Persistent concerns about the first new Alzheimer’s treatment in more than 20 years are curbing access more than four months after regulators approved it. Several major medical centers remain undecided on whether to use Biogen’s Aduhelm. Big names like the Cleveland Clinic and Mass General Brigham in Boston say they’ll pass on it for now. One neurology practice has even banned Biogen sales reps from its offices over the drug. Doctors say they need to learn more about how Aduhelm works and what will be covered before they can decide whether to offer infusions of it.