One cutting-edge neuroscience issue is whether a vaccine can cure Alzheimer disease (AD). A much-ballyhooed clinical trial recently sought an answer. But a mistrial was soon declared, and scientific sleuths now face a fresh mystery: Why did 15 trial subjects get sick?

The vaccine, developed by Elan Corp., contained Ab, the peptide widely believed to trigger AD by forming brain-clogging amyloid plaques. When Elan researchers vaccinated transgenic mice that had developed AD-like pathology, plaques melted away. Two-and-a-half years of animal experiments yielded further encouraging results. The vaccine prevented and possibly reversed cognitive deficits.1

Last September, Dublin-based Elan and its partner, Wyeth Pharmaceuticals, began a Phase II trial of the vaccine, AN-1792, injecting 375 mildly to moderately afflicted AD patients. In mid-January, the companies suspended further dosing because four patients showed signs of central nervous system inflammation. On March 1, the companies halted the study after 11 more...

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