U.S. Will Offer Anthrax Shots For Thousands
Federal health officials will offer military's anthrax vaccine, as well as extra medicine, to thousands of Capitol Hill workers and postal employees who were exposed and remain at slight risk even after 60-day antibiotic treatment; shots will be made available as part of experimental study run by Centers for Disease Control, first time government has offered ordinary citizens vaccine against biological weapon; Sec Tommy Thompson explains 'abundance of caution'; few studies in monkeys have found that spores can linger in lungs 75 days after exposure, and that vaccine protected animals; potential side effects, mostly minor, noted (M)