Elderly Americans lined up this week to get flu shots before the limited supply ran out. A 79-year-old Lafayette, Calif., woman died when she fainted and hit her head after waiting five hours for a shot. In Concord, Calif., two elderly women were hospitalized after collapsing in a vaccine line. The rush started after health officials announced that half the nations vaccine supply had been contaminated and would not be available this year. Federal officials urged Americans not to panic and promised there would be enough to inoculate those at greatest risk, including the elderly and small children. Experts said the shortage could cause millions more Americans to get sick than in a normal flu season. Lost productivity and medical costs could reach $20 billion.