In a race to beat the flu season, medical institutes across the United States will begin human trials for a new H1N1 flu vaccine starting in early August, the University of Maryland announced Wednesday. In the hope of getting the vaccine to those who will need it most by October, the clinical trials will enroll as many as 1,000 adults and children at 10 centers nationwide, said officials at the Center for Vaccine Development at the University of Maryland School of Medicine, which will lead the effort.