An experimental vaccine can reduce both the incidence and the severity of shingles by more than half, doctors reported yesterday, in a development that could spare hundreds of thousands of elderly Americans from an extremely painful disease. The effectiveness of the vaccine was determined in an unusually large clinical trial involving more than 38,500 people over the age of 60, the group most prone to shingles, a skin and nerve infection. "I think the results are quite clinically significant," said Dr. Michael N. Oxman, the leader of the study, which is being published today in The New England Journal of...