LOS ANGELES (MarketWatch) — “Yeah, it’s that good.”

That was the simple, yet typical review of one of several Wall Street analysts who gushed Friday over Biogen Idec Inc.’s BIIB, -0.32% Phase 1 study results for its Alzheimer’s treatment, now known under the test moniker of BIIB037.

The review came from Baird Equity Research’s Christopher Raymond, who, like several other followers of Cambridge, Mass.-based Biogen, raised his price target on the company’s already pricey stock—and may have to raise it again if current trends continue.

Shares of Biogen jumped nearly $30 apiece to $463.22 in recent action, up nearly 7%, and had been as high as $480 at one point for a 10% gain early in the session. Raymond’s previous price target of $445 quickly became obsolete and so he jacked it up to $498, saying that the long-awaited data from Biogen’s testing was “well worth the wait.”

“Bottom line, the data, in our view, are even better than we had hoped, and as such expect shares to open strong this morning,” Raymond wrote in his note before the market opened. As it turned out, he was right.

RBC Capital Markets’ Michael Yee, who attended the conference in Nice, France, where the presentation was made, said the drug showed a 95% reduction in amyloid.

“We believe BIIB037 data shows very impressive data including strong efficacy and very good safety,” Yee said.

But is Wall Street overreacting? One fund manager seems to think analysts are a little too enthusiastic.

Andrew McDonald, founder and chief executive of LifeSci Advisors, and manager of its BioShares Funds, McDonald’s BioShares Biotechnology Product Fund BBP, +0.14% includes Biogen among its stable of 36 companies in its portfolio.

“Quite honestly, I don’t think it’s as good as people have been writing about,” McDonald said.

McDonald points out there are a high incidences of what are known as Amyloid-Related Imaging Abnormalities, or ARIA, with higher doses, though the effectiveness is much greater. He says Biogen is “threading the needle” in trying to find the right dosage.

He adds that the number of patients used in testing is small, and cautions the drug won’t prove effective on patients who have had the disease for an extended period.

“It has to do with selecting the right patient population,” McDonald said.

Mark Schoenebaum, analyst with Evercore ISI, concurred, for the most part that Biogen’s results were impressive. “The magnitude of the efficacy improvements is really striking, in line with what [the] company has been sort of implying [through] their enthusiasm, I think,” he wrote in a note.

But Schoenebaum agreed with McDonald there are concerns over ARIA. He says high doses of the drug at 10 milligrams had high rates of ARIA at 41%, and roughly half of all the patients who got ARIA dropped out of the study. The company is looking at the prospect of using a smaller dose at 6 milligrams. Schoenebaum said that Biogen presenters see the ARIA incidents as manageable.

Biogen says the villain in the fight against Alzheimer’s is amyloid plaque, considered to be key in the development of the mentally crippling disease. By targeting that plaque, it can cut back on symptoms.

“This is the first time an investigational drug for Alzheimer’s disease has demonstrated a statistically significant reduction on amyloid plaque as well as a statistically significant slowing of clinical impairment in patients with [developing] or mild disease,” Dr. Alfred Sandrock, Biogen’s chief medical officer, said in a news release.

Biogen officials said the findings do not affect the alliance it joined with four other drug makers to develop Alzheimer’s treatments in the U.K. earlier this week. Biogen teamed up with GlaxoSmithKline GSK, -0.53% , Johnson & Johnson JNJ, +1.36% , Eli Lilly & Co. LLY, +1.11% and Pfizer Inc. PFE, -0.51%

“They are actually separate initiatives,” Biogen spokeswoman Kate Niazi-Sai said in an email response to an inquiry. “Our candidate findings do not affect this alliance. The alliance is to help fund promising additional research. We are committed to working with public and private sectors to make a meaningful difference, and today’s news is only one part of that commitment.”