As many as 5 million in US infected with H1N1-study

* Up to 20,000 people hospitalized through July 23-CDC

* 6 percent of hospitalized people die, study finds

* CDC says 24.8 million doses of H1N1 vaccine available

By Julie Steenhuysen

CHICAGO, Oct 29 (Reuters) - As many as 5.7 million Americans were infected with the H1N1 virus between April and late July, U.S. researchers said on Thursday, offering the clearest picture yet of how quickly and widely swine flu can spread.

Researchers used computer models to estimate the number of people who have contracted swine flu, which began infecting Americans in April.

They estimated that 1.8 million to 5.7 million cases of swine flu occurred between April and July 23, sending between 9,000 and 20,000 people to the hospital.

About 6 percent of people who were hospitalized with the virus died, the team, led by Carrie Reed at the U.S. Centers for Disease Control and Prevention, reported in the journal Emerging Infectious Diseases.

This suggests that as many as 1,300 people died from their infections between April and July. Officially, 1,000 U.S. deaths have been attributed to H1N1 since April.

Dr. Anne Schuchat of the CDC said on Thursday the agency does not have an update beyond July 24.

"We do believe many millions of people have already contracted this virus in the United States," Schuchat said.

"It's probably now well more than 20,000 hospitalizations," she said. "Really, the priority is to minimize illness and death."

Part of the U.S. plan to do that was through widespread vaccinations, but manufacturing delays have stalled those efforts. "We had all hoped to have more vaccine now than we have," Schuchat said.

Earlier government estimates had suggested there would be as many as 40 million vaccine doses available for state and local health authorities to distribute by the end of October.

Schuchat said 24.8 million doses of the H1N1 vaccine are available, 1.6 million more doses than on Wednesday.

The United States has ordered up to 250 million doses of H1N1 vaccine from five companies -- MedImmune, a unit of AstraZeneca (AZN.L), Sanofi-Aventis (SASY.PA), Australia's CSL (CSL.AX), GlaxoSmithKline (GSK.L) and Novartis (NOVN.VX).