WEST PALM BEACH, Fla. — The group most threatened by swine flu and most in need of the new vaccine, world health authorities say, is that of pregnant women. For example, Aubrey Opdyke. On June 27, Opdyke, a 27-year-old waitress and former high school swimmer who weighed 135 pounds before her pregnancy and had no health risks other than a smoking habit, came down with mild flu symptoms. She just came home from the hospital three weeks ago.