Group Health Cooperative will recruit hundreds of volunteers to test whether the vaccine will protect against H1N1 virus. The Seattle health co-op will join research centers in seven other cities in fast-track clinical trials directed by the National Institutes of Health.

Some health experts fear the virus may return to the Northern Hemisphere in a more virulent or more contagious form. And because most people have no natural immunity against it, an effective vaccine is considered the best defense.

Even if the clinical trials show the H1N1 vaccine to be safe and effective, health experts don't know yet if it can be manufactured quickly enough to inoculate everyone in high-risk groups, including school-age children.

Group Health expects to get the test vaccines the first week of August and to begin giving shots to volunteers the next week, said Dr. Lisa Jackson, a senior scientist with Group Health Center for Health Studies.

Would-be subjects must be 18 or older and in generally good health. They would need to make five visits to Group Health's research clinics in downtown Seattle over six weeks.

All would get two swine-flu shots three weeks apart, and have their blood drawn to measure their antibodies to the virus.

If the vaccine appears safe initially, the trials would expand to include children 6 months to 17 years old.