Health officials are considering mass vaccination clinics this fall to ward off a possible swine flu epidemic.

Washington Health Secretary Mary Selecky said Thursday that five companies are expected to produce 100 million doses of a vaccine designed to combat the H1N1 virus, aka swine flu. The vaccine should be ready in October.

Washington state’s initial share, she said, would target especially vulnerable groups, including children in schools and day cares, their families, pregnant women and school and health care workers.

The vaccines could be administered at schools or other large gathering places, said U.S. Health and Human Services Secretary Kathleen Sebelius.

Separate vaccinations for the seasonal influenza virus are expected to be ready in September.

Selecky was among about 500 health officials and others who attended a swine flu summit Thursday called by the White House.

The vaccines will be paid for by the federal government. Clinical trials will begin next month as officials fear mild symptoms of H1N1 could become more severe as the virus mutates and spreads.

Three strains have shown resistance to Tamiflu, a prescription drug that reduces the symptoms. There is no cure for influenza.

The federal government has 52 million treatment courses of Tamiflu and Relenza in its strategic national stockpile. Tamiflu, or oseltamivir, is made by Roche under license from Gilead Sciences, while Relenza, or zanamivir, is made by GlaxoSmithKline under license from Biota Inc.

The H1N1 virus has stayed around longer than expected. Influenza cases normally ebb over the summer and early fall.

“This new virus has found a home in Washington state, and it’s sticking around,” Selecky said. “We’re still seeing people hospitalized every week.”

Globally, the virus has killed at least 429 people, according to the Centers for Disease Control and Prevention. It is the first pandemic of the 21st century.