In the most significant sign yet of a broad shift in the focus of Alzheimer’s research from treating to preventing the disease, the federal government announced on Wednesday its largest grant so far to test an Alzheimer’s drug on healthy people at greatest risk for the most common form of the disease.

The $33.2 million grant, and several other prevention studies awarded federal money in the last year, follow years of unsuccessful trials of treatments on people who already have dementia. Those failures have led to the realization that these drugs appear to be ineffective by the time memory and thinking problems have taken hold. At the same time, scientific advances have allowed researchers to identify people at risk for Alzheimer’s long before symptoms emerge.

With five million Americans suffering from Alzheimer’s and their ranks projected to surge as baby boomers age, federal health officials consider the disease such a priority that Dr. Francis S. Collins, director of the National Institutes of Health, scraped money together when forced budget cuts slashed the Obama administration’s promise of $100 million in additional funding for Alzheimer’s for 2013. Dr. Collins said he dipped into the budgets of the 27 N.I.H. agencies to supply $40 million awarded Wednesday for several Alzheimer’s research projects. Another $5 million was provided by the National Institute on Aging.

“The worst thing we could do would be to just hunker down and hold off tackling very important problems,” Dr. Collins said, adding, “Obviously, this is high-risk research, but goodness, the stakes are so high that we felt we had to go forward even in the face of the most difficult budget environment that anyone can remember in the N.I.H.”