Fears about swine flu have spurred more than 3,000 people so far to volunteer to take part in upcoming clinical trials of a new vaccine. That’s already more than the number scientists will need at eight different sites around the U.S. “We don’t generally ever get a response like this,” Dr. Lisa Jackson, the principal researcher in charge of the clinical trials being readied at Seattle-based Group Health Cooperative, which fielded more than a thousand calls from people in two days.