The coronavirus vaccine made by Moderna is highly protective for adults and prevents severe cases of Covid-19, according to data released on Tuesday by the U.S. Food & Drug Administration. Based on the encouraging findings, the agency intends to grant emergency authorization for use of the vaccine on Friday, people familiar with the F.D.A.’s plans said. The decision would give millions of Americans access to a second coronavirus vaccine beginning as early as next week. The review by the F.D.A. confirms Moderna’s earlier assessment that its vaccine had an efficacy rate of 94.1 percent in a trial of 30,000 people. Side effects, including fever, headache and fatigue, were unpleasant but not dangerous, the agency found.