WASHINGTON DC, July 15 -- The first COVID-19 vaccine tested in the U.S. revved up people's immune systems just the way scientists had hoped, researchers reported Tuesday -- as the shots are poised to begin key final testing.

"No matter how you slice this, this is good news," Dr. Anthony Fauci, the U.S. government's top infectious disease expert, told The Associated Press.

The experimental vaccine, developed by Fauci's colleagues at the National Institutes of Health and Moderna Inc., will start its most important step around July 27: A 30,000-person study to prove if the shots really are strong enough to protect against the coronavirus.

But Tuesday, researchers reported anxiously awaited findings from the first 45 volunteers who rolled ...