Patients taking the drug, known as lecanemab, showed a 27% decrease in cognitive decline compared to a control group, according to developers Biogen, based in Cambridge, Massachusetts, and Eisai, headquartered in Tokyo, Japan.

“Starting as early as six months, across all time points, the treatment showed highly statistically significant changes in [dementia severity] from baseline compared to placebo,” the companies announced.